A True Infinite Baffle Subwoofer

Effect of flat surfaces facing IB drivers.

2012-01-18T17:31:00.002+01:00

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It has often been recommended that IB drivers should not face flat surfaces. Particularly close walls.

The subject was raised on AVForums with regards to an IB project. (DIY Subwoofer Build forum)

It was desired to have a pair of well spaced manifolds but this brought the 18" driver face to about 18" from a solid wall. There was some discussion (and disagreement) of the likely effect of their relatively close proximity to each other.

It seemed worthwhile to try and prove, using REW, whether an IB is really affected by flat surfaces. Reflected pressure wavefronts returning from the wall could theoretically affect the large driver cones. Or the wall might increase the pressure seen by the cone. Not a desirable effect in an IB if it affects the TS parameters.

I decided to place cardboard boxes on the floor to provide nominally flat surfaces facing my 15" drivers. The boxes reached almost to the top of the lower four drivers but were open at the top and sides.

The flat surfaces were placed about 6" from the faces of the lowest four drivers in my 8 x 15" driver manifold. Due to the access door to the IB enclosure I could not place the box on that side any further away. Only the bottom four drivers of the IB manifold were driven.

My Galaxy 140 SPL meter was placed on top of a pile of cushions. About 6" from the mouth of the manifold and about a foot above the bottom of the manifold.

The lower 4 x 15" IB was swept 3 times using the longest 1M REW frequency sweep both with and without the boxes in place. Each half of the manifold is physically separated buy a shelf midway up the 6' high box.

A clear and repeatable difference of about 1.5dB (maximum) existed between each set of graphs. The individual traces were identical within each set above 8Hz. The set with the boxes in place consistently reproduced the sweep at the higher level.

Only at 12Hz did the sets match: Below 8Hz variations are commonplace even using the longest REW sweep. This could be due to random air movements or draughts.

It seems rather surprising that such a clear result was possible from such a simple test.

There was a difference between the waterfall graphs too. Mostly at infrasonic frequencies:

With boards

No boards in place.

Now I ought to repeat the test using more massive boards of much greater size to see if this affects the outcome. Also to examine the effect of distance of the flat surfaces from the cones.

There is also the question of whether a diver would be similarly affected if the magnet faced the wall. Rather than the driver cone.

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BTW; I am still suffering from intermittent switch problems on the cheap CX2310 Behringer active crossover. Perhaps some specialist electronics switch cleaner would help? I have never felt the use for any until now.

Feel the vibes.

2011-10-22T21:16:00.003+02:00

I haven't been adding much to my IB blog recently. My wife and I are still enjoying a couple of new (hired) films each weekend. The IB subwoofer continues to surprise and delight when the film asks for it. The system is also switched on for the odd, favourite TV series. Though TV soundtracks are never remotely on the scale of action films. Why should they bother when the average viewer is listening to the TV's own speakers?

All our TV is sourced from our digital satellite receiver and two dishes. Terrestrial TV has never been successful locally due to our location in a hollow in the landscape surrounded by low hills. The Danish government chooses to code the Danish TV channels on satellite. This despite the highest TV license fee imaginable. 2260DKK = £264 GBP or $418 US. Is an exclamation mark really necessary?

Without a smart card one cannot watch Danish TV by satellite. The companies which provide Commercial TV channel "packs" just add in the Danish channels as an afterthought. Without paying for a TV pack one cannot get a free smart card just for the Danish channels.

We can receive about 2000 different TV channels and probably almost as many radio channels . I've lost count of how many of these are free of coding. Literally hundreds and hundreds including HD channels. But no Danish channels at all.

For receiving Danish terrestrial TV we would need to take a chance on buying a digital tuner, antenna and possibly a mast to receive the digital Danish terrestrial channels. So we never see Danish TV. Nor listen to Danish radio. Not ever.

The irony is that the majority of immigrants to Denmark have satellite dishes to watch their home programmes. The government complains that they do not integrate fully into Danish society. (if at all) I wonder why? Could it be that the only Danish culture they ever see is in the checkout queue at the local supermarket?

This wasn't the real reason I started scribbling this post. We were watching a hired DVD of "X-men First Class" this evening. I went down to the kitchen to make a cup of coffee without pausing the film. The infrasonics in the kitchen and bathroom were quite remarkable. These rooms lie partly under the IB enclosure. So it is hardly surprising that one can hear the bass through the wooden floor. Which is boarded both above and below. Thus offering little attenuation or damping.

These infrasonics seem reduced upstairs. Or they are masked by higher frequencies. There is no shortage of weird pressure effects and the air being "shredded". (as I usually like to describe it) What seems to be missing are the very slow beating effects. Where each pressure cycle can be clearly sensed. Giving the illusion of almost being able to count the beats. Though one would need to be incredibly fleet of tongue to do so aloud. They are certainly present on classical organ music but not so often on films. Perhaps it is simply a case of being able to focus on the infrasonics downstairs in the absence of other frequencies?

It occurs to me to try a few REW sweeps with the microphone downstairs. Just to see what actually gets through the floor. My wife has often mentioned the kitchen ceiling rippling as she prepared a meal. Usually during one of my classical organ music, listening sessions. Or when she nipped downstairs to check a meal's progress during a film. Not quite the nausea that the array used to cause in the bathroom. But still worth trying an REW sweep or two:

NB. All doors between rooms were closed during testing. Peak frequency levels were lifted in REW to match the listening position. A Galaxy 140 SPL meter was used as the test microphone with matching REW .cal file. Meter set to low range, C-weighting on a 10m twin coax cable to Creative Live! external sound card: Only the subwoofer was in operation.

Too much complexity despite peak levels being matched?

Listening position ^

Some 9' from the manifold opening in the left wall.
Measured with the SPL meter placed on a cushion on the seat of my chair.
At normal, seated, ear level, with the SPL meter on a tripod the lower bass dominates more than this.

Middle of IB enclosure ^

Only 3 feet from the 6' tall, 8 x 15" driver manifold. The enclosure is about 600cuft minus open storage. It is still very leaky to the outdoors at the very edge of the roof. Plans are afoot to finish the last row of insulation between the final pair of rafters before winter. The 45 degree sloping ceiling will then be finally boarded over.

Bottom of open stairs ^

There seems to be a distinct tuned port effect from the open stairwell. 20-25Hz is only borderline audible using sinewaves. My own audible barrier is at 25Hz as a very soft, fluttering effect.

Downstairs hall ^

A small space with a boarded ceiling below the IB enclosure. The hall doors to bathroom, kitchen and living room were closed. A double resonance.

Bathroom ^

Thinly boarded ceiling hiding the joists. It lies directly below the IB enclosure. A distinctly narrow plateau from 15-20Hz. Hardly surprising the vibes can be felt rather than heard!

Kitchen ^

Boarded ceiling between exposed joists partially under the IB enclosure.The peak is at 20hz with strong extension to even lower frequencies.

Other end of listening room ^

A triangular opening, about the size of a door, leads from one half of the attic listening room to the other. The room is split almost exactly in two by the chimney. A broad peak with an emphasis on the very low bass. 9-15Hz is all infrasonic.

Middle of downstairs living room ^

Open to the listening room via the open stairwell. Actually a simple rectangular opening in the floor/ceiling. There may be a bandpass effect at work here. A 9Hz peak!

As expected, the lower frequencies dominate. With an emphasis on the infrasonics. This is hardly surprising given the ready willingness of boarded surfaces to resonate and retransmit the acoustic (cyclic pressure variation) signal which excited them in the first place.

From memory, the levels downstairs were 10-15dB below the listening room levels. Though I made no real effort to record them. REW has recorded all calibration levels as the same. So I had better get busy and record actual levels relative to the listening position. The information might be useful in that it will give some idea of bass "leakage" between rooms with wooden floors.

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I have discussed contractor's, plate vibrators here before. Today I came across another. Outside a supermarket I was visiting was a very deep trench. A contractor was working at consolidating the gravel backfill. The noise he was making was all low frequency. My chest was resonating strongly which usually means somewhere up around 140hz. Though the clearly audible sound reminded me more of my high level tests with sinewaves at 40hz. Very unpleasant it was too! It was actually far more comfortable to go much closer than to stand 20 yards away loading my shopping. I don't know why this should be.

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As promised I have checked the noise levels leaking downstairs and elsewhere:

I opened REW and chose Preferences, Subwoofer, Check Levels. Then I switched on the audio system. I set the level at 85dB(C) at the listening position using the system's volume control and the Galaxy140 SPL meter. This was set to the 80-100dB range C-weighting - Slow. I shut all the doors as I moved from room to room with the meter held aloft like an Olympic torch.

The levels were roughly 8-10dB down in the other half of the room. 10-15dB down in the downstairs living room, bathroom and hall. And, all of 20dB down in the kitchen. Turning off the speaker amp and repeating the exercise made no real difference. There was some variation (about 3-5dB) as I moved around in each space. There were measurably higher sound pressure levels against solid room boundaries.

The ceiling vibrations downstairs must be quite narrow band. On film LFE the IB easily hits peaks of 110dB+ as read directly on the SPL meter. This would easily equate to 90dB+ levels even in the kitchen.

I had no desire to set the system to a steady state SPL much above 85dB(C). It is not at all pleasant to subject oneself to loud, pink noise. Nor would playing high levels of sinewaves be any more comfortable.

No doubt one could eventually reproduce the resonant frequency of the downstairs ceilings. Provided, of course, one wore ear defenders while in the listening room. An aid downstairs to report any resonant effects, as frequency was adjusted up and down via the mouse, would speed things up. I would imagine these resonances lie deep in the infrasonic region rather than the audible.

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I tried something I haven't watched for ages. Lord of The Rings: Return of The King. I laid the Galaxy 140 SPL meter down on a cushion about 9 feet from the IB manifold. It was left on Max Hold, C-weighting, 80-130dB range. By the time the film was finished it had hit 119.7dB(C). My wife was outside gardening for the first hour. She said the bass was very audible and rather repetitive. Boom-boom, rattle. Boom-boom, rattle! :-)

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A small victory was gained against a continuing, slight problem of mould on the inner back panel of the manifold. I simply stapled a black, closed-cell foam, camping mattress onto the outer back panel of the manifold. (from top to bottom) The extra insulation, provided by the foam, seems to have cured the problem of continuous dampness. This only occurred on the lower back panel inside the manifold. The drivers and side panels seemed unaffected.

Only thin, office-type staples were used to fix the foam. To allow easy removal if desired. This panel is invisible from the audio room but I used black foam to match the manifold anyway. Checking with a torch shows that the inside back panel remains perfectly dry now.

I really must finish insulating the last bay of the roof above the IB enclosure. This requires working from above. Weather conditions never seemed suitable this year. With constant winds. Having fallen off the roof before I am not allowed up there if there is the slightest risk.

Click on any image for an enlargement.

Faulty CX2310 mute switches

2011-07-24T14:10:00.005+02:00

Just a warning that the push button, channel mute switches (two of which are arrowed above) can be temperamental.

Click on the image for an enlargement. Back click to return to the text.

Last night we watched a film on TV and we could hear no bass at all. Since we couldn't pause the film we watched it to the end and then went to bed.

Checking the next morning, we had completely lost one channel of bass and what remained was scratchy as hell. As if all four drivers had bottomed out on that channel! Which should have been impossible given the usual tiny excursions on an eight x 15" driver subwoofer.

I turned everything off and pulled every plug in the entire system and reinserted them all to clean the contact surfaces. That is quite a lot of plugs!

Then I switched everything back on and twiddled with every knob. I waggled every driver connecting wire and speaker cable. Nothing made any audible difference.

Time to get serious! So I took off the organ CD I had been playing and put on Bass Outlaws. Just to ensure I had a strong and continuous bass signal to play with.

Finally, I found that one CX2310 mute switch was electrically stuck in the mute position despite the light being off. It was very noisy when pressed in and out. While the other switch was scratchy and intermittent in its switching behaviour. It was this channel which was producing the rattly, scratchy noise in the bass.

A few repeated presses and both switch faults cleared. Suddenly all the bass came back with a vengeance! I often use these mute switches when testing individual channels with REW. So they may have had rather more exercise than is usual.

So now we know where Behringer saved the pennies on this inexpensive active crossover: On switch quality and reliability. Worth bearing in mind if you have similar symptoms.

The beast is loose at last!

2011-02-19T22:50:00.012+01:00

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Mixing in the LFE has released a violent monster. We have just watched "The Last Air Bender" with terrifying levels of bass suddenly unleashed from nowhere.

After the first warning, when the whole house shook like a leaf, I turned the EP2500 controls right down from 12 o'clock. When the house was still about to collapse I turned the LFE right down on the mixer control. A cut of about 10dB. I was till getting occasional red flashes on the BFD with every surface vibrating violently. I have never experienced anything like it before.

Years of messing about with downmixing had not prepared me for this. The highest I ever registered (during a film) before, was about 111dB(C) on the Galaxy SPL meter. It was brutal and certainly impressive but almost a whimper compared with the present performance.

Being completely dark at the time I had no way to tell what the cone excursions looked like. Eight 15" drivers moving in unison never felt so dangerous as they do now. I know the IB can do 135+dB on sinewaves above 30Hz without much sign of anything happening at all.

That seemed like mere child's play compared with what we just experienced. The walls, ceiling, floor and windows were shaking so hard I feared for the safety of the structure. The vibration through the floor was literally painful. The air around us was being shredded by an incredibly forceful shaking.

Every moment of film watching, prior to mixing in the LFE, feels as if it has been completely wasted. I hadn't a clue what I had been missing even on DTS. I had sound quality aplenty but the infrasonics were almost absent. My wife kept saying that the SVS cylinder was far more impressive. I am ashamed to admit she was right. I had been fooling myself that SQ was more important than the physical violence of which a large subwoofer is so easily capable.

I was so keen to believe that my IB was impressive that I shut out the memories of rattling eyeballs. The weird, slow waves of bass which passed eerily through the room. The terrible shaking of dragon's wings on LOTR. All this from a puny 12" driver in a cardboard cylinder covered in a thin rug.

Not even a better model either. My SVS was the cheapest (though tallest) of their range when I imported it. No wonder it had turned my wife onto serious bass and infrasonics. We were both incredibly impressed right out of the box. We went around with a silly SVS grin for literally ages.

Then came the demand for more. For greater safety on the really serious peaks. The SVS seemed to peak at about 107dB on the meter when there was a loud bass roar. The IB seemed like the obvious answer at the time. It went a bit louder than the cylinder but was hampered by very non-spec drivers and a flimsy baffle wall.

Then the array gave way to a manifold and a serious gain in output and drop in vibration. Then newer drivers arrived to replace the old ones. Then I built all eight into a box taller than myself. Quite honestly it was all a complete waste of effort for films! We watched literally hundreds of films without the bass which the director had intended.

I was very seriously handicapped by the lack of LFE. On music the IB went loud and deep. Only fear kept levels reasonably modest. On films the bass only went loud when the dialogue and everything else was far too loud. I read other people's descriptions of film bass and buried my head in the rock wool lining of my enclosure. I was in serious denial.

The weird thing was that I could easily make myself nauseous on organ music. Films were a loud disappointment. The bass was certainly there but not like it should and could have have been. It was exhausting sitting through an action film at reference level without the special bass sound effects.

Now I don't know whether to feel like a complete fraud or a complete chump. So much potential was just sitting there with so little reward. Until now. The mixer has finally allowed me to inject the LFE into the front channels.

The IB must have hit 130dB (briefly) on the effects at the start of the film we watched last night. "The Last Air Bender". Without any exaggeration the vicious vibrations were really very frightening. My arm shot out to cut the bass on the mixer. Then I went out to the EP2500 and cut that to zero on both controls. I was still seeing occasional red (clipping) lights on the BFD. The air, floor and every surface in the room were still shaking incredibly violently at times. It sounded as if everything was literally going to come apart. An odd film with good points and bad.

I shall have to go back to the beginning and recalibrate the bass channels and the IB relative to the speakers. I remember leaving the IB/speaker calibration flat on "The Calibrator" DVD after discovering it was a a few dB "hot". The speakers didn't sound as loud as usual when we started watching the film and I turned up the volume to get clearer dialogue. That may be where the problems started.

Another update 26/2/11. Just watched XXX with Vin Diesel on DVD. The avalanche scene hit 117dB(C) Max hold on the Galaxy 140 with no red lights on the BFD. It sounded strong without the imminent threat of damage. The mixer BFD control was at 12 o'clock. I should have measured the explosion at the earlier drug bust helicopter scene. I think it was a bit louder. Exterminator Salvation played at up to 111dB(C).

March 2011 RED and Takers both peaked out at around 115dB with no red lights on the BFD. The hotel gunfight on Takers was spectacular. It was this scene I checked on the Galaxy 140 SPL meter afterwards. Both films were good fun.

Another update at the end of March 2011: Instead of previous attempts to balance the drivers I am trying balancing the orange lights on the EP2500. Previously I have tried to match the frequency responses of the two very different sets of AEIB15 drivers. This didn't work too well because it robbed the drivers of energy in their respective power bands. This greatly reduced impact and had me searching for alternative ways to get it back. OBs and small sealed boxes were tried but discarded on SQ and other grounds.

I am now using only one filter on the BFD. +16dB @ 20Hz with 120BW on the older 32Hz, vinyl coned drivers. They don't do deep bass without massive boost. Now both sets of drivers are still very different. The older 32Hz drivers are stronger higher up. The newer, paper coned, 13Hz drivers roll off early at the top end. How best to match them so they share their output to best effect? I have tried matching their excursions on programme material but this isn't ideal as it is so frequency dependent.

One day I was watching the orange lights on the EP2500. Just checking for clipping on very loud LFE effects. It was obvious that one channel had a continuous orange light while other only flashed on now and then. I have yet to see a red clipping light despite all my audio games. The two sets of drivers not only have very different physical characteristics but also different impedances. It occurred to me that I should try to match these orange lights on broadband LFE effects. The EP2500s controls ended up at 90 degrees to each other. I then physically checked the excursions with my fingertips on the outer suspensions of each set of drivers.

On bassy music (like Bass Outlaws) the drivers show quite serious variations in excursion. This is unavoidable. On deep and continuous LFE "roars" the drivers are much better matched. This suggests (to me at least) that they are now sharing output duties more evenly. The sound and feel very strong on films now. With some amazing infrasonic effects on recent films. I must find time to try some of the old bassy film classics like LOTR. I have never heard them played properly on my various IBs.

Of course nobody sensible would use two very different sets of drivers in one IB. I had no choice in the matter. I was originally supplied with a set of four very poor quality, very badly machined and cosmetically unfinished IB 15s.

After much fretting about the missing infrasonic bass I discovered they had a totally non-spec Fs of 32Hz instead of 16hz. I had followed a thread on an audio forum where another owner was complaining about his IB15s. So I tested my drivers myself. Using REW, a small amplifier and a series resistor I discovered Fs averaged nearly 33hz instead of the claimed 16Hz.

Later, when John H @ AE offered me four of his latest 16Hz IB drivers I discovered these were non-spec too. Measuring at 13Hz average Fs. He must have dumped some of the pre-production prototypes on me. At least they were properly finished this time. They cost me a lot of money to get delivered and cleared by the freight company.

John had originally told me he would pay my costs but didn't. After posting a favourable review on his AE forum I have now been banned. Presumably for speaking up about his non-spec drivers on another forum. It seems supplies of his excellent IB drivers have now dried up. He seems to be incredibly unlucky in his choice of workers and suppliers. Last I heard he was working alone to catch up on backlogs of unfilled orders going back for over a year! Dé jà vu!

Ironically, Fi, the only other supplier of specialist IB drivers, is restructuring. Henceforth production of all their HT drivers will be under the Blueprint name. It seems too many customers were using their HT drivers in car audio applications and breaking them. Several months without any IB drivers being available is causing some disquiet in IB circles. Blueprint hasn't made any public announcements for several months.

If there is any doubt about the latest boost in performance of my IB you may find the following amusing: My wife had gone downstairs while I set up the system for a new hired film. She came back up with a bowl of yoghurt just as an LFE moment hit. She was so shocked she poured her yoghurt all down the front of my system rack! Of course I switched everything off as quickly as possible. Then there was an intermission while everything was cleaned up again. No damage was done but it is revealing of the incredible shock value we are now getting from the IB. :-)

Mixing it.

2011-01-10T19:02:00.071+01:00

It has been a bit quiet around here so I ought to bring things up to date: Don't get too used to the new blog format or colours. I will be playing for a while yet. Somebody messaged me that they couldn't read the text. I had some sympathy with their plight. I had been trying to adjust the transparency of the post background for days but couldn't find anything about it online. In the end I have reverted to an opaque background. Highly legible but just a tad boring compared with a translucent background. I may play with these colours some more. I have now taken the (flash) shine off the AEIB15 drivers to avoid glare. Feel free to comment if you have anything constructive to add on how you think the appearance may be improved. I have tried most of the Blogger templates without success on a slightly transparent text background.

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To the point: I was becoming ever more tired of a lack of bass impact on BluRay and Dolby Digital 5.1 films. Setting "All Bass to Mains" on my old Yamaha E800 amp/processor was robbing the IB of its full potential. The downmixing to put the bass through the front two channels compressed the dynamic range and/or discarded the LFE. The E800's SubW out socket had remained (almost) unsullied since purchase. Though I had tried connecting the SVS cylinder once or twice.

RX1602 16 Channel Mixer

When I was first playing with IBs I had bought a cheap mixer. A Behringer Eurorack Pro RX1602. It had remained in its box unused. The original idea was to use it to push the missing LFE into the system somewhere. As I had concentrated on music and automatically chose DTS, when watching films, I had left the mixer simmering on the back burner. I hated having it just sitting there unused but a long series of roundtuits had intervened in its promising career. I had no real desire to add yet another box to the music chain. So if it was used it could only be used on the subwoofers. The speaker signal must be left well alone.

Rear of RX1602 Mixer

On music IB bass can be as thunderous as desired. It can blow out windows and lay waste over a wide blast radius. 135dB+ on the SPL meter on sinewaves? No problem at all, Sir! The room and downright cowardice usually set the limit on how load I could play real music. A steady 110dB(C) on Metallica is usually my limit.

On films it was a different matter altogether. Without the LFE input I had to play everything very loud to get any real bass. It was no use just turning up the IB wick. That just made everything ponderous and heavy without increasing the dynamic peaks. Playing very loud may have produced plenty of loud bass but it was exhausting to experience a whole action film. Coming back from an errand in the kitchen the film always seemed shockingly loud. (pardon?)

CX1310 2.1 Channel Active crossover

Because it is intended primarily for music my system is run as stereo throughout the chain. All the way from from the source to the two sets of drivers in the 6' high IB manifold.

Films use dual mono L&R on the front mains fed by the E800 surround processor. For both music and films the IB bass is split off from the Front Main speaker channels with an active crossover at 80Hz. A Naim NAP180 drives the Mission 753Freedom, floor standing, main/stereo speakers from the Hi channel of the Behringer CX2310 active crossover.

Stereo music ignores the E800 surround receiver/processor and the power it provides for the Centre and Rear speakers. So the E800 is left switched off for music.

Rear of CX1310 active crossover

How to get the missing dynamic range into my very powerful IB subwoofers system had largely eluded me since I first started. The system worked fine on music CDs. It worked well enough on DTS DVDs. Dolby Digital DVDs needed to be played too loud but were still rather gutless in the way of bass dynamics. If I wanted loud bass then so was everything else! BluRay disks were usually a bore from the bass point of view. Which may explain why I have bought so few of them. Apart form the eternal problem of rarely wanting to see a film twice within two years. With the unique exception of Hot Fuzz. Of which we never seem to tire. :-)

It may be that I need a new receiver with all the latest "bells and whistles" to maximise BluRay's audio potential. ((I hope I'm not being too technical for some of my audience here. ;-) ) However, I have absolutely no desire to buy a new receiver, with its built in obsolescence. I'm not prepared to upgrade constantly as manufacturers vie to make up silly names for their latest hyped-up formats. If I really wanted an electric room heater I'd buy a heater. Not an AV receiver.

So, finally, I have decided to drag the unused mixer into the system. It would handle both stereo and dual mono on the Front Main bass channels only depending on source. (i.e. CD Player or BDP) The only real difference is the mixing of the missing LFE into the front two channels on films.

At last I no longer need to set the E800 to "All bass to mains". It is now set to 5.1 for both Dolby and DTS. This immediately precludes the automatic compression on DD5.1. Nor is LFE discarded any more.

At this point, I have only had a chance to squeeze the mixer into the middle of the rack and swap over to the new cables. The mixer has stereo jack sockets only, for balanced operation. As I had no stock of stereo jack cables in my collection I had to go shopping for cables first.

Once I had everything safely in place, "Expendables" on BD was my first test subject. It had sounded strong and powerful on a hired DVD. But fell flat on the BluRay disk which I had bought my wife for Christmas. She loves "things blowing up" so the disk seemed like a good way to feed this unusual taste in films. On the BD disk I had played the same scenes repeatedly without the mixer but it just would not come to life at any level. I kept wanting to turn the overall volume down. Not further up!

Despite the very rudimentary mixer set-up I was getting intermittent red flashes on the BFD bars. That was a new experience! Instead of peaking at 103dB on my test scene (with over-loud dialogue) I hit 109.7dB(C) on the Galaxy 140 SPL meter on the very first trial. 6dB is not to be sniffed at when one is hungry for more impact! More importantly this was at far more comfortable overall listening levels.

The mixer LED bars hardly showed any signal so I still have some settings to play with. The EP2500 power amp on the IB was showing steady orange lights but no red for clipping. No sign of cone excursion on the eight 15" drivers so there's plenty in reserve if I can only tease it out.

Here's a diagram of what the basic system looks like so far: (ignoring sources of course)

Don't ask me why I drew it upside down, compared with reality, but that's just me. (PhotoFiltre drawing and text)

Note that the mixer only intervenes in the front mains/stereo speaker, bass channels coming from the CX2310 active crossover. So it has no effect on the Stereo/ Front Main speakers' SQ. The Missions go down to 40hz at full power. So an 80Hz crossover suits them fine. On full range material they are supposed to be good for 110dB. With an 80Hz crossover they can probably go louder as an easier load on their Naim NAP180 amplifier.

Just as before, not switching on the E800 has no effect on music. The mixer must be left on because it passes the stereo channels onwards to the big EP2500 amp which powers the IB subwoofer drivers.

Behringer Feedback Destroyer two channel parametric filter.

I have a choice over BFD input levels (domestic or pro) but haven't examined this possibility in conjunction with the mixer yet. I'd like a lot more headroom before the red lights come on! Later on. I found that the push button selection was the answer to removing the red clipping bars on the BFD.

Rear of BFD.

With so many different level controls in the chain the possibilities are endless. On films the Naim preamp no longer seems to have any effect on bass levels until well advanced. (far higher than I would ever listen to music) The E800 volume control affects everything now. Before the mixer was inserted it had no effect on the subwoofers. I shall have to play around with level settings to maximise dynamic range without increasing the dialogue and ordinary sound effects and film music levels. It will be a real pleasure to enjoy shots and explosions at thunderous levels without actually having to listen at reference level.

The real question is why did I wait so long? My old SVS cylinder used to manage 107dB(C) peaks on the Galaxy SPL meter. It was rare indeed that I ever saw the same peaks on the IB on films.

More details will follow when I have had a chance to play with level controls. Pictures too, when I have tidied up the "knitting" behind the stack of boxes in the rack. You have no idea what is involved in a rack tidying session with 10 boxes vertically arranged! Many of the cables are yards long and coiled up with zip ties (tie-wraps) keeping the weight off the plugs and sockets. I may even pull the whole thing out from the wall once the mains is safely switched off. Fortunately most of the XLR signal plugs are marked with Dymo labels. Maintaining L&R between boxes is always a problem without labels.

Front and back of the system rack. (after tidying up!)

Stop laughing at the back! :-)

As all the mains sockets are on the right and all the wall sockets on the left I dressed each mains cable along the horizontal bars and tied them to the rails with several zip ties. I try to avoid tying more than one cable in at a time. There are always reasons to remove one box or cable and snipping and replacing multiple ties holding several cables quickly become a bore. Hum has never been a problem but having the mains cables at right angles to the signal cables is probably good practice.

My usual musical instrument and cable shop only has minimum 2 metre lengths on display. I could do with 1 metre (3') or even less, for most of the connections behind the three Behringer boxes. Though not all. Lots of longer cables go off to the big IB amp, the Front speakers and the LCD TV.

The Fortec Star Passion HD satellite receiver has finally lost its place in the rack and will go under the TV once I have a coax coupler to extend the cable from the dishes outside. Or, more accurately, from the DiSEqC box which the dishes feed into.

RX, CX & BFD stack. Mixer, Crossover and Subwoofer Equaliser.

My rack is just the right width for these boxes but I prefer zip ties around the tubular frame uprights to hold them in place. I can guarantee that if I drilled the rack I would want to move the boxes up or down within a week! Not fixing them to the rack means the boxes slide backwards when turning them on or off with their press switches. These Behringer boxes should always be turned on first and turned off last to avoid loud thumps in the speakers and subs. Completely unforgivable these days, if you ask me.

I have yet to try setting the Front Main speakers to Small for films. With the former system I had no choice. The bass for the IBs was split from the Front speaker channels. With the mixer in place I now have a choice. Though I shall have to be careful about not cascading crossovers.

A quick test of the "Expendables" motorcycle scene (on BD) was very satisfactory with Mains still set to Large. With thunderous bass at appropriate moments but without any "heaviness" in between. Realism above all else. I'll happily swap 10dB of potential headroom for 3dB of actual dynamic range. A dull roar has no impact (at all) compared with a sudden loud shock from nowhere. Which is why heavy metal is so boring for some listeners. Everything is at the same level without any contrast. Metallica excluded. They know how to build to a crescendo from a quiet riff.

The BFD wants to stay with domestic input level settings. Switching it to Pro settings produced two vertical row of bars on almost no bass. I'm using only a 1/4 of available level on the mixer L&R Main Out knobs. The yellow LEDS on the EP2500 are about right with only 1/4 setting on the twin control knobs. Bass sounds quick, deep and absolutely effortless. "Brutal" is my preferred term for the realism of IB bass but I may have overused it just a tad.

A quick check of the Bass Outlaws CD provided massive bass when called for. There seems to be no limit as to how loud it will go if desired. Not that one wants to swamp the speakers. Balance in all things. It also saves destroying the house. The rattles from the walls, floor, windows and other items sometimes make me wish I had a sealed concrete cellar to play in.

Following the recent changes I have finally fitted trunking across floor in front of the double doors. In doing so I managed to pull the bananas out of the Naim power amp. Then I discovered that one set of IB drivers wasn't working. I had been stripping out the Component cables to the TV and HDMI to the satellite receiver and had dislodged an XLR plug. More fiddling with the help of a pencil torch and all was well again.

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I was just testing my mixer controls with organ music. The bass on Guillou's, "Cesar Franck Complete Organ Works" is shockingly good. (Rather oddly, I have read critical comments on both Guillou and Franck on YouTube organ videos) This double CD is a DDD recording: 92292 from Brilliant Classics.com.

The detail, texture and weight are in a class of their own. Franck is easily my favourite organ music composer and this my favourite CD. I used to prefer Vierne but was never that keen on Bach. Though he does have some very pretty tunes. I have access to a large, free collection of CDs at the Odense Music library. My ISP also offers loads of free downloads of organ music CDs.

My vinyl collection of organ LPs is also steadily increasing thanks to occasional finds at charity shops. I haven't counted my organ LPs for a while but the numbers must be close to 60 by now. Few of them remotely match the best CD's for bass. Though many LPs have a uniquely "live" and emotional quality which CD never matches. Perhaps I should repeat my SQ listening test comparisons between the CD63SE and the LX70A on Franck instead of Clannad? :-)

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Interlude:

I noticed something interesting while out on my daily bike rides in the recent days of thick mist. The low frequencies were heard earlier and much later as vehicles approached and receded respectively. The sound also carried far better than normal. So I was made aware of a vehicle coming long before normal. Conversely I had to suffer their racket for long after they had disappeared into the mist.

My guess would be an inversion layer. One which stopped the low frequencies from escaping vertically to be scattered and absorbed in the atmosphere. Low frequency (long wave) sounds are known to travel much further than higher frequencies. Large animals use deep sounds to communicate at great distances. Particularly in the forests where higher frequencies would be rapidly absorbed by the intervening twigs and leaves.

I wondered at the time whether the inversion layer was performing as an acoustic boundary (layer). Further augmenting the low frequencies by providing additional boundary gain. This strange acoustic effect was certainly very noticeable but was completely absent after the mist had lifted several days later. It may be that forests provide similar conditions. The bare trunks of the trees would offer a clearer pathway for lower frequencies. While the dense mass of branches, just above, would form a partial barrier to vertical absorption. It may be that a form of low absorption, transmission line or wave guide is involved. Further reinforcing the effect at even greater distances.

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Meanwhile, back at the ranch:

A few more films later and I am still very happy that I added the mixer to get "real" 5.1. I still haven't tried setting the Fronts to Small but I have the 80Hz active crossover which probably amounts to the same thing. I'm still puzzling over this one. While more bass may be sent to the subwoofer channels the bass is already in the Front Main channels. Which are then split at 80Hz. Probably a "swings and roundabouts" situation. I may have a play if I have a boring few hours to kill.

BTW: Film music is also much improved with new bounce and drive. Maiden Heist has a catchy number at the beginning of the credits. It sounded great wound well up. Though not the sort of music I would ever sit down and listen to seriously.

Well, we have just watched "Jonah Hex" at spirited levels. A total bass orgy! I only put the SPL meter on towards the end and immediately hit 117.5dB(C) on Max hold! Absolutely amazing! The baffle wall was shaking like a wall really shouldn't. The shocks through the floor were downright nasty. My CD storage was threatening to throw the entire contents onto the floor. It will have to be moved. And the wall clock too! :-)

Later, I checked the speaker/subwoofer level calibration using The Calibrator DVD. I discovered that the IB was running about 5dB "hot" on the pink noise test tracks. I have reduced this now to avoid hitting red lights too often on the BFD!

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Here's a great YT video about an audio/HT system with an IB containing four 18" Fi IB318 drivers built into a solid block wall: Try not to drool on your keyboard at the sight of four 18"s arranged vertically. ;-)

Click on any image for an enlargement. Back click to return to the text.

Odense Elektro Days Show

2010-10-17T14:41:00.011+02:00

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I attended the annual Odense domestic electronics show with high hopes for some great audio and plenty of visual toys. The vast majority of the show seemed to revolve around two large halls of large, flat TVs. 3-D headlines were splashed everywhere but I counted only four pairs of 3D glasses to go around several thousand wanna-sees.

Somebody really should be publicly thrashed for the long delays when there was nothing to see on many of the screens. If they can't get rid if the endless cues, long blanks and silly credits, for every short film sequence, then they really should seriously consider the attention span of the average customer at such a show! Especially me!

Many of the screens showed jerky sideways movement on moving images which wasn't making me at all hungry for a better screen than my modest JVC 100Hz 37" LCD. My LCD doesn't judder on fast moving sideways action!

There was one huge, wall-sized screen but the image quality, while acceptable, wasn't exactly crisp. Nor, sadly, was my photograph of it. Most of the other screens varied considerably depending on source material.

The "real" audio was tucked well away from the main halls in much smaller rooms at the far end of a long corridor. A stack of Naim electronics, which in total would probably cost as much as a house. It was all racked up on their incredibly expensive Fraim support system.

The kit was playing through white, fire hoses to Audiovector S3 "Avantguard Arretté" speakers at over £5k a pair. Smart floor-standers with two 6-7" bass-drivers and a fancy looking tweeter above, consisting of a series of sub-elements on a recessed plate.

A picture of the very pricey Naim kit. Probably no change from £2k on any of the boxes on display. With several running from £12k for the CDP to well over £15k each for the pre and power amps.

I failed to capture the computer monitor perched right up on top. This automatically displayed track details and the album artwork after dropping a CD into the player. Not forgetting the inevitable puck. Naim kit is always hard to photograph but the room wasn't well lit anyway. I should have moved further back because the wide angle lens has distorted the geometry of the kit. The room was a bit too crowded for relaxed photography.

It all sounded very Hifi with plenty of drive, wallop and the ability to go quite loud. Bass was easily strong enough but it completely lacked any real extension, realism or physical presence. There were a few tracks with acoustic bass but it didn't sounds as if it was in the room. Unless it was a cello being plucked. All thoroughly enjoyable if you don't mind the obvious distortion on vocal pushes and the poor imitation of real bass. It would easily satisfy a great many listeners who know no better. People like investment bankers, estate agents and footballers. Basically anybody who can drive an Audi badly while suffering delusions of grandeur as to their true value to our world.

As usual, my attention was immediately drawn to the slightly brash Naim signature sound. I have owned a modest preamp/PS/power amp Naim combo for nearly fifteen years. So I'm not just picking on the Naim kit for the sake of it. The forwardness is soon forgotten but can make some speakers sound "too loud" and tiring to listen to. Though not in this case. It was very well balanced without actually being at all wonderful.

There was the usual grainy SQ from CD and the tizzy edge to vocals. There seems to be no escape from the forward presence regardless of the wide range of music played. Somebody had brought along a favourite artist's CD and this sounded very HiFi too. It was well recorded but certainly didn't make me want to go out and buy it like some systems do. Nor could I tap my feet reliably to anything which was played. This is completely unforgivable in any Hifi system! Particularly one from Naim! PRAT! (Pace, rhythm and timing, to the uninitiated)

Basically it sounded just how Hifi is expected to sound. Exciting and loud in places. Unfortunately it didn't sound at all real. The imagery told us there was a soloist surrounded by a group of musicians up front somewhere. That didn't mean it sounded as if there was somebody actually performing right in front of the seats. There was no sense of being able to put one's hand out and touch anybody. I moved about a bit as seats became available but could not improve on the basic offering.

Just to add to the fun somebody was demonstrating HD audio via a computer right outside the open door! A tragic case of mismanagement by the show organisers. One expects to be able to wallow in peace and luxury at this price level. It seems I missed some demo rooms due to closed doors being completely unmarked. Why the hell was Bose allowed into this area to take the largest room available? It speaks volumes of how the organising electrical chain has moved down-market into white goods. Probably in a desperate attempt to stay afloat in a recession. I think one may safely assume Bose offers a good mark-up and a ready willingness to pay for exhibition space.

In the high-end "chatter room" I found a massive, thread-driven platter, a 3/4" bearing shaft and a weird 12" SME 3012 clone. With (horror of horrors!) a detachable headshell!?! I have a couple of old 3009s knocking about somewhere. One with a detachable headshell. The other the 3009 Improved. (Non-detachable headshell) Including the silicone damping trough on one of them. I think this was an attempt to kill the knife-edge rattles from the new breed of low compliance moving coil cartridges. These were more usefully attached to a Linn Sondek/Ittok. Which broke SME's stranglehold for those who could afford this level of kit. Or was it all about damping record warp related, cone movement? It all seems a very long time ago now but I remember it all being horribly expensive.

The room had several Avid turntables, lots of flashy, traditionally chromed, valve (tube) electronics and a variety of speakers. I recognised no names on most of the other tin boxes except for Tim de Paravicini. (from a mention in a magazine) A pair of promising, blond, Spendor, floor-standers were pushed towards the front. Though a modestly-sized pair of OBs were playing when I went in. At least they looked like badly undersized OBs. Though they weren't grabbing anybody's attention at all. In fact the room was stuffed with Hifi bores all talking very loudly! I could hardly hear the music. 'Nuff said?

Then we had to stand up while they rotated all the chairs by 90 degrees so we could be subjected to a new lot of kit. This time with a large pair of forward leaning, piano black speakers and another, more stealth-like, black Avid turntable. This system immediately sounded so dire that I had to leave the room to stop myself from bursting out laughing! Or complaining out loud in my pidgin Danish! You know when you put a bit of greaseproof paper around a comb to make a simple kazoo when you're a kid? Well, now you've got the picture. :-)

When I glanced back into the Naim/Audiovector room it looked as if the kit had packed up. Or some fault had developed in the demonstrator's skills. Probably just tired batteries in the terrifyingly expensive Naim remote.

So, all in all, the free pass to get into the show was well worth the asking price. ;-)

The best part of any show is always coming home to a modest system which completely trashes anything you heard at any price on the day. Bung on "Gaucho" to make sure you weren't terribly mistaken about the high-end kit's graininess, total lack of warmth, realism or depth. Now wallow in that sweetness. "Hi, Honey, I'm home." :-)

Me negative? Only in the sense that I didn't need to spend any more money to remain happy with my music. If I get a free ticket next year I'm definitely taking my favourite Franck organ CD. That'll show 'em! :-)

For the humour-challenged, obsessive-compulsive, hi-fi phreak, none of the above should be taken too seriously. It's just that some of us have moved on from treating overpriced boxes as sacred objects. We no longer erect shrines in our listening rooms. Now we listen to the music and pretentious hi-fi just gets in the way. As a magazine equipment reviewer I'd last about two minutes. I have no respect for anything which doesn't play music. I certainly heard nothing today which could manage it. Until I came home.
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As you were.

2010-08-28T06:34:00.009+02:00

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Well, I have now replaced the older drivers back in the bottom four holes of the big manifold. Since I had spent some time adjusting filters on the BFD for the experimental "sealed" boxes I had to start from scratch. It would have been more sensible to have started a new filter series but one thing led to another. A small change just to see what happens and then another...

A quick check with a 9V battery on the speaker cables, at the EP2500 amp end, confirmed that the drivers were all moving in unison. I had wired them in series-parallel again to obtain a combined load of 4 Ohms.

I started with an REW sweep of the newer drivers with no BFD. (or rather with the BFD bypassed and no LED lit on the top, right button.) No obvious problems with 1/3rd octave smoothing. I'm not going to bother trying to improve this with the BFD.

The older drivers proved not to be quite so accommodating. I added two filters. A boost at 20Hz and a -5dB cut at 40Hz to flatten the hump. That was all that was required for the result below:

Interestingly(?) there is a phase anomaly between the two sets of drivers. The new version of REW can now display phase though I omitted to record it. The effect of this anomaly is that the older drivers suck out the bottom end of the newer drivers unless their phase is reversed. I chose not to reverse the phase because of the unwanted effects higher up the frequency range.

Here is the combined effect with the drivers level matched and in parallel:

Not a bad result from just two BFD filters on the older set of drivers and leaving the new ones quite untouched. No cheating here. This is exactly as measured at the listening position at ear height.

My next objective is to better match the IB to the speakers. The images above are with a much higher crossover point setting on the CX2310 to match the different needs of the "sealed" boxes. I shall have to experiment with a lower crossover point to ensure there is no bass source localisation. Though none was noticed upon listening to a few samples of music. The crossover is probably somewhere up around 200Hz, at the moment, but there was no more time to play after dinner.

Below is shown the awful "raw" result from having no BFD but with the speakers playing as well as both sets of IB drivers:

The deep trough, centred on 180Hz, has returned with a vengeance. I have now decreased the IB level relative to the speakers but need much more time to play. It still sounds amazing compared with the previous "sealed" boxes. These were intended as mini IBs but I was never allowed to make new holes in the walls for heat loss reasons.

I have had another idea though. The ceiling below the AV stage needs to be redone. I have already boarded the other half of the ceiling in the sitting room with pine. I could use the stage area as an OB (open baffle) with the manifolds opening into the room below. The distance to the edge of the baffle would only be about six feet to the open stairwell at my feet. Still enough to be well worthwhile if deep infrasonics are not required.

The older 32Hz drivers don't do very deep bass anyway without massive boost. The best thing about this idea is no connection to the outside world. So no heat loss. The area below the stage is a transition area to the main sitting room area so a couple of ceiling grills will pass completely unnoticed.

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ib + sealed boxes

2010-07-25T18:01:00.015+02:00

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Having built the two new, 2 x15" manifolds some domestic resistance was encountered to cutting holes in the walls. So they were simply planted open side down on the carpet at the back of the stage for the moment. The true IB remained as the top half of the tall manifold with the four newer drivers. A shelf halfway down isolates the drivers from the rest of the box. Heavy cardboard has been temporarily stapled over the open driver cut-outs.

I did some REW sweeps to see how the new boxes were performing alone and together with the IB. As usual these older drivers (Fs32Hz) rolled off very early without serious bass boost. So I added their usual +16dB @ 20Hz 120BW steroid injection with the DSP1124 BFD. This lifted the bottom end nicely leaving a very broad hump peaking around 90Hz. I doubt they will suffer from over-excursion in such small boxes if they aren't harmed in an IB. Besides, they have the four 15" drivers in the IB to support them.

This is the IB + the two boxes + the main speakers up to 500Hz with 1/3 octave smoothing. 120Hz indicated setting on the CX2310, 24dB/0ctave, active crossover. This probably means they are crossing over a bit higher going on past measurements. The idea of the two boxes was to allow a higher crossover point. I shall experiment further with this. The new manifolds/boxes contribute nicely to fill in the former deep troughs between the IB and the speakers.

I have had a quick listen to Loreena McKennitt and some Franck organ music but no films yet. I normally have the subwoofers rather more "warm" than this. Early days yet as I can easily add more bass to taste. The Behringer EP2500 is throttled right back on the control knobs. So has plenty in reserve.

As an afterthought I have switched input channels on the EP2500 to bring the boxes onto the Right channel and the IB on the Left. This should ensure what remains of any stereo information in the bass is maintained. The IB is on one channel and the two boxes are on the other. Bass sounds "drier" and lighter than before but still with good depth. Detail and weight is certainly not lacking.

The REW Waterfall graph generated from the response curve above.

I shall bide my time until resistance crumbles and I can open up the plywood wall to let the manifolds breathe as true IBs. The reason for not opening the manifolds to the under-roof space is thermal loss in winter. I'm considering a plywood slide to close off the openings when the subs are not in use. The drivers are fitted magnets outwards to the boxes so that the metalwork will remain in the room to avoid condensation. I could also hinge the boxes to the wall. Then slip a piece of thin plywood in to seal the wall when the subs are not in active use. I'll think of something to satisfy the "Management Committee Chairperson".

I moved both of the new manifolds right to the back of the stage today and then watched The Matrix Revolutions rather loudly. The manifolds are still operating as grossly undersized sealed boxes with opposed drivers. The combined subs managed to "melt the floor" under our chairs a few times. The polythene DPM out in the enclosure was rippling in time to the visible driver excursion of the IB. I probably saw +/- 1/4" cone excursion (+/-6mm) during the big fight scene in the rain. No doubt it would have been even more exciting with a "hotter" setting on the subs. I suppose I should have used the SPL meter but I was too busy enjoying the film. It's ages since I last saw it. What a truly remarkable example of special effects. One which actually works. Even the actors are well up to the task for an SF action film.

An update: I have fiddled with the phase, crossover point and levels of the two sealed manifolds and the IB relative to the speakers. The idea was to fill the trough around 180Hz. The new curve looks slightly worse than the last one. What is much worse is the sound quality. Lifting the levels on the sealed boxes has muddied everything.

Switching between the subs using the push buttons on the CX2310 it is easy to hear the difference in SQ on organ music. On films and rock music there is more drive and attack. We watched S.W.A.T last night which has lots of rock music tracks. Drum and bass both sounded better. On classical organ there is no detail or subtlety. Just loud "subwoofer" bass.

I'm still getting resistance to the idea of cutting more holes in the house. So I may just return the older drivers to the big manifold. I am thinking of switching the newer drivers to the bottom of the box. The older drivers can go up in the top section. This may not achieve anything useful but it's something I haven't tried yet.

I have a couple of 10" SEAS 25F-EWRX drivers which I built into a stereo pair of 6th order series bandpass boxes. They have been resting unused for years but could be fitted into a couple of sealed boxes just to fill the 180Hz trough. The problem always comes down to the necessary 3-way crossover and amp to drive them. They might add the missing "wallop" which the present 180Hz trough is denying me. I have tried every imaginable crossover point from around 40hz up to several hundred Hz but nothing helps to fill the trough. It is obviously room related but with a twist. The interaction between the main speakers and offset IB is still a problem.

Logic suggests that I need a balancing IB on the right of the stage. There just isn't anywhere to fit one sensibly. There is a box dormer to the right of the stage where I sit at my computer. Beyond that is solid cupboard space on both sides of the central chimney. The ceiling of the dormer is flat with the open roof space above the insulation accessible but thermal losses come into play. It's a shame because it would be otherwise ideal. Probably with room for a compact 4 x 15" manifold up there. I have been over this so many times in my mind but nothing suggests itself. At least nothing that I can easily get away with and still have a clear conscience. :-)

Update: 25th Aug. I have returned the four older drivers to the big manifold.

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Labyrinthine musings:

2010-05-08T12:52:00.026+02:00

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As usual I am just thinking aloud again:

Having failed with an OB sub I am still feeling an urge to do something constructive with the four, older, AEIB15 drivers. (Vinyl cones, big magnets, Fs 32Hz, Xmax~15mm, remember them?)

While the present IB performance is fine I still want to reduce the IB's Total Vas Ratio to a more normal figure. I would like still more impact on drums and bass guitar than I have at the moment. A more "Hifi" sound if you want to be really picky. My Mission 753F, main speakers have only four 4" diameter bass/mid drivers with only the bottom two reflex ported.

Which is where the four older 15" drivers could excel if given the opportunity. Ideally they should be moved from the big manifold to a central position between the main speakers. Where a higher crossover point could be used if necessary or found desirable.

I may have achieved a reasonable balance between the two sets of drivers with the BFD. But it is obvious that they are still very different in electro-mechanical character. The newer drivers are being handicapped in a much reduced enclosure volume to driver ratio. Which is rolling them off early. The older drivers can't do low bass without massive boost. So the older drivers have to come out of the tall manifold but still be used for something useful. I hated to have them just sitting in boxes while I listened to the new drivers. It was such a waste of (huge) potential. Which is why I built the big manifold in the first place.

Behind and above the TV and main speakers is a 45 degree, sloping wall. Above this wall is the roof itself. There is at least 4" wide continuous eaves ventilation from end to end of the roof. With 12-16" of rockwool resting above the flat ceiling and sloping surfaces of my attic AV room.

I am now thinking of fitting the older sub drivers so that they exhaust backwards through the knee wall. Either from baffles or manifolds. This will depend whether I decide to have two drivers in front and two behind me in an identical arrangement. (this might need a separate amp and EQ for each pair) Or have all four drivers in front which is easiest.

The rear wave would find itself opening upwards into a very wide, and quite deep, wedge-shaped area above the insulation but still under the roof surface. Some sound will inevitably escape through the continuous roof vents but I doubt this will matter given our rural situation. There would be no air resistance (at all) other than that required to negotiate the upward bend just beyond the knee wall. So in some senses it is an IB but with an extended path to freedom. Hence the term "labyrinth" mentioned in the title.

I am thinking about thermal losses while still having my audio "cake and eating it". A very low position in the room will avoid the higher temperatures up near the ceiling due to stratification in our two story, open plan home. Open plan probably sounds a bit too posh for an open stairwell in the middle of my AV room. This stairwell is what has been putting me off fitting an IB in the ceiling. I could not possibly tolerate the heat loss through the cones in winter. The drivers already sweat in the unheated IB enclosure if the magnets are placed outside the manifold.

Any shutter on the warm side of ceiling-mounted drivers would cause the drivers to cool rapidly to outside temperatures. With subsequent and massive condensation upon reopening the shutter! Seven feet lower down at first floor level should reduce this problem considerably. Moreover, the drivers will not be directly subjected to outside temperatures. Nor will they be radiating room temperature to the open roof above the insulation. A manifold could provide an exit to the cold outdoors as small as 17" square per pair of drivers. Far better than having the driver cones fully exposed to the open roof space itself.

The baskets would be placed on the warm side of the wall so that they enjoy the heat input from the room's ambient temperature. Driver orientation makes no great difference to sound quality with subwoofers. Placing the baskets in the room allows a simple, flat, fine wire mesh screen to protect the driver cones from any birds or other wildlife which might get past the careful screening at the eaves. (I had a tiny wren exploring the IB enclosure one day!)

The list of advantages goes on: The drivers would find themselves placed in a dark, invisible, storage area behind the TV stand. No special finish would be required as an existing curtain drops in front of this area to conceal the few storage boxes hidden back there. The very low position at the throat of the large horn, formed by the floor and sloping ceiling, should enjoy considerable boundary gain. The central position should help to reduce the present troughs between the speakers and the present IB.

While no offset bass effects are noticed in the soundfield, with the present IB location, placing drivers centrally can't do any harm. To avoid stereo channel confusion I think I will use the 300 watt Bash amp from my SVS cylinder with all its extra controls. Though it is limited to a 120Hz maximum crossover point with a 12dB/octave slope this should be enough to let the IB15s provide a bit of extra impact.

Shrinking the big IB manifold down to four opposed drivers would let much more light in through the glazed doors. It would allow a stiffer manifold, closer to the floor, for yet more boundary gain and better SQ. The big amp could go back on top of the manifold out of the way. I am also looking at ideas for a sand-filled manifold rather like St.Louis Bob's superb OBs. Or even cast concrete panels. Or paving slabs for shelves or manifold reinforcement, cladding or resistive mass. A four driver manifold allows much more space between the drivers for such fun and games without the manifold box increasing too much in size.

You might be asking why not move all eight drivers to this "very desirable situation" in the sloping wall? Well, the present IB is a proven SQ winner with just four drivers. The enclosure volume is ideal for four drivers but not eight. The peculiar quasi-IB/ labyrinthine/ bandpass arrangement, which I am discussing here, may not offer the same quantity or quality of bass. It remains an unknown quantity until it is built and tested half to death. Eight drivers would also take up much more room and require four manifolds.

I already have enough ready-cut baffles to throw two compact, two-driver manifolds together to check if this idea actually works.

27June 2010: Update: I built a quick and dirty opposed 2 driver manifold out of 3/4" plywood but have yet to cut new holes in the construction of the "happy home". Instead I turned the box opening downwards to achieve a sort of seriously undersized, sealed box. This sounded louder and deeper than having it open to the room. I tried reversing the polarity relative to the remaining two in the big manifold but could hear no difference subjectively. Have yet to try REW on the new box. The two sets of older drivers sound much quieter than the four newer drivers in the big manifold when I mute each box in turn on the active crossover.

Having removed a small area of the house roof covering I was able to peer down into the insulation filled void from above. Just to remind myself what I did when I was building the new roof. Clearance between the inner side of the 4" external, block house wall and the present plywood knee wall is only about 7". I can most easily add a new knee wall/ baffle further into the room to allow more clearance if necessary. I could use weld mesh to fix the insulation on either side to achieve a very wide and deep, insulation-lined slot to the underside of the roof covering. (and thence up over the top of the 12"-16" of rockwool roof insulation) Any leakage to the great outdoors (at the well ventilated eaves) would be on the opposite side of the house to the nearest neighbours. I deliberately opened a dormer window while playing a loud and bassy CD today and found sound leakage well down. Not the least bit problematic.

It is quite shocking how much room two 15" drivers with magnets outwards on a modest 12" wide box take up indoors. For greater compactness, I am sorely tempted to build a horizontal line array despite my previous negative experience with such long baffles. I am thinking that the driver assembly will be working against the bottom of the rafters, floor joists and the entire roof mass above. (rather than sitting in a flexible wall between flimsy, original studs in the former vertical array) The bulky magnets and baskets can best be lost in the space under the 45 degree sloping ceiling behind the baffle. Though this does raise thermal issues with cold drivers in winter. I can easily add more mass in the form of paving slabs and local bracing if necessary. As well as fixing the baffle/ knee wall firmly to the surrounding timbers.

Avoiding the present, +16dB bass boost at 20Hz will reduce reaction forces dramatically. I confirmed this today when the temporary sealing boards started rattling over the empty driver cut-outs in the manifold. Removing the boost instantly silenced the vicious buzzing. I was then able to raise the boost to +6dB without further problems. An hour of listening to Bass Outlaw's CD "Illegal Bass" was both interesting and entertaining. I found it impossible to localise the bass to the new box even when I had my head right beside it. All of the bass seemed to be coming only from the nearest speaker two feet away on the other side of my head. A very odd sensation since I had already raised the crossover point to 120hz.

I built the second manifold and placed that too open side down on the carpet behind the other main speaker. It was time to fire up the new [2010] version of REW.

N.B. No effort was made to EQ the results. Only to roughly balance the various units and ensure they were all connected in the best polarity relative to each other. The manifold boxes were still much quieter than the remaining 4 x 15" IB but I adjusted the gain until they matched well enough for listening tests. The new boxes peaked at 90hz rolling away slowly to below 20hz. The various combinations appear in the image below:

The red trace is all subs together using a 80Hz 24dB/octave active crossover but no speakers.

Yellow is with the speakers connected but deliberately set out of phase for comparison.

Blue is the best match between speakers, 4 x15" IB and the two box subs. (2 x 2 x15")

The 180Hz and 280Hz troughs are reduced relative to the 8 driver IB alone though not absent.

There is more energy in the upper bass now. Which is exactly what was hoped for with music.

The question now is whether it is worth hacking holes in the sloping wall behind the speakers to allow the two new boxes to breath as quasi-IBs? Or should I build a new horizontal line array (IB) for all four older drivers? The two new manifolds owe me nothing in time or materials as they were thrown together from scrap 3/4" plywood and simply screwed together.

As the rear wall slopes at 45 degrees I have various options regarding the height of the final design. The higher I place them the greater the likely heat loss through the cones. Though this does bring them nearer to the plane of the main speaker baffles. If I raise a line array just high enough it won't upset the present storage arrangements at all.

A simple (long) box with bottom and front at right angles would bring the driver baffle to the vertical. Triangular ends would seal the box to the sloping wall. Internal triangular braces would help to stiffen the whole arrangement.

Placing the array box three feet off the stage floor would offer the greatest freedom for air movement between the sloping indoor wall and outer roof surface. I have a weird roof construction which I built myself to make room for lots of rockwool insulation. By overlaying a new set of rafters at a lower angle I gained plenty of space without needing hugely deep rafters. The original roof was 45 degrees and very probably thatched when built. The new roof surface is 40 degrees. The small difference in angle provides plenty of space for well ventilated insulation. The large overhang was designed to allow heavy external insulation but I haven't got round to doing this yet. This rough drawing is not remotely to scale: Green is 12-16" of rockwool.

As can be seen; exhausting the box to the space above the insulation would constitute a true IB as near as matters. Not only is the free volume above the insulation very large in area but the eaves are also well ventilated. The only thing which stopped me going ahead years ago was worrying about heat loss though the driver cones. The 12" insulating blanket would have to be pierced to let the drivers breathe. The area where the manifold/array would sit would lie behind existing, concealing curtains. I could always have a hinged, super-insulated, drop-down flap to cover the driver cones when not in use, I suppose. Though I would greatly prefer the insulation on the cold side of the drivers.

Getting such an idea past the "Head Gardener" is the greatest hurdle to fruition. Her patience with my IB building is wearing rather thin. Nothing I have tried so far reduces her love for the old SVS cylinder. At least this tension offers me some leverage. I can always claim I am only building new and better IBs in her interest. Nah, I wouldn't buy that story either. :-)

Click on any image for an enlargement. Back click to return to the text.
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Introduction

2007-04-25T00:00:00.009+02:00

*THIS IS THE START OF THE ORIGINAL IB BLOG.*

Posts run forward in time from this point.

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Throughout the life of this blog I will constantly discover new spelling mistakes or find better ways of sharing my thoughts. Try reloading the page occasionally. Or run CCleaner regularly as I do.

The default spelling checker I use here is UK English but is rarely working properly as I rattle away at the keyboard. Only later do all the red lines appear like bright poppies in the fields of text.

If the text seems too small for comfort try holding down the CTRL key and then pressing +. This enlarges the page including text and images and is very useful. CTRL and 0 (zero or nought) will return the text to normal size. CTRL and - will reduce it.

By the way: The dates of all posts are completely fictitious. I deliberately date each new post so that it follows a logical chronological order. When I started blogging it was impossible to run a blog "backwards". i.e. So that new posts appeared at the bottom of the chapter list rather than at the top. Dating posts backwards into the past to achieve my aims has continued.

If you find yourself running out of new chapters to read try clicking on "Home" at the bottom of the page. Clicking on"Older Posts" will usually provide the next chapter or chapters.

I chose the blog format because it is so flexible, provides enlargeable image hosting and can be continuously updated and corrected. It is also free. It's strange editing problems are a trial but beggars can't be choosers.

Blogging seems unlikely to suffer the sudden death common to ISP's free web pages as more avaricious companies buy them up. I lost 5 websites to changes of ISP ownership ending in a final toxic dose of agent Orange.

Although it doesn't seem very likely this blog actually gets more interesting as it goes backwards in time! If you have already dismissed March '07 as out of date then you may like to know that this was written in mid summer '09 just as I doubled the size of my IB to 8 x 15".

If you are regular visitor do remember to reload the pages regularly. I go back and change things all the time. Often adding new images as appropriate.
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The IB Array

2007-04-23T10:49:00.000+02:00

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My first attempt at a True Infinite Baffle Subwoofer proved to be slightly flawed. It performed incredibly well compared with my large commercial SVS cylindrical subwoofer. It was just that the array seemed to lack the giant-killing power that I had read about in posts from other delighted IB subwoofer owners on The Cult of the Infinitely Baffled. (A specialist Internet forum dedicated to True Infinite Baffle Subwoofers)

http://ibsubwoofers.proboards51.com/index

The major problem was the flexibility of my baffle wall. This old planked-over-vertical-studs wall forms part of an even older timber-framed house with later additions. When the four big 15" driver cones moved ... the wall moved the opposite way! Newton's Law of action = reaction was biting back with a vengeance.

The original array had a remarkable performance but had shaken the wall and adjacent double doors far too much for comfort. In fact the whole house seemed to shake from the mechanical vibration of the drivers. The bass sometimes made me feel nauseous in the bathroom below the IB enclosure! Yet despite the available power and four large drivers the output on programme material was just a little disappointing. It failed to vibrate the floor on films no matter how much I played with BFD filters. Despite an easy 120+dB capability and 8Hz extension on test tones the IB just did not deliver quite as expected. I also had absolutely no headroom to play the subwoofer louder ("hotter") than the speakers on programme material. Something had to be done!
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The IB Enclosure.

2007-04-22T19:28:00.000+02:00

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This is what the 4 x 15" vertical array looked like from the enclosure side.
I think you will agree that it looked very impressive.
The IB enclosure is a large prism, triangular in elevation and has a volume of about 600 cu.ft or 17 cu. meters. This small room was once an open balcony in the gable end of our old house. After the Danish Storm of the Century in late 1999 I had closed off the gable end with large, double-glazed windows to make a pleasant sitting room with a view out over our garden.

My wife suggested this space for the IB enclosure since it avoided clambering into the roof space and cutting holes in the planked ceiling of the AV room. Luckily the space offered a suitable volume and very easy access from the listening room via the original French windows. (glazed double doors)

The basic concept of an IB subwoofer is very simple indeed. The large drivers (loudspeaker units) are fixed into a wall (the baffle) between two adjacent spaces. The listener sits in one space and the IB subwoofer enclosure is the entire space beyond the shared wall. The loft can also be used as the enclosure with the ceiling acting as the necessary baffle. Underfloor IBs are also popular using the listening room floor as the baffle and the crawl space (or cellar) as the enclosure.

The black box on the steel table in the image above is a Behringer Europower EP2500, dual mono, power amplifier. Providing 450 watts RMS per channel into 8 Ohms. (650 Watts RMS into 4 Ohms) Designed for professional musician's instrument amplification it offers remarkable output for mere pocket money in domestic hifi terms. It closely matches the power requirements of my IB subwoofer. Its incredibly noisy original cooling fan has been replaced with a much quieter one! This is a popular and easy modification of these amps for domestic audio use.
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Inside the box.

2007-04-21T12:39:00.000+02:00

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The term "infinite baffle" really means an infinitely large, rigid panel with a loudspeaker drive unit fixed somewhere in the middle. The great size of the baffle forces the sound waves from the driver to to travel a very long way before they reach the edge of the baffle. When the sound waves reach the edge of the baffle they would be of opposite phase which would cause cancellation at a frequency depending on the length of the wave which matched the distance from the driver. The positive pressure wave from the front of the driver cone cancels the negative partial vacuum of the rear wave. Since frequency is directly related to wavelength the baffle must be made very large indeed so that cancellation can only occur at very low frequencies. Hopefully at a frequency below that required to reproduce music and film effects with full power.

In real-world situations it is far easier to use a large, enclosed rear volume as an IB enclosure. The infinitely large baffle then becomes the wall between the listening room and the rear enclosure volume. The edges of the baffle have been effectively wrapped around to seal off the rear space from the AV room. Since the sound waves from the front and rear of the cones cannot interact with each other there is no out of phase cancellation at any frequency. Because the rear volume is so large there is no back pressure on the drivers as there would be in a small sealed box. This lack of back pressure allows the IB to reach very low frequencies indeed at high output levels while maintaining low distortion and very reasonable efficiency. When the drivers are housed in a box manifold sealed into the dividing wall the IB can be thought of as a local folding of the (infinite) baffle.

The listening room itself must be sealed from the IB enclosure to avoid out-of-phase sound wave cancellation. Open doors, serving hatches or windows between the two spaces would cancel the bass at certain frequencies depending on the wavelength at which the sound waves met each other at the opening in the baffle wall. Airtight sealing is not that important though. The large volumes of air movement required at very low frequencies would struggle to squeeze around the narrow gaps of a closed door for example. A perfectionist might like to draught seal the door so they can sleep soundly at night. If nothing else it will help to stop the door from rattling against its frame. You'd be amazed how a door can flex when driven by cyclic low frequency pressure waves.

A view inside the open manifold from the AV room: Click for a larger image. Backclick to return to the text.

The drivers in my own IB are now fixed in pairs on either side of a manifold (box). The box is completely open on only one side to the listening room. The open area (or vent) of the manifold is calculated to match the total cone area of the 4 x 15" drivers (or 4 x Sd) to avoid any compression effects in the manifold. (In my own case the opening is 40" x 13.5")

The driver cones in an IB must all move in the same direction (towards the open side of the manifold ) when a signal is applied. If any cones moved in opposite directions they would cancel each other's output. A torch battery connected briefly to the speaker cables can be used to check that all the cones jump in the correct direction. The cables can then be inserted into the power amplifier speaker terminals observing the correct polarity.

The unusual two-facing-in, two-facing-out arrangement of the drivers in my manifold in this image is supposed to cancel out some distortion components. The drivers which face into the box will not produce exactly the same audio response as the drivers which face out of the box. This imbalance helps to cancel out any variations between them. It is considered a better arrangement than having all the drivers facing inwards or all facing outwards. (Where unwanted driver effects would be additive)

Opposing the drivers on either side of the box cancels mechanical vibration due to cone movements.

Mechanical vibration was the weakness of my original vertical, line array IB. The downside is that a rather large box has to be housed next to the baffle wall. Whereas an array takes up no space in either room. The box can be hidden in the enclosure as I have done. Or it can be placed in the listening room. In which case it can be made to look like a large box subwoofer. This type is usually referred to as an "outie". My manifold could have been made smaller but I wanted to avoid any chance of compression effects in the box by making the opening into the AV room as large as possible. The opening into the room can be covered in breathable cloth without affecting the IB's performance. Some IB builders use decorative screens to cover the manifold opening. Grills are more practical with underfloor IB manifolds where the manifold opening might be walked on accidentally.
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The manifold box.

2007-04-20T20:55:00.001+02:00

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The lack of "killer" bass from my IB required a serious rethink. The array was sitting in a weak wall which would never improve no matter how much I tried to reinforce it by fixing on more 4" x 2" timbers with 6" long screws.

So after much procrastination I finally decided to rebuild my IB as an opposed driver manifold. (box) Just to see if mechanical vibration really was my IB's Achilles heel. The box would be made of 3/4" plywood and I would use a single layer. I would judge how it performed first before beefing up the box with a second layer. I was still full of doubt abut the advantages of a manifold at this point.

The desired manifold size could be cut from a single sheet of 18mm plywood with little or no waste. I carefully selected a nice, flat sheet from the local timber merchant then brought it home in the car trailer. It didn't take long to cut everything to size and rout the mounting holes for the drivers.

Having built my heavy manifold in my garden workshop I suddenly realised that I couldn't get the large box up the narrow stairs to my attic AV room! I should have glued, clamped and screwed the box together right where it was going to be sealed into the baffle wall.

The heavy 48" x 24" x 16" box had to be run across the garden on a sack truck. Then slid up a gently sloping ladder and in through an open window. Straight into the IB enclosure. After a lot of heavy panting (and rest for a cup of coffee) I could push the manifold between the vertical timbers of the stud wall. A piece of advice: Wall studs are rarely straight without some twist or bend. Make your manifold width slightly undersized or be prepared to hack some studding away with a large chisel! This noisy exercise will not endear you to your partner. Who will assume that the confidence you had in your careful calculations was completely unfounded. They will think that the IB will be a total waste of time and money! It will not be, but don't give them any rope to hang you by at this critical stage. Build a slightly undersized manifold and then use wood or plywood packing strips. Or strips of foam plastic pressed well in. These deliberate ruses will impress your better half no end. You will appear to have everything completely under control no matter how poor your woodworking skills.

The maintenance of the balance of power is vital in an HT/AV/Hifi household. Diplomacy before a war of words will avoid expensive concessions which might be much better spent on equipment upgrades. Being allowed to make large holes in the house to install subwoofers goes completely against the nesting instincts so tread very carefully to avoid breaking too many eggs.

After the struggle to get the manifold between the twisted old timbers I had drilled holes for the speaker cables and fitted the drivers. Depending on your driver orientation it can help to fix a small batten to support the driver rim safely while you get the first fixing screw in. Once achieved you can take your time as you go around fitting the rest of the screws in place. Drivers are strangely unbalanced objects and can often benefit from a bottom fixing screw first rather than a seemingly more logical top one. It all depends on whether you fix your drivers from the front or rear of the basket rim and whether you have somebody handy to help you at this stage. I found myself having to climb right into my manifold while my wife gingerly handed me the drivers. A situation of the utmost gravity!

Once safely in place the drivers could be wired up to the amplifier again and double checked for phase. The drivers must all move towards the centre of the manifold or they will cancel each other out. Having managed this error a couple of times when moving the drivers around or rewiring after REW tests I can vouch for the lack of bass if one gets the polarity wrong.

The effect of changing to a manifold from the array was quite simply staggering! I nearly fell over with shock the first time I fired it up on a favourite bit of bassy rock music! I had mistakenly used the same amplifier settings as the baffle array. I suddenly had the bass that an IB subwoofer is famed for. The floor shook like a diesel truck on test tones. (or testosterone if you prefer)

Now I had to reduce the bass considerably to match my floor standing speakers. With the array I was always fighting for more SPLs from the IB to match the speaker output. Now I had far more bass than I could tolerate without quickly losing my nerve. It really was a night and day difference.

More importantly the wall and old French windows no longer vibrated an inch back and forth! The structure around the manifold remained almost completely free of vibration regardless of output level or frequency. The single layer of 3/4" plywood seemed quite stiff enough until I tried a 100dB(C) sinewave @ 10Hz. Even then there wasn't enough flexure to worry me unduly. The bass out in the IB enclosure also seemed to have dropped considerably and was very much less audible in the rooms below.

Just in case there is still any doubt, the manifold in an IB is not like a sealed speaker box. The manifold is certainly not the IB enclosure. The manifold merely holds the speakers rigidly in place. One side of the box is completely open to the AV room. All the drivers cones are exposed on the other side to the large IB enclosure. (Which is about 17 cu. meters or 600 cu feet in my case) The open side of the box behaves like a single, large, compression driver. The air vibrating in the mouth of the manifold produces all the bass in the AV room. Doing so with the combined power of all 4 x 15" loudspeakers simultaneously. A safe minimum number of drivers for an IB which can be exceeded at will.

The large volume of the IB enclosure means that the drivers' natural resonant frequency is not raised by an increased air pressure cushion pressing back on the cones. This allows an IB to perform to much lower frequencies at much higher SPLs than any other conventional subwoofer design. It also uses much less power into the bargain. The IB has the unique and often enormous advantage of not taking up any space (at all) in the AV room. Those who take their music and films seriously are often torn between building a huge subwoofer and their partner's natural reluctance to having such things standing in "their" living rooms. Under such circumstances the IB may be your giant subwoofer get-out clause.
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Equalisation and frequency response.

2007-04-19T21:30:00.000+02:00

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Equalisation of my IB subwoofer response was carried out using the popular DSP1124 Behringer Feedback Destroyer. Popularly known as the BFD. The REW test software was downloaded from The Home Theatre Shack Forum along with a calibration file for my Galaxy 140 SPL meter. The Galaxy was used as the test microphone at the listening position. It has proved in tests to be highly repeatable and accurate down to 5Hz using the matching calibration file. Far better than the commonly used Radioshack SPL meter.

I used a +16dB BFD filter at 20Hz to lift the extreme bass using the maximum filter bandwidth of 120/60. (two octaves) The BFD has a lowest frequency filter option of 20Hz. This heavy boost was necessary because my original drivers had an Fs of 32Hz. Fs is the Thiele-Small parameter for the free air resonance of the driver.

The graph below shows the frequency response of my IB and my Mission 753 Freedom speakers after subwoofer equalisation. The curve has since been slightly improved after repeated test sweeps with the REW software and changing the filters slightly on the BFD parametric equaliser. Note that my IB subwoofer reaches 12Hz at full power before falling away quite rapidly. This extension into the infrasonic is rarely matched by any commercial subwoofer. The bass is deliberately louder (hotter) than the output from the speakers. An 80Hz 24dB/octave active crossover is used between the IB sub and the speakers. (Behringer CX2310)

My Mission 753F speakers are driven by a Naim NAC72-Hicap- NAP 180 stereo system. Surround sound (DTS & Dolby 5.1) for films is provided by an old Yamaha DSP E800 processor/amplifier. The resulting sound quality leaves affordable AV Receivers for dead.

Click on the graph for a larger image.

My 27 feet long attic AV room is very heavily damped acoustically. Because of a central chimney the listening triangle is arranged across only one half the room. The whole area behind the speakers (concealed by a curtain) is used for clothes storage with bookshelves behind which greatly adds to the room's acoustic damping. There is an open stairwell in the floor between the listener and the main speakers! All room surfaces are boarded. All this combines to absorb a very wide range of frequencies totally devoid of echo.

No attempt to fill the obvious troughs in the response by altering phase or crossover frequency has had any real effect so far. The system still sounds wonderful to my ears. The quality, massive weight, lack of colouration and startling reality of the bass is beyond all expectation. Whether thundering on a great organ pipe or the "War of The Worlds" film DVD I could not be happier with the sound quality. There is simply no comparison with any commercial subwoofer I have ever heard. The IB totally outclasses them in every respect.
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New AE IB15 drivers

2007-04-18T12:16:00.001+02:00

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Acoustic Elegance kindly sent me some new drivers to replace my originals which had the wrong specs. John, the very knowledgeable owner of AE had suffered some personal tragedies over the last couple of years. This had affected his control over the company's products. He has now restarted his business on a more secure financial footing with far better quality control and redesigned products

These new AE IB15 drivers are currently available in sets of four for $400US plus freight. Four is probably a safe minimum with these 15" drivers in an IB. I found that muting any two of my four drivers makes a serious difference in output. That means more excursion is required to handle peaks. More excursion may be beyond the limits of only two driver's in an IB.

Warning! Click this image at your peril! The enlarged image is 400kB so don't attempt it on a slow modem! I posted this large image to show the sheer quality of these AE drivers. Smaller images just don't seem to do them justice.

The new drivers have a lower Fs of 16Hz which means they should produce more deep bass straight out of the box. I had been using maximum boost of +16dB on the BFD at 20Hz on my old drivers which had a measured Fs of 32Hz.

The graph below clearly confirms the massive improvement. The blue trace represents just two new drivers in the manifold. The other curves show the old drivers without bass boost in pairs and all four. Levels have been matched to make comparison easier. The 11-12Hz peak is a room boundary effect common to both old a new drivers. As it is free bass and not requiring extra power from the amplifier, or extra cone excursion, I think I can safely ignore it. Or even consider it a bonus. There aren't many subwoofers which can claim 11Hz at well over 100dB(C-weighting).

I had no problem getting 100dB(C) at 10Hz [uncorrected] on my Galaxy 140 SPL meter except for the physical discomfort. Though completely inaudible the feeling of pressure in the room is intense. The Galaxy cal. file adds nearly 11dBs of correction to the REW response at 10Hz. This suggests that my IB is producing a true 110dB(C) (and over) at 10Hz without reaching the driver excursion limits. The room gain factor at 11Hz is probably helping here though I have no idea where this gain is coming from. There seems to be no linear dimension between boundaries which would have this free and beneficial effect unless the sound waves are folding at the stairwell. It is also possible that the room below is acting as a Helmholtz resonator with the open stairwell as its port. The IB manifold opening is quite close to the stairwell and may be driving the room below like a giant reflex loudspeaker enclosure.

The new drivers show obvious signs of much greater care in manufacturing and have had a complete facelift. The cones are now made of reinforced paper instead of the original poly material. The cones are now heavily flared compared with the straight-sided cones of the earlier models. The basket has been given a black finish with bright edges to the legs of the basket. The spider is also black instead of yellow. The earlier screw terminals have now become a sprung push-release type. Much easier to use, perhaps, but I would have liked much more room in the cross-hole for thicker cables. I'm using original 79 strand to connect the EP2500 amps to the drivers. The push terminals will not quite accept doubled wires no matter how tightly they are twisted together. The magnet backplates now have rounded shoulders and a smoothly flared pole vent. The quality of the machining is very much higher than on my old drivers.

At first I was running the new drivers in series pairs per channel because that was how the old drivers were wired. The new drivers have an 8 Ohms impedance which meant the amplifier was seeing 16 Ohms per channel. Despite hitting over 112dB on WOTW "machine rising" I felt the new drivers sounded a little quieter than the old ones. The old drivers were 4 Ohms each in series pairs for 8 Ohms and 450 Watts per channel. I have now rewired the new drivers in parallel pairs for 4 Ohms and 650Watts per channel or 325 watts per driver. This has increased output nicely over the former series pairs arrangement. Output now matches the older driver with plenty of weight on my favourite classical organ CDs and the bass-heavy pop and rock numbers I use to confirm bass quality and quantity.

This is an REW frequency response trace measured at the listening position using the Galaxy 140 SPL meter as test microphone with its matching calibration correction file loaded.

Here my BFD has been reprogrammed to match the new driver's output in the very deep bass. The trough between 12Hz and 25Hz has been filled with a small but wide boost filter at 20Hz. Further small filters at higher frequencies have smoothed the response nicely. The constantly rising slope to 12Hz could hardly be called a house curve. More of a ramp. This final curve seems to suit my ageing ears and my room. The image above is a subwoofer only response graph from REW. [Room Eq Wizard] This test software is available as a free download to members of Home Theatre Shack forums. John Mulcahy, the brilliant and generous software author, is usually able to answer specific questions about the software itself. General questions about its use are usually fielded by the very knowledgeable moderators of the specialist REW forum at the HT Shack.

Below are the manufacturer's T/S Parameters for the new AE IB15. Based on my own experience with these new IB drivers I heartily recommend them in sets of four for IB use. The sheer quality and quantity of bass they provide in a manifold IB is staggering using an EP2500 amplifier and 80Hz crossover. Your main speakers are much more likely to be the limiting factor in any sound reproduction system involving an IB. Given the amount of crossover overlap at high listening levels, even with a 24dB/octave crossover, requires a good quality floor stander with 110dB capability and at least 40Hz extension.

Thiele-Small parameters of the all new Acoustic Elegance IB15.

Fs: 16Hz
Qms: 6.8
Vas: 439L
Cms: .45mm/N
Mms: 220g
Rms: 3.239
Xmax: 18.5mm
Xmech: 25mm
Sd: 830sqcm
Vd: 3.07L (p-p)
Qes: .78
Re: 5.5ohm
Le: .33mH
Bl: 12.49
Pe: 500W
Qts: .7
1WSPL: 86dB
2.83V: 87.3dB

Here is one of the best online lists of Thiele-Small parameters and their meanings.

Car Audio: Thiele-Small Parameters

Click on any image for a larger version. Back click to return to the blog.
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IB performance on organ music.

2007-04-17T13:44:00.004+02:00

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I have been listening to my favourite classical organ CDs all morning to assess the new drivers.

These include: Amongst many others of:

"Bombarde!" French organ classics: Liverpool Cathedral: Ian Tracey. Chandos CHAN 9716

"For Weddings" Chichester Cathedral : Kevin Bowyer. Nimbus NI712

"Tsar of Instruments" Organ music from Russia: Winchester Cathedral: Iain Quinn. Chandos: CHAN 10043

Cesar Franck's Complete Organ Works , Jean Guillou, organ at St.Eastache, Paris. Brilliant Classics 92282. On 2 Cds this 1989 recording is simply stunning for its clarity and capture of the building's reverberation at all frequencies. The bass quality and weight is phenomenal with the timbre of pedal stops and pipes laid completely bare by a good subwoofer. The inner detail of the big pipes and their complex interactions is entrancing.

The deep tones from the great pipes sound suitably heavy and effortless with fine rendering of timbre. The way the pipes stop and start is vital to enjoyment in my experience. Some pipes have a slow way of breathing themselves gently into audibility. Others chuff or start and stop abruptly. Some change pitch as they play their tone. All have different tonal qualities and this should be heard without effort. It is never enough to just reproduce low frequencies. The reproduction must be done with low distortion and great subtlety of tone, timbre and dynamics. It is no use just filling the room with loud sinewaves. (or square waves for that matter) That is not what organ music is all about.

The tones one hears must be nimble to match the footwork of the organist on the pedals. Most cathedral organs have various ranks of great pipes which can be played alone or in unison. The 16 foot pipes sometimes dance on a foundation of 32 foot stops. These huge pipes can produce inaudible tones which modulate the sounds of the shorter pipes. Sometimes the great pipes beat together producing a "tremolo" or "warble" effect with varying frequency. Sometimes the beating has clear gaps between the beats. Which themselves are constantly changing in speed and strength. I have heard beating as slow as a half second cycle. Putting one's head in the manifold brings out even more detail suggesting that there is much more to be had if only it could be brought out into the room. The danger is in overemphasis of the bass by playing the IB too hot. Very impressive but a little unrealistic.

All this detail must be reproduced for the listener's ears without artefact or distortion and without making him or her work hard to hear it all. The only way to do this well seems to involve many large drivers working in unison in an IB.

A single 12" driver can reproduce low frequencies in large quantities particularly if placed in a large, ported enclosure. Though it does so with too many false harmonics. Which often mask the fundamental with unwanted distortion. All detail is lost to a muddy rendition of loud sound waves which have little to do with the original musical performance. It could be described as a black and white cartoon rendition of the music. Bold, but completely lacking in the fine detail which we take completely for granted in the cathedral, hall or church when listening to live organ music.

The IB subwoofer fills in the details like a colour cine film. Everything is constantly moving, changing in tone, colour and contrast. One can hear the great pipes start, sustain and decay. The IB adds huge weight to the sound no matter how briefly a pipe may speak. The incredibly deep tones dance nimbly on the jumbled infrasonic ice field hidden just beneath the more audible surface.

Here's an image of the new drivers installed in the same IB manifold. The drivers have been brought magnet to magnet inside the manifold to avoid the metalwork "sweating" in cold Danish winters. My IB enclosure is unheated, though frost free and subject to rapid temperature changes in afternoon sunshine. A perfect solar oven were it not for the white curtains drawn across the glass gable end. Endlessly delayed plans to fit exterior louvres to block summer sunshine have never matured into reality.

Warning! : Large 300KB image if you click on the above: Not for slow modems.

It has been pointed out that my T-nuts are not properly bedded into the plywood to lock them firmly into place. This is because I originally planned to line the box with OSB or double the thickness of the plywood. As I may have to replace the original manifold I haven't bothered to secure the T-nuts properly yet.

The next plan is to build another manifold to stack on top of the existing one. This new box will support the four original drivers. Bringing the tally up to eight fifteen inch drivers. The Total Vas ratio to enclosure volume will fall to around 4.5:1. It will be interesting to hear how this arrangement performs when driven by the same amplifier. I may need to examine other installation ideas if it fails to please. These plans have been put on hold while I examine further alternatives for the four old drivers. I might build an open backed baffle over on the right hand side of the main speakers. The idea is to boost upper bass/midrange to allow higher levels and dynamic range. With the present performance level of the IB I don't need to do anything in reality but would like to use these older drivers for something useful.

I am examining ways to avoid the magnets coming so close together. I may stagger the drivers relative to each other in a new box manifold. To make the magnets alternate from one side to the other. This will make cross bracing between the drivers difficult or impossible though. Not that I've ever noticed much flexure in the manifold sides so far despite the use of only single layer of 3/4" (18mm) plywood.

It would not be too difficult to arrange something to go between the magnets to stiffen the structure further. Any inter-magnet bracing would need to allow the pole vents to function normally. It will have to wait for warmer weather before I am really tempted to start making more sawdust. In the meantime I shall be considering different ways of doing things. I am an inveterate modifier and a martyr to perfectionism.

There is little doubt that, should they ever suffer the misfortune of my arrival in any sort of heaven, I shall be offering ideas for improvement from day one. True perfection would be intolerable for me to enjoy. Others sometimes think I am being negative or criticising when the complete opposite is true. I automatically seek alternative and possibly better ways of doing things. This is the habit of a lifetime. Ideas are simply thrown up in the air. To be shot down or caught and given a bit more spin before being lofted again.

If I should ever earn a proper gravestone I shall have a very simple message carved there:

"IF IT AIN'T BROKE: DON'T MEND IT!"

And carved just below: "No tools to be left near this grave!"
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More thinking aloud

2007-04-16T14:05:00.001+02:00

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If the infrasonics really are the most important part of the IB's performance then adding a high pass filter at (say) 25Hz should cripple the IB's advantage over box subs. In fact when I engage the 25Hz filters on my CX2310 crossover there is only a very slight loss of impact on low frequency, electronic programme material.

I tried Bass Outlaws: Illegal Bass: "Extreme Woofer Test" at a steady 95 and then at 100dB(C) read straight off the Galaxy 140 SPL meter at the listening position. The non-filtered output was only a little more full with a few more rattles from the room's surfaces. So much for my essential IB infrasonics theory! There was zero visible cone movement with the 100dB "Woofer Test" yet the whole room was shaking and most of the house along with it.

My guess is that the IB's SQ advantage lies with very low distortion and remarkable transient performance. The large cone area is able to respond instantly to programme input due to the very small linear cone movements usually required. The individual cones each have adequate motor power to closely follow whatever is called for with minimum air disturbance.

A single heavy cone in a sealed box is constantly fighting against the resistance of the air in the box and/or the inertia of that in the port. The single cone must move many times as far as the multiple IB cones to achieve the same output. The single cone is under constant heavy acceleration which must make life difficult if it wants to add smaller nuances on top of the major acoustic output frequency. So detail is inevitably lost unless serious cone breakup comes into play. At low frequencies it is difficult to imagine the amount of breakup required to achieve simultaneous low frequency sine waves at different frequencies. The air movement required for each frequency is considerable at high levels. It is no wonder the multi-driver IB is so capable since it can move so much air with no visible come movement whatsoever.

Even a low frequency sinewave at higher levels is demanding in itself. The cone must move to the maximum excursion required for the acoustic output demanded. It must do so in both directions without undershoot or overshoot. How closely the cone can follow the signal is vital to the waveform reproduced. Would you trust multiple, large driver cones (each with their own motor) moving a tiny fraction of an inch? Or a single cone moving well over an inch in each direction and trying to follow and reproduce a number of perfect waveforms all at the same time at various sound pressure levels? (SPLs)

It is interesting that some authorities deny that subwoofers have variable speed depending on the design. They suggest that the rise time for the signal is far too slow for a subwoofer to know the difference. They obviously haven't felt the sharp edge of an IB exciting the floor through acoustic pressure alone on an LFE pulse.

Doppler distortion would seem very likely with a single long-throw cone in comparison with multiple cones moving over tiny distances in unison.

Mechanical noise and turbulence must be higher in a long-throw "air beater". Imagine a small fan whizzing round to move as much air as a large fan moving much more slowly. You can't even hear the large fan as it effortlessly moves hundreds of cubic meters of air per minute. The small fan quickly drives you out of the room with its racket. Or you try to turn down the speed to get some peace. Only to lose the required air movement to keep you cool. So what do you do when you get fed up with fan noise? You put up a huge, slow, ceiling fan for a bit of quietness. The large ceiling fan doesn't even need carefully shaped blades to achieve its low noise cooling effect. It can make do with flat plates arranged at a suitable angle of attack. No wonder a multi-driver IB wins over a single cone.

Ignoring displacement, the question is whether matching cone area by purchasing a stack of large box subs adds up to an IB on sound quality. My immediate thoughts would be that any colouration would be additive with the boxes. The cost of going this route would of course be many times that of the IB with similar cone area. The power requirements of multiple box subs would need a special cable to be run. All of these potential gremlins may be subjectively inaudible of course. So it is just as well this is just a thought exercise and none of my own money changes hands.

If reality follows my musings then the Thigpen fan subwoofer should have a much larger but much slower moving fan. To kill the unwanted noise. The fan blades of this clever subwoofer must deliberately change their angle of attack with frequency. But each angle of attack must require a completely different aerofoil section to avoid stall and/or turbulence. Carefully shaped cowl rings on both sides of the baffle in which the fan rotates would no doubt help reduce noise. Even if I had a fan sub to play with my home is just too structurally noisy to tolerate such serious output at deep infrasonics frequencies. Just playing "The Ultimate Woofer Test" at serious levels already finds every rattle and point of flexure. This strange track has vastly more serious LF content than any other track I have found so far. It sounds completely different and almost innocuous through headphones. No doubt it sounds very different through box subs and car subs too. Played at 110dB through my IB it has the impact of a wrecking ball! Rather like the server room scene in the film "Pulse" but considerably more musical.
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Subjective performance

2007-04-15T14:56:00.000+02:00

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I have been watching a lot of films and listening to lots of music since I fitted the new drivers.

The new drivers have greatly improved the performance at the bottom end. The IB is also incredibly quick on LFE impacts. I watched Elektra for the first time and the shocks through the floor were almost painful to my feet. Brutal hammer blows with zero overhang! Since my IB is built into the wall this is purely an acoustic effect of pressure waves acting on the floor. The manifold itself does not move physically to driver reaction forces so the floor is not receiving any vibration by mechanical conduction from that source. Nor is the floor well connected to the IB enclosure.

The much smaller BFD filters I need now are not sapping the power as much either. I have gained about 8dB over my old drivers which had used massive bass boost at 20Hz. (+16dB) The new drivers are also seeing more power because they are in parallel pairs for 4 Ohms impedance per channel instead of series pairs per channel for 8 Ohms. This has offered a very worthwhile increase in output. On a couple of films with really "hot" bass levels I have even backed off the bass by a further 6dB to increase comfort levels and headroom. Relative bass levels are very much a matter of taste. Which depends a great deal on programme material and one's mood.

I love the way the IB locks onto the bass drum and bass guitar on rock music. There is absolutely no sense of the subwoofer being off-centre. Listening to Satriani CDs at a steady 100dB(C) is certainly entertaining these days. The clarity and impact on drums is excellent. The IB cones aren't visibly moving at these levels. Perhaps a fraction of a millimetre. It can be felt with a fingertip on the cone surround but it is not usually visible even watching from close to.

Only film LFE provides the levels and frequencies which get the cones moving and then only rarely to a serious degree.

An update:

Well, I seem to have found my new drivers' absolute limits. The four AEIB15s seem to be capable of nearly 120dB(C) (Galaxy 140 SPL meter uncorrected) measured at 8 feet from the manifold. Watching the Fantastic 4 DVD I heard a couple of deep knocking sounds when the heat seeking missile exploded on the river barge. When I checked the levels again (after reducing the subwoofer gain by 6dB for safety) I hit 112.8dB(B) on Max hold/Fast on the SPL meter during this scene. Adding the 6dB back on, there is no doubt that the IB subwoofer was giving out well over 120dB(C) if the REW SPL meter correction factors are also added in. Until then the whole room had really been shaking. There were times when the floor went completely soft beneath my feet and the air itself could be felt shaking vigorously.

I approached the manifold at one point in the film with a torch to check the drivers' excursion and found I could hardly catch my breath during LFE rumbles. The acoustic vibration at the manifold opening was incredibly fierce yet the drivers were hardly moving. This reminds me of a kind of forced air heater used to warm or dry out large halls and buildings after a flood. These industrial, forced air fan heaters produce vast quantities of VLF and infrasonics. The effect of standing near the manifold is rather similar. In fact an acoustic furnace is quite a good description of the manifold during action films.

Occasionally I notice some strange transient cone movements on films which produce well over 1/2" (12mm) of excursion but nothing audible is heard. There seems to be nothing on-screen to suggest any reason for these rather odd transients. The speaker cones shoot out simultaneously and then return slightly more slowly.

Needless to say I am absolutely delighted with the performance of my IB despite the slight possibility of having bottomed the drivers. Though it may well have been a structural rattle. The bass levels were truly shocking until that point and I see no reason (whatsoever) to criticize this level of performance.

Since I seem to have found the safe limits of my IB I ought to press ahead with the second manifold to gain some extra VLF headroom. The LFE effects I like. Risking these amazing AE IB15 drivers is not my purpose. John E Janowitz the designer and builder of the AE speakers claims that these drivers can't be physically bottomed so the knocking sounds I heard may be sympathetic structural movements. The double doors out to the IB enclosure are the most likely culprits here. It is amazing to see glazed wooden doors bend back and forth by an inch as they used to do with my array. Leaning my full weight on them and pressing hard with my shoulder or hands had no effect at all on this flexure. It may be that the doors still flex to sudden acoustic pressure waves on film LFE transients.
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The Electronics.

2007-04-14T18:32:00.001+02:00

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Here are illustrations of some of the kit in my system.

The Behringer CX2310 active crossover sitting on top of the "infamous" DSP 1124 BFD equaliser. The BFD [Behringer Feedback Destroyer] is used by many home AV enthusiasts worldwide to improve the performance of their subwoofers by adjusting the in-room frequency response. Behringer was so uninterested in this vast customer base that they discontinued it! Then resurrected it again. One popular HT forum is trying to get Behringer to offer a specialist equaliser for the home subwoofer market. Don't hold your breath...

The image was cropped from my own rather poor photograph of the entire rack using flash. The Behringer CX2310 24/dB/octave active crossover is set at around 80Hz to split the analogue stereo signal to the Main/Stereo speaker channels. The High channel goes to the Naim power amp and the main/stereo speakers. The Low channel goes to the EP2500 and the IB subwoofer. It is difficult to tell where 80Hz is exactly on the marked frequency scale. Note the rack mounting ears and the total lack of rubber feet for domestic use. Removing the ears leaves open holes in the case which could expose high voltages to probing children and idiot adults.

A view inside my Naim NAC72 preamp. Though not obvious in this image the sub circuits (or cards) stand up on edge towards the viewer.

The view inside the Naim NAP180 stereo power amplifier which drives my Mission 753 Freedom floor standing loudspeakers. 90 wpc RMS into 4 Ohms is claimed. Neat workmanship with the large smoothing capacitors hidden away under the PCB. I find my Naim amps offer a rather hard sound on massed orchestral strings. Perhaps this is just transparency to the CD63SE source?

Inside the Naim Hicap power supply. It weighs and costs a ton! It replaces the internal power supply offered by some (but not all) of the Naim power amps. Naim claims improved sound quality from driving their pre-amps with plenty of clean power. Most owners seem to agree.

The view inside the rather untidy Behringer EP2500 Europower amplifier. Note that the cooling fan shroud has been reversed to show where the fan is situated inside the backplate. The shroud should form a continuation of the heat sink. [EDIT] As supplied, it pushes cooling air through the tubular heat sink and out through the front panel slots. Many owners prefer to replace the 24Volt fan with a much quieter model. Some like to reverse the airflow to pull it through the heat sink from the front panel. [/EDIT] I have never noticed more than a couple of degrees above ambient temperature whatever the film or musical punishment is thrown at this amp. 110dB(C) continuous for an hour on bassy rock music and car audio "tuning" tracks had little effect on temperature.

The manufacturer claims 650 WPC RMS into 4 Ohms. This inexpensive pro-power amplifier has proved very reliable for IB use provided it isn't given foolishly low impedances to work with. 4 Ohms per channel or 8 Ohms bridged is a sensible load for any amplifier. Not just the Behringers. Competitors against the EP2500 are considerably more expensive for the same power output. The slightly cheaper EP1500 offers lower power for little saving in money except for those on a very tight budget.

My tall rack is always dimly lit as far as natural light is concerned. So has to be photographed with flash. Unless it is photographed at an angle the fascias reflect the flash back to the camera. Making it difficult to capture without distortion. There is a remarkable amount of barrel distortion in the heavily cropped image below.

Click on any image for a larger view. Back click to return to the blog.
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An organ recital

2007-04-13T19:43:00.000+02:00

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Nearly ten years have passed since I last enjoyed live organ music so I thought I'd better remind myself of the real thing. An online search provided an opportunity to enjoy a half hour recital. It was mid afternoon only a couple of days later in a large church in the city. Having a free day I took along my Galaxy 140 SPL meter (and my wife) and placed myself half way along the body of the church. I read through the programme and details of the organ on the leaflets kindly provided as I waited for others to arrive. It seemed very odd to turn my back on the organ. So I reversed and sat on one of the chairs provided at the end of each pew.

Though certainly not of cathedral proportions the beautiful organ sported several 16' stops. Right on time the organist launched straight in and averaged 80dB(C) throughout. Only on the quietest moments did the sound level drop down to 70dB(C). Background noise level was 45dB(C) with occasional 50dB(C) peaks when a member of the tiny audience coughed. I believe we were only five in total which really did not do justice to the excellent playing! This Danish organ was quite unlike anything I'd ever heard in the UK. Very much brighter and considerably more nasal at times than I remembered. The Walther variations provided an amazing range of stops with many different voices on display.

My IB is certainly capable of playing much louder and deeper than this particular organ on the programme material we enjoyed today. The IB's detail and weight in the bass was easily on a par with this real organ. Very realistic indeed!

What I had not expected were the high average levels of the performance covering 4 composers and as many superbly contrasting works. Though I only saw a few maximum peaks at around 86dB(C) I now realise that I listen at home with the bass rather hotter than a real organ but with my average listening levels a few dB lower than reality. This makes for a much more comfortable experience. Particularly when spread over several hours rather than a short recital. Now I'd really like to find a cathedral organ with 32' stops to compare with my IB.

Another day, another organ.

2007-04-12T20:07:00.000+02:00

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Haderslev Cathedral main organ. My apologies for the monochrome image. I was still finding my way around a new compact digital camera.

My search for the authentic organ sound to compare with my IB brought me to Haderslev Cathedral in southern Jutland for a performance of Messiaen's "L'Ascension". Vibeke Astner was playing the organ for a moving rendition of a work which I had never heard before. I tend to shy away from 20th century composers just as much as I avoid the works of 20th century artists. The audience must have numbered perhaps 40 by the time the last, lost, elderly soul wandered in towards the end of the half hour recital.

The music was certainly different. I read the supplied leaflet to discover that Messiaen was something of a genius. He invented his own scales and sought inspiration in the music of other continents. He was particularly interested in birdsong and would use his own notation to describe the sounds they made.

I thoroughly enjoyed the piece from start to finish. The playing was flawless and beautifully expressive. The full power of the organ had my solid oak pew vibrating against my back. Even my chest vibrated to the pedal organ at one point. Like the last, this organ has only 16' stops. I had rather fixed the idea in my mind that it took a giant organ with a 32 foot stop to produce the really heavy effects. However after reading a fascinating organ website I discovered the effects of playing pipes a fifth apart produced a new tone an octave below the lowest pipe. This may well explain the results of running organ tracks through SpectraPlus software where one can often see lots of 8Hz content. There are very few real 64 foot pipes or even folded 32 footers but there are always subharmonics. One odd effect I noted was a slow, half second beating effect on the deep tones occasionally. Whether the organist was producing this deliberately or it was a natural acoustic effect of combining tones I wasn't sure. It is certainly something I have heard on some of my English cathedral organ CDs.

From this same website I even found that clever organ designers had really invented the Tricolumn, a folded horn in a simple pipe form full range speaker. A design I had enlarged considerably to make a subwoofer to listen to organ LPs in the late 1960s. I was using a Mullard 510 valve amp back then which produced about as many useful watts as a birthday cake candle to drive my 10" Whitely, cambric-coned, loudspeaker unit. Cambric was a fine, light cotton material which pre-dated Kevlar and carbon fibre reinforcement material for paper loudspeaker cones by half a century or more.

But I digress. The more organs I hear the more I am delighted with the bass from my IB. Both have much the same sound quality, the power, the breath and the exquisite detail as each other. There is still a brightness and airiness missing from my speaker's attempts to reproduce the smaller pipes. The reverberation and "airiness" within the vast spaces of the buildings which house the organs is missing to some degree. I was never fond of bright speakers so must learn to live with this for the moment.

I was able to buy a Vibeke Astner CD of Messiaen's "Les Corps Glorieux" "Sept Visions brèves de laVie des Ressuscités" on the Helikon label after the recital . [HCD1056]

The sound quality on this CD is simply stunning. Absolutely state of the art. The 7th track: "Le Mystère de la saint Trinité" is the last of these mostly rather quiet, meditative pieces. Here the upper voices are underpinned by the most amazing, deep, quiet bass tones one is ever likely to enjoy. The clarity is extraordinary and makes the CD worth obtaining simply for this gorgeous, reference-quality track. The other tracks are all beautiful too in Messiaen's own rather individual style. Helikon is a small Danish record label.

http://www.helikonrecords.dk/orgel.htm

Another organ CD I must mention is Felix Hell's "Organ Sensation". The sheer weight of bass on this stunning CD is tremendous! Certain tracks found the resonant frequencies of my walls and floor and even shook the kitchen ceiling downstairs. There is considerable infrasonic content to be found on this superb album with (probably) very few subwoofers other than an IB able to make really good use of it. Felix has an excellent website which is a must for the pipe organ enthusiast. His videos are a unique opportunity to watch an organist actually playing a large, fully-exposed console. Normally the organist is hidden away. Often high up on a screened balcony. Where one might only see the top of someone's head swaying along to the music.

http://www.felix-hell.com/index.shtml

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Revelation, at long last.

2007-04-11T19:16:00.000+02:00

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Another trip in the car and another organ. This time it was to Svendborg in southeast Fyn where a young organist showed off his talents at St. Nikolai's Church. It was immediately apparent that this organ had plenty of seriously deep bass compared with the rather more subtle organs I had recently enjoyed. Three 16' stops were listed this time. The audience was far larger than previous recitals and provided suitable applause to encourage the clever young man at the console. The 4 + 1 pieces played within the half hour recital produced a feast of exhilaratingly deep bass tones. Listening to the descending notes as they dug ever deeper was a revelation to my ears. I had waited for so many decades to hear such stunning clarity, weight, depth and sense of limitless power. Finally, I had a reference with which to compare my IB subwoofer back at home.

I knew immediately that had I allowed myself to enjoy live organ music on a regular basis that my speaker system would have left me deeply disappointed for every moment of those 40 long years. Until, that is, the last moment before I had fired up my IB manifold. My pair of DIY 6th order series subwoofer could do wonders but were just too muffled and coloured in retrospect. The big SVS cylinder had plenty of output down low but was just too muddy on organ music. The IB had been the breakthrough in bass quality which I had been seeking since the late 1960s. Until now I had nothing with which to compare it.

My IB speaks the language of the organ as far as my ears are concerned. It can chuff or breathe itself slowly into life. It can bellow with raw power and still whisper the subtle truth in the next moment of inky black silence. It provides the same immense weight and grandeur as suddenly and effortlessly as a real organ. Easily conveying the power of the great pipes which congregations and audiences have held in awe for hundreds of years. I am beginning to realise that I need new, more revealing main speakers to capture the clear, sharp tones and airiness of the smaller pipes in these vast spaces.

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Brace yourselves...

2007-04-10T16:04:00.000+02:00

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Seeing the massively thick IB manifolds illustrated on the IB Cult forum has finally nudged me into action. I bored two holes and fitted a piece of studding as a cross brace between the two sides of my 3/4" plywood manifold. The studding (or screwed rod) is 1/2" in diameter. (12mm) With very large, square, thick, galvanised washers (inside and out) to spread the tension and compression loads from the driver reaction forces in the side panels. I had carefully chosen a brace position which lay midway between the drivers and the open side of the box.

I carefully tightened the nuts hard without distorting the box and put on a favorite organ CD to judge the effect. The improvement seemed well worthwhile on the subjective level. With plenty of detail in the deep bass beautifully exposed. However, a quick check with REW produced zero difference between the nearfield response plots whether the nuts were tightened hard or completely loosened. Absolutely no change could be seen between output levels or response curves. Well, at least I made the effort and it took only a few minutes to gather the tools and materials to make the modification.

A standard of 2 x 18mm (2 x 3/4") layers for a laminated box manifold is still probably a very good idea for most IB installations. Ensuring a solid but very heavy box well able to resist reaction forces between the opposed drivers. Giving them the best chance of moving air rather than "flopping about" uselessly. (like my array)

My own manifold used an entire (metric) 8' x 4' sheet of 18mm (3/4") high quality plywood with very little left over. This gives a pretty good idea what the bare manifold weighs. Add the weight of another sheet to bring the manifold up to proper 1.5" thickness. Then add the weight of the four drivers and one should plan an IB installation very carefully. Fortunately my manifold sits on the floor and has easy access both inside and out. In a ceiling installation one should make certain that the joists can support the load without sagging.

When I think about it; the back, top and bottom of my manifold add plenty of stiffness on three sides of the driver panels. The open sides of the manifold are also being reinforced by the wall studs between which the manifold fits. The surface covering on the adjoining walls adds further resistance to bowing at the manifold mouth provided the manifold is well screwed to the studs.

If one doesn't like the look of a naked threaded cross brace then a pipe or even a length of opaque hose could be cut to fit between the inner nuts. Then slid over the screwed rod as it is assembled in the box. Perhaps a plywood "window" brace might be better. Though if the brace is glued in the drivers become more inaccessible. It is usually so dark in my own manifold that appearance hardly matters and the performance is still superb. After a few days there still seems to be greater weight and detail in the bass I'm still not sure whether it is a purely psychological improvement, or not.

Here's the simple studding brace in place showing the big, load-spreading washers to resist crushing forces on the plywood.

Having had time to think about it I now realise that REW is a rising sinewave sweep at a fixed level. The reaction forces of the drivers when subjected to sudden acceleration from severe transients is hardly the same thing. So it is hardly suprising that I saw no difference in the REW response plots.

I really ought to know better and should build a more solid manifold because I so enjoy the subtleties and microdynamics in the bass. Anything which helps to resist driver reaction forces should aid sound quality at all levels of output. However, I am still uncertain how to proceed beyond this point with regards to adding more drivers for greater dynamic headroom. So the manifold remains at 3/4" (18mm) for the moment. If I beef up the box with another layer of plywood or MDF I might want to scrap it soon afterwards to build something else.

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More measuring.

2007-04-09T11:48:00.000+02:00

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Last time I found some rather odd variations between my pair of RS SPL meters and my Galaxy 140. I wanted some new nearfield comparisons so took this opportunity to do a completely new set of REW measurements. I also included my new ECM8000 microphone and Xenyx 802 mic preamp in the tests. It soon became clear that the REW Cal file for my two RS meters was seriously out of step with the Galaxy and ECM8000. So I deleted the 33-2050 cal. file and downloaded the 33-2055-4050 instead. The latter file gave curves much closer to the Galaxy and Behringer microphone. Now I could progress with my testing.

All devices were tested twice without being touched to exclude possible environmental variations. (wind, distant traffic, people moving around in the house, doors opening and closing, etc)

The red trace below is the ECM8000 and those above this line are the Galaxy and then the two RS meters respectively. All traces have the BFD out of circuit. All devices used the matching REW calibration file freshly downloaded from the HT Shack forum. The CX2310 active crossover remained in circuit with a 24dB/octave cut at around 70-80Hz. It is difficult to read 80Hz accurately on the frequency setting dial. The speakers were muted in all cases at the crossover. One of the advantages of having all those push buttons.

These nearfield curves are all quite smooth and each pair of tests per device match each other closely enough to give me confidence that there is nothing untoward in my test methods this time.

A cynic might argue that the ECM8000 is reading low in the deep bass since it does not match the three other devices which all match each other closely. However, the high cost of professional microphone calibration will not have me following that route in a hurry. For my subwoofer testing purposes they all do well enough. The Behringer mic and Galaxy SPL meter are better for fullrange speaker testing at higher frequencies. These two are also better at reading deep infrasonic output accurately since the correction factors are very much smaller than the cheaper SPL meters and the cal file extends down to 5Hz in the case of the Galaxy 140.

It should be noted that some of my other graphs show much closer correlation between all these measuring devices. I have convinced myself that any of these four devices is suitable for measuring most domestic subwoofers. Only when speakers or IBs are to be measured should the Galaxy 140 or the ECM8000/Xenyx combination be seriously considered.

All the devices were laid horizontally and individually on a cushion on the floor at the mouth of my IB for these tests. The small variations seen could well be down to slight changes in position or minor resonance in the bodies of the SPL meters. The very solid, metal-bodied ECM8000 is probably immune to this but compressed the cushion much more than the other devices because of its comparatively small cross section and considerable weight.

From Left to Right: RS 33-2050, 33-4050, Galaxy 140 SPL meter and ECM8000 calibration microphone. The ECM8000 needs a mic preamp with phantom power adding considerably to the cost of this inexpensive microphone. I believe some computer sound cards offer phantom power so that may be worth exploring.

I chose to buy a Behringer Xenyx 802 to go with the microphone because this was the unit suggested by the HT Shack experts. This offered a proven device with settings already worked out for the new user. Here I have plugged the microphone into the input XLR socket. Normally an XLR cable will fit in here and the microphone would be mounted on a tripod for testing. Note the positions of the control knobs on the Xenyx for REW testing. The HT Shack has a better image for confirming these settings.

The background noise level reading on the Galaxy in this shot is due to the proximity of my computer and the C-weighting option being selected. A-weighting background noise in my room is around 31.5dB(A) at the listening position or 43dB(C-weighting) . Both readings were taken with the Galaxy resting on my listening chair with the rather noisy computer running about twelve feet away.

The nearfield curves in the REW graph show little or no room gain. At the listening position the lower frequencies are boosted considerably by room gain. The nearfield curves are reasonably flat from 20Hz upwards and speak volumes about the quality of these AEIB15 drivers.

Later reading suggests that there is some spread to the frequency response of the inexpensive Behringer ECM8000 microphone. Those who wish to enlighten themselves or have their microphones properly calibrated could do worse than check the HT Shack threads on this subject.

Remember that you can click on all images for a larger version. Back click to return to the blog. .

Calculating IB enclosure volume.

2007-04-08T12:56:00.000+02:00

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I have received a comment asking for IB enclosure volume calculations. Thanks for the timely suggestion.

So: Total Vas x 10 = minimum enclosure volume for an IB subwoofer.

That's all there is to it really. But I suppose you want more? :-)

Until now I have largely avoided putting real numbers on my IB. Though, in fact, the design calculations are very simple indeed and can be (almost) ignored if one stays well within the usual IB design parameters. Which is just another fancy way of saying a 4 x 15" IB can be built to standard dimensions and it will work very well indeed. It will also stay safely within any cone excursion limits which might endanger the drivers. It will also have a reasonably flat frequency response down to well below 20Hz. Or even 10Hz if your room allows it.

First things first: An IB subwoofer is critically dependant on the chosen drivers: So choose from the small range of 15" or 18" IB-specific drivers and most of your design problems cease to exist right away. The specialised IB driver will offer a realistic maximum cone excursion. Which usually means a minimum of 18mm Xmax. Avoid car audio drivers with flashy paintwork and chrome finishes claiming ultra long stroke. Unless Xmax is clearly stated then any claims are meaningless sales hype. So, which drivers do the real experts use for competition car audio dB drag racing? Those manufactured by exactly the same companies which offer serious IB drivers at lower cost. Not the over-hyped, over-Xpensive paint and chrome items intended to appeal to the young, the inexpert and the gullible.

Driver displacement is vitally important in IBs. Cone excursion multiplied by the cone area multiplied by the number of drivers should come to around 10-12 litres minimum total displacement. For 15" IB drivers: 4 x 3 =12 litres. This displacement can also be achieved with two good 18" IB drivers. So we've already learned that 4 x 15" IB drivers = 2 x 18" IB drivers. Since Xmax is measured as a one way come movement we use 2 x Xmax x Sd to give the displacement for one driver. The larger cone area of the 18" means a smaller Xmax will provide a similar displacement to a 15" driver. Better still is a large Xmax giving the 18" IB driver around double the displacement of the typical 15" IB driver.The Fi Car audio IB18 offers 30mm Xmax!

What if you want to use 12" drivers? Then you must calculate their total displacement to achieve a minimum of 10-12 litres just to be safe. Displacement obviously favours large cones so your 12" drivers had better have a seriously long stroke and you'll need at least 8 (and preferably more) to compete safely at high sound levels at very low frequencies with a typical 4 x 15" IB. Twelve or even sixteen x 12" drivers is a better starting point but likely to be rather expensive using high quality drivers. Most 12" drivers wont have such a low Fs as proper 15" IB drivers.

BTW: For those who have forgotten: The area of a circle = radius x radius x Pi. (Pi = 3.142)

The squaring of the radius is the obvious clue to the rapid increase in driver effective area with increasing diameter.

Reproducing low frequencies need lots of air movement. Fanning the air slowly with a large board is obviously far more efficient than waving a postcard a yard backwards and forwards each way. So it is obvious that bigger drivers move far more air per stroke than smaller drivers. Big driver often play loud and low without any visible cone movement at all! So more cone area = higher efficiency at low frequencies. The IB is naturally efficient with falling frequency thanks to a lack of back pressure and large cone area. It also has a motor for each of those big cones to keep everything nicely under control.

A 15" driver is a reasonable size for true pistonic movement at low frequencies. Bigger drivers may suffer from cone breakup. Very large cones almost certainly will. Large plastic/vinyl cones are generally regarded as too soft to be ideal for a sub-bass driver. Particularly in straight-sided conical configurations. Steeply flared, reinforced paper cones are greatly admired by those in the know for their lightness and stiffness. Aluminium cones are highly dependent on the skills of the designer and manufacturer and disliked by some purists who claim the cones can "cry" audibly at higher frequencies. Only testing with software like REW would prove this to be the case. All the Cult recommended IB drivers have reinforced paper cones.

IBs are so efficient that with quite reasonable power they will go very loud and very deep on a typical musician's power amp. The Behringer EP2500 amplifier is a popular choice for IB use with 4 x 15" drivers. Two 8 Ohm AE IB15s in parallel is a comfortable 4 Ohms impedance per channel. The manufacturer claims 650 watts per channel RMS into 4 Ohms. So the drivers are seeing 325 watts each. The EP2500 has also been used satisfactorily with 8 x 15" drivers thanks to the greater efficiency of the larger combined cone area.

Acoustic Elegance made my IB15 drivers and they are deservedly popular in sets of four at $100US each plus freight. Even in Europe they make good sense despite the heavy freight charges and import and sales taxes. One day, global tariff agreement will be reached or tariffs will be scrapped altogether. Then we'll only have to pay for freight to Europe for US drivers without all the unwanted extras. Check with AE for latest pricing before placing an order though as this blog may eventually grow as many grey hairs as its author.

Fi Caraudio makes excellent 15" and 18" IB specific drivers. Four Fi IB18s would probably be bulletproof no matter how wild action DVDs may become and will probably leave nothing to upgrade for several decades. They also make more extreme models at higher, but still attractive, US prices. They are a very small company and each driver is handmade to order.

Other popular drivers are listed on the IB Cult's recommended driver page:

Driver manufacturers come and go. Their model range may also change. So check for the latest drivers and manufacturers before making any final driver decisions. Also look for Mach5's IXL18.4 and and the DIY Cable Tempest X15 for further popular models for IB use. The Tempest X15 is available from a European distributor. (Streaming Audio of Sweden) Prices are probably competitive in sets of four with the AE IB15s once freight and taxes are added to the AE drivers US prices. The Tempext X15 claims 27mm of Xmax for slightly more displacement at 4 liters and more headroom than the AE IB15's 3 liters. At a glance these two drivers look remarkably similar to each other in appearance except for the outer roll suspension. The AE claims lower distortion for their IB15 due to the careful motor design.

There is a link below to the list of drivers currently on the IB Cult's forum website. Use the US dollar price per litre of displacement as the surest guide to driver value for IB use in the USA. Elsewhere in the world you'll have to do the maths depending on local availability and any additional import and freight charges. These can easily double the US online purchase price.

http://pages.sbcglobal.net/ginmtb/woofer_comparison_chart.htm

Loudspeaker drive units (or 'drivers' as they are now commonly known from US usage of the term) are built to very exact specifications. Thiele-Small paramaters are used to describe drivers in precise detail and in units which don't mean much to most people. Here's the Wikipedia entry: You can safely ignore all the terrifying equations and scroll straight down to the list of Thiele-Small parameters list much further down the page under Qualitative Descriptions:

http://en.wikipedia.org/wiki/Thiele/Small

Some useful terms for IB design: (with simplified descriptions) Feel free to wallow in the fine details from Wikipedia and other online resources. You really don't need to know more than appears below to design an excellent IB. Remember that buying recommended IB specific drivers will save a lot of extra work and worrying over details. Choosing these IB drivers is a short cut to success where these recommended drivers are available and affordable.

Fs is the free air resonance of a driver and measured in Hz.

Hz replaced cycles per second some years ago in honour of Hertz but the two terms mean exactly the same thing. Having an Fs somewhere between 15 and 20Hz is ideal for an IB driver. Go much higher in frequency for Fs and you might need to start boosting the bottom end to get serious infrasonic output.

Infrasonics = sounds below the normal frequency threshold of the human ear.

Though usually completely silent, at the levels we listen to at home, infrasonics are good for making the room shake on action films and deep organ music. The lower audible frequency threshold for adults is usually around 20-22Hz) A low, free air resonant frequency driver will avoid the need for low bass boost. Room gain will help. Leaving a nice flat, extended response to below 10Hz in most cases. (provided the 15-25Hz range for Fs is adhered to) The AEIB15's Fs = 16Hz = Ideal for IB use. The IB specific drivers mentioned above all fall safely within this range. This is another good reason to seek out IB specific drivers. Everything is taken care of by the designer saving you having to worry about specific details.

Provided we stick to standard IB driver designs and use a recommended number of IB specific drivers we only need to know the numerical values of the following T-S parameters: There is absolutely no need to understand how these numbers are obtained by the manufacturer unless it really interests you. It won't make a better IB but you may be able to impress your lucky visitors with more than your world class IB bass.

Vas= Equivalent air volume of driver suspension stiffness. Vas is used to calculate the size of the rear volume or backspace of the IB. In normal loudspeaker and subwoofer terms we'd call this back space the enclosure. The same holds true for IBs. IBs just use huge volumes for their enclosures.

(N.B. Enclosure size does not relate to the manifold volume. The volume of the manifold is largely irrelevant as long as it is big enough to house the drivers. A driver manifold is really just a local folding of the baffle with serious mechanical advantages which you should not ignore.)

Vas is given in litres by the manufacturer in their driver specification. The AE IB15 has a Vas of 439 litres. (15.5 cu.ft. in old money) If you are metrically challenged just divide litres by 28.4 to convert them to cu.ft.. BTW: Liters and litres are interchangeable names depending entirely on your own geography. Divide litres by 1000 if you want volumes measured in cubic meters.

Qts = Is a unitless measure of quality of a driver. It usually lies between .2 and .8 for loudspeaker units or drivers. The AE IB15 has a Qts of .7. Which is fine for IB purposes. You can go lower than .7 but avoid going much higher than .7 if you can. Experts consider .5 to be the optimum where electronic equalisation may push up the overall Q of the system. Q can be pushed up by accident or design. It cannot be lowered.

Vd = Cone displacement measured in litres = Cone area (Sd) x (2 x Xmax)

Vd is calculated from the following two parameters:

Sd = Useful cone area in square centimetres (usually including part of the outer suspension roll surround which contributes to the driver's output)

Xmax = One way safe cone excursion measured in millimetres. (there are other factors involved but we don't need to delve deeply because we just use the manufacturer's claimed Xmax in millimetres)

All these figures should and will be provided by the high quality IB driver manufacturer. So all you need to do is pick off the numbers you need and you can go ahead and design your theoretically perfect IB.

Ideal IB enclosure minimum volume = Vas x Number of drivers x 20.

Enclosure volume for 4 x AEIB15s = 439 Liters x 4 x 20 = 35120 liters = 1236 cubic feet. (Wow! That's a 10.75 x 10.75 x 10.75 foot cube!) Say you only have a typical 7 feet domestic headroom you're going to need a space 13ft x 13ft to house your IB optimally.

A multiplier of 20 x total VAS is the calculated figure which will not raise the Fs (natural resonant frequency) of the IB drivers any higher than if they were measured in free air. As you reduce the rear volume (enclosure size) the natural F of the driver(s) will slowly rise. This might cause you to lose very deep bass and decrease system efficiency. This will theoretically require more amplifier power to reach the same output levels at very low frequencies and might also need some low bass boost. You may lose subtle sounds and dynamic effects in the deep bass which make an IB sound so real.

How low dare you go with an IB enclosure volume? Ten is quite a safe Vas multiplier but the risk is yours, not mine. My own IB enclosure is now down to a Vas multiplier of about 7. (at a very rough guess) Unfortunately the enclosure space is just too handy for storing packaging. Insecurity makes one keep the packaging in case the item needs to be returned to the dealer. In an ideal world all those boxes would be nested and left wide open. (To help reduce compression effects within the IB enclosure and to help scatter any standing waves) Do I notice anything from adding yet more empty boxes? Nope.

Note: My enclosure space is quite poorly sealed to the outside world which probably helps the cause. I also have 12"-16" of almost bare rock wool insulation above the sloping surfaces of the enclosure with only a DPM covering at the moment. Polythene DPM is probably quite transparent to very low frequencies if they are allowed to bounce around in the enclosure. Such a thickness of rock wool is quite good at absorbing very low frequencies. If you have a badly undersized concrete box for an IB enclosure you'd better not use it for storage of anything but fibreglass or rock wool wadding!

So, in summary: In a perfect world, you aim for at least 20 x Total Vas (Total Vas = Vas x No. of drivers) as your ideal enclosure volume. Then hope you can find enough rear volume in your crawl space, loft or cellar. As discussed earlier, the IB enclosure can be a spare room which shares a wall with your AV room. It can also be the underfloor (crawl) space or cellar. Or the loft above the AV room ceiling. Thankfully crawl spaces are usually very large areas covering the entire house floor plan. So the lack of depth isn't usually too much of a problem where volume is concerned. Lofts are often the largest room in the house when they go unused. They are often asymmetrical, triangular in cross section and well supplied with thick absorbent insulation into the bargain.

The real problems start when you really want to have an IB but you only have a small closet or cupboard available for your enclosure. Assuming you want to use 4 x AE IB15s and your closet height is only 7 feet then you need to think very seriously about alternatives. You could spread the necessary volume around between several smaller IBs in different places around the room if that helps you get more enclosure volume. The extra enclosures could be more cupboards, underfloor spaces or above the ceiling.

Some IB builders have removed the ceiling of an undersized walk-in closet to exhaust into the room above. Not ideal if it has to exhaust into the loft space though! Not unless the closet enclosure can be very well insulated in very cold climates. You must insulate or the heat in the house will wick rapidly away through the closet walls and probably cause massive condensation in winter. The opened closet ceiling can't be blocked with insulation or the IB drivers will still think they're trapped in the small closet. You wouldn't want to leave the open hole unguarded or people and wildlife will continuously fall into your IB enclosure. The exit hole can't be small either or the very low frequencies won't notice the hole is even there. Steel weld mesh over the hole down into the closet is a good start to avoid nasty accidents and wont affect the necessary cyclic airflow through the open closet ceiling.

Air movements are so large at VLFs that they need lots of room for free flow. Sound pressure waves move backwards and forwards. Not up and down like a skipping rope making waves. Imagine low frequencies as a string of huge balloons being cyclically stretched and then compressed back together. The scale is awesome. The usual crossover point of an audio system with a subwoofer is 80 Hz which has wavelength of around 15ft. 20Hz has wavelength of nearly 60 feet!

Now try to squeeze the vast balloons through too small a hole and the balloons will object. Greatly increasing in friction until no movement is possible. You might even think of the squeals of protest from the giant balloons as harmonic distortion components. This is why you don't want to port your IB enclosure. The IB can do bass like no other enclosure. "Gilding the lily" with a port is not a good idea. Using the same number and size of drivers (as in your intended IB) the enclosure size for a reflex design would have to be completely different. (usually very much smaller than an IB enclosure) The port would have to pass so much air that it would be difficult to avoid turbulence, noise and distortion. Moreover the ported enclosure would have to be airtight and rigid enough not to flex unduly to work as an efficient Helmholtz resonator. The air in the huge port required must have some deleterious effect on the sound quality due to turbulence and inertia.

The IB doesn't mind if you make its enclosure much larger than 20 x Vas. Nor how leaky it is. You are not really tuning an enclosed volume like a ported box. With an IB you are merely blocking the rear pressure waves from meeting the front pressure waves being produced by the driver cones.

In theory you could have your IB open to the outside world. Just fix your drivers into an outside wall and turn up the volume! Fine, if you live in the woods and are half a mile from your nearest neighbours. Not a good idea (at all) if you live in town. It's best to cut down some of the energy in the rear waves before they reach your neighbours right next door. So enclosing the rear waves is a vital exercise for most IB builders. Containing the IB in an enclosure also limits the bass leakage to other rooms in the house. Obviously this will vary and the rear waves aren't the only culprit. Any powerful subwoofer will share its output with the rest of the house unless you have thick concrete walls and floors, brick or block walls and heavy soundproof doors.

IB design in a nutshell:

A regular box of any shape has a volume of Breadth x Length x Height .

A roof prism shape (attic or loft) has a volume of 1/2Base x Height x Length.

A trapezium (attic or loft) with a flat ceiling width A and floor width B has a volume of 1/2 (A+B) x Height x Length.

Let's work out an example to check my own IB enclosure volume. It is a simple roof prism which goes right up to the peak of the roof without any form of flat ceiling.

Height is 10 feet. Base = Total width across the floor = 18 feet. Length between the two triangular walls is 6'6".

Volume = 1/2 x B x H x L = 10 x 18/2 x 6.5 = 10 x 9 x 6.5 = 585 cubic feet.

It comes to exactly the same answer if you multiply all the dimensions together then divide by two. (B X H X L)/ 2 = (18 x 10 x 6.5)/2 = 585. You can halve any one of the dimensions of a roof prism and still get the same answer. 1/2 x Base is the same as half x Height or Half x length provided you halve only one of the dimensions in the formula for volume.

Don't forget to multiply the Vas of your chosen drivers by the number of drivers to obtain Total Vas. Or your rear volume calculations won't be of much use. Using Vas for only one driver in the volume calculations seems to be quite a common mistake and leads to unfounded optimism about the minimum allowable volume of an IB enclosure.

If you can only find enough rear volume using Total Vas x 5 then I'd probably still go ahead. You can put some thick absorbent material in the rear space which will fool the drivers into thinking they have a little more space available. It's not ideal but you'll still have a subwoofer which leaves most others for dead on sound quality, frequency extension and sheer power. The downside with such a small enclosure is that the enclosure walls will need to be solid or well braced in comparison with a much bigger volume. The walls of a bigger space are not subject to such fierce cyclic compression and rarefaction effects as the smaller rear volume. Though the surfaces will still vibrate if they are flexible enough even in a very large space. Wall vibration may be out of phase and cancel your bass at particular frequencies. Unwanted structural noise produced from vibration is bad because it draws attention to itself. Spoiling the pleasure of listening to music or a film soundtrack.

Simple Basic Rules for IB construction:

A: Use an opposed driver manifold unless you can build a very stiff and/or massive wall for an array. If you are determined to have an array then keep drivers low down in a horizontal row just above the floor. So the stiffness of the floor plate directly contributes resistance to driver movement. Or put them in a horizontal row up high. Using the lateral resistance of the ceiling joists in the same way as a floor. Or arrange your drivers vertically up a corner between two walls to gain the resistance of the side wall. Worst case scenario is when an array is arranged vertically in the middle of a lightweight partition wall. Even worse is having a door beside the array! The drivers will sh-sh-sh-shake the wall and door like you would not believe! Been there. Done that. Didn't like it! Have you seen a door literally bending rapidly back and forth in its frame by a full 1" from flat? I have. That's why I now have a manifold! Do not underestimate the reaction forces from 15" or 18" drivers!

B: Use Vas x Number of drivers x 20 for your enclosure volume. Cheat on volume if you must. We won't tell if you don't. But big is still beautiful. 10 xVas is fine in practice.

C: Use (at least) 4 x 15" or 2 x 18" IB specific drivers for a minimum of 10-12 Litres of total driver displacement. Make no excuses and you won't bottom your drivers on the first action film you watch! As ThomasW of the IB Cult says: Drivers make poor (but expensive) paperweights. Say you cheat and try to use only two x 15" drivers and break the first two. Then you have to buy four more to be safe next time! That's six drivers you have to buy in all! Even I can understand the maths of such an economic disaster. Ouch!

D: An impedance load of 4 Ohms per channel is optimum. Eight ohms per channel is acceptable but will limit output a bit. Two Ohms per channel is far too hard on many amplifiers. Bridging an amplifier is the same thing as halving the impedance per channel seen by the amplifier. So eight ohms bridged is okay. While 4 ohms bridged is very hard work and overheating or instability may occur in the amplifier. Some amplifier manufacturers will specifically tell you not to wire your drivers for a 2 Ohms load per channel or 4 Ohms bridged. It really is far safer simply to avoid 2 Ohms per channel or 4 Ohms bridged. If your amplifier goes into heavy clipping it will burn your voice coils with nasty distortion. More paperweights! Even if you can afford all these paperweights try and think of the planet's finite resources.

E: Two, four and eight drivers are usually easy to wire up for a sensible amplifier load. Three or six or nine drivers each of 4 or 8 Ohms impedance are difficult to wire up sensibly. You can't wire them in series, parallel or series-parallel to obtain a suitable impedance around 4 Ohms for your amplifier to drive.

F: Feed your IB drivers with a popular/reliable musician's power amp for clean headroom. If you want more output then double the number of IB drivers rather than doubling the RMS wattage. Avoid plate amps if you can. Most lack the power you need and are not such good value for money as pro power amps anyway. Your AVReceiver will usually act as the crossover between subwoofer and speakers. So the plate amp's crossover will be bypassed and becomes superfluous.

G: Test your proposed IB location with a conventional subwoofer and REW before you start making large holes in your home. If a smaller subwoofer suffers from massive nulls or peaks at the listening position then it is certain an IB will suffer from them too! Moving a finished IB is not the easiest thing in the world and loses you major house points with your partner. So check your proposed manifold position first even if you have to borrow a box subwoofer and learn to use REW. Or build one of your IB drivers into a box and use that for testing. If you are really confident you want an IB could build your manifold and use that with the opening blanked off or resting on a carpet or rug to obtain a seal. It really will be worth the hassle to test the proposed IB location first. If the conventional subwoofer sounds okay to your ears at the chosen spot then you may be okay. Though using REW is very much safer because you can see exactly where particular frequencies are missing or peaking in the response curve and this isn't always easily audible. Not unless you listen very carefully to a wide range of different material during your tests and know what to listen for.

H: Last but not least: Join "The Cult of the Infinitely Baffled" forum. All of the above information has been gleaned at the feet of ThomasW, the IB Cult's high Wizard. He has forgotten more about audio than I will ever know. (or can still remember) So if you are really serious about building an IB join the IB Cult and get stuck into the IB Cult's priceless FAQs. Each of which is individually hand beaten from 21st Century alchemist's gold by the fabled high wizard himself. Seriously though, Thomas has popularised the IB subwoofer almost single handedly with his IB Cult forum. His decades of audio experience remove much of the hearsay and bullshit you will read elsewhere. Reading some of the nonsense posts on others forums where the members try to talk about IBs from a standpint of total ignorance can be more excruciating than amusing! There really is a lot of ignorance surrounding IBs even by those who try to build them without knowing any of the simple rules.

I advise you to read the Cult's FAQs right through first. Then look at the galleries for lots of working examples. Only then ask questions on the forums. It's only polite, given the enormous effort ThomasW has put into them.

And finally: Don't ask about car audio infinite baffles on the IB Cult. There are plenty of car audio forums for discussing all kinds of subwoofers. The IB Cult is for discussion of properly designed, HT/AV/Home Audio, True Infinite Baffle subwoofers. Whether for film, or music, or both, the IB will deliver bass sound quality and clean power like you have never heard before. Provided you follow the simple rules.

In case you have any lingering doubts about an IB's sound quality: The Cult is littered with examples of owners who have built IBs to replace stacked or multiples of expensive, commercial subwoofers. The IB has always sounded better except in one or two rare cases. Most IB builders are absolutely delighted with their results provided they followed ThomaW's advice and adhered to the basic rules.
*

Organic vinyl

2007-04-07T15:46:00.000+02:00

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Finding a space for my turntable on top of my rack had eluded me for more years than I could easily remember. I had simply lost interest in my modest vinyl collection at some point. Most of the records I might have wanted to enjoy were too well worn from decades of repeated listening. Surface noise had driven me inexorably towards the evil digital CD replay. The occasional secondhand vinyl purchase had proved a great disappointment due to surface noise, wear and distortion. So my Linn had rested unused on top of a cupboard for years. Like some long forgotten, ageing sportscar in a backstreet garage. It even had its own cloth throw-over to keep the worst of the dust at bay. Sadly this had not excluded the slow oxidation of the aluminium alloy platter and general dulling of the muscular, Ittok VIII arm.

My system, with a series of subwoofers culminating in the IB, had reproduced endless CDs to a very reasonable level of satisfaction. I had online access to a vast national public library of CDs completely free of charge. All I had to do was find a CD in the online catalogue then place my order with a few clumsy taps of the keyboard with subsequent editing for the all too literal server dummy. A few days later I'd be able to collect my chosen CD(s) from my local library on receipt of their email notification of arrival. It was almost intolerably easy even for one so pathologically lazy like myself.

CDs had fuelled my system for years. But there were still the live organ recitals and street musicians constantly nagging at my subconscious.. These often left me feeling that my speakers had been stuffed with thick winter socks. Or I that I had succumbed to a head cold. Yet I hate bright speakers. So upgrading didn't look like an easy option at any price this side of the national debt of a small African country. Having made so many speakers in my time I had scant respect for the commercial products which are held in such awe yet seeming to need constant upgrading. Without which, last years model was merely a tawdry copy of perfection.

One day, while wandering around yet another flea market looking for cheap CDs, I found a pristine looking organ LP with an excellent pedigree. The price was foolishly low. Nah. Well..? Possibly? It was cheap enough to throw away if it should it prove a dud. As had others, equally promising, in the past. The purchase was made.

Yet, despite my potentially interesting find and deep, unsatisfied curiosity about vinyl organ bass the LP had rested untouched but safely vertical. Sitting beside the rack you see above in its latest configuration. No two shelves have the same free height. That makes for interesting arrangements of expensive metal boxes and the potential for yet more if only the endless grid-locking of available rack space could be overcome.

Then, only yesterday, while wandering around a charity shop in a distant town, I found a Rega turntable shelf. It was hiding amongst the junk furniture in a back room. It was so ridiculously cheap that it simply could not be ignored. Nor left behind!

Next day, after a particularly stormy autumn night, I had decided that it was time to spring clean the rack. There was much untangling of the huge bundles of knitting to do prior to an exploratory setting up of the infamous LP12 turntable. So several backbreaking hours later I had moved everything up or down in the rack. I actually had a clear top surface for a turntable. Unfortunately the enticing Rega metalwork had proved impossible to hang anywhere useful due to a complete lack of suitable vertical walls in my attic AV room. The Rega shelf however had served a far more important purpose. That of providing the impetus to try vinyl again. A larger piece of toughened glass was quickly found amongst the reserve collection. (rack owning audio fans should never ignore nice bits of glass begging for a comfortable home) The glass chosen was floated on hard foam strips on top of my tall rack to support my old LP12. I hadn't even bothered to level the rack at this point. Sorry about the dust. Try to imagine this is an antique in appearance if not strictly within the hundred year old category.

The large, black and shiny, organ LP was reverently placed onto the thick felt mat. The Ittok LVIII arm, with attached DL304 cartridge, was swung across. In best building-crane-like fashion. Cued, and the "needle" dropped safely into the run-in groove in the well-rehearsed manner. I may be showing my age here. Easy peasy, I thought smugly to myself. Just like riding a bike! Until, that is, I trod one step back to admire the warmth of the age-darkened, afrormosia woodwork of my decades old Linn plinth. Oh, and to increase the volume considerably. The CD volume setting on the Naim NAC72 preamp is considerably lower than the low output moving coil cartridge requirements despite the matching low output moving coil internal cards.

It was only then that I finally remembered why I had stopped using the turntable. The floorboards up here had much the same quality as a trampoline but offer only a vanishingly small fraction of the fun. With some practice and a delicate touch I was finally able to tip-toe hesitantly to my listening chair without the pickup arm bouncing completely off the record. Every time I moved it was behaving like some old biplane trying to land on an unimproved meadow. You should have seen see the exaggerated cone movement on the IB! We are talking seismographic quantities of excursion here. I might even sign up to become a monitoring station for North Korea's nefarious activities.

I hadn't even waved my magic carbon fibre dust brush above the over-20 year old old organ LP beforehand. There was certainly some of the detested surface noise present on the run up to the usual leap of vinyl faith into the void of potential audio perfection. I braced myself instinctively as Vierne's magnificent Carillon de Westminster started. Expecting only the worst. Pessimism is a learned trait in analogue audio. Particularly where vinyl is concerned. One's ego is easily as fragile as the medium itself.

The ensuing sound was certainly very different from my old Marantz CD63SE CD player with all its daft copper screws. All that glisters is not often gold! The record sounded brighter yet warmer than CD. If that makes any sense. Far more descriptive of the echoing venue without actually shouting about it. I liked this new subtlety. When the bass finally arrived it was warmer and softer too. Without that hard-edged, intricate, infrasonic, balls-out detail I usually get from my better CDs. Still, the weight of the great pipes was really quite shockingly good! I'm a great believer in weight where organs are concerned. If you do not own an IB subwoofer nor attend live organ recitals you probably won't have clue what I am talking about where "weight" is concerned. The overall dynamics were also very pleasing indeed.

The strange thing was the sense of naked realism. There was none of your fine, spacial, 3D detail spread out up front like unique pigeonholes which one often gets from a CD. More a natural feeling of general shape and great solidity at a modest distance. A real organ was actually playing somewhere in front of me. Rather than a cardboard cut out. With its high definition artwork glued to the front and brightly lit with lots of little flashing diodes. Real life does not have pinpoint imaginary in my experience. One senses only direction, distance and the "roundness" of the source. No audible singularities exist in reality. Yet these are treated as the holy grail in some equipment reviews.

No matter how I wound up the volume the vinyl sound just got louder without becoming unpleasant. With CD I find one just has to set one's teeth on some of the brighter bits if you give the system its head. Things get all grainy and can easily sound distorted. The Linn just sounded so completely free of harshness and distortion by comparison.

There was also a much greater sense of rhythm from the vinyl. It actually sounded all over the place. Yet oddly realistic and very easy to follow. One moment the organist sounded slow and ponderous and really dragging his heels. Get on with it, man! The next he was galloping across the downs with a following gale and the hounds of the Baskervilles in tow! Both tempos sounded peculiarly correct for their own particular theme. I was able to follow the inner detail of the musical strands and stops far more easily on the record than on my CD player. One could flick one's attention from one set of pipes to another without ever losing the overall plot. CD draws attention to itself like painting by numbers. Listen to this! Then that. Now back to this! Repeat as necessary. Haven't they heard of organic music?

Warning: The enlargement of all the LP cover images is up around 300kB to give a real feeling for the sheer scale of vinyl LP sleeves and their superb artwork.

Swell actually sounded (strangely) like swell. Though I could not easily explain why in simple terms. I had almost forgotten about swell on CD. On vinyl it became not merely louder but suddenly more insistent and far more expressive. On CD swell just sounds as if the volume has been turned up on the amplifier for a moment and may be safely ignored at will. That is if you manage to notice it at all. The character of the music does not suddenly change on CD. I tried the only other example of the Westminster Carillon I had in my CD collection. (on a DG Gold CD) This was played on a French organ and far too fierce to enjoy at the serious levels at which I was playing the record. At that point my wife asked why the CD was so horribly distorted compared with the LP. Always a good sign as I greatly respect her hearing acuity. Far more than I do my own in fact. Pardon? I then had to put on a Felix Hell CD to ensure the CD player wasn't actually broken. It seemed not. Though the sound quality wasn't remotely up to the Linn level of performance by several leagues.

Of course there was always a huge difference in the price between even a modest Linn LP12 and a CD63SE. The Denon DL304 moving coil cartridge alone probably cost as much as the once-popular Marantz CD player when both were common Hifi fodder at the dealers. If I wasn't very afraid that all CD players sound exactly the same in A/B/X blind listening tests I would probably move up market in the CD player department. Possibly one of the new Regas? Nah. Upgrading digital sources is a mug's game. Isn't it? Naim CD Players are now right outside my, deliberately-self-limiting, pocket money allowance. Particularly since Naim sold out to scary greed and started catering exclusively to oil sheiks, investment bankers, pop sluts and overnight software billionaires.

You can skip the next bit if you have a weak audio stomach:

If one invests heavily in Hifi one's expectations rise to match the expenditure. One subconsciously demands that the perceived performance matches the price paid. Value for your hard earned money is required. It's just the way you are brought up and largely unavoidable these days. So, instead of listening to the music you listen to your system. Inevitably criticising every failure to provide intense pleasure at every listening session. You expect a new dose of noradrenaline from the shrill treble. Or preferably dopamine washing around your brain's reward centres just for buying your latest kit. But the chemical bath soon wears off and you find yourself back at your friendly, smiling (dope) dealers (again). Or searching the online ads for even more high-end Hifi equipment instead of looking for more music to enjoy. This road leads to misery.

As I eventually discovered in my youth. (I use the term loosely as I was late developer in the Hifi wisdom department) I found that a combination of DIY and limiting myself to a level of equipment which I could still enjoy finally worked for me. My system stabilised. One might even suggest that it stagnated. But I didn't care. I was too busy listening to the music rather than the system. I was in the enviable position that I had a system at home which was hard to beat by anything on my annual visits to a large local hifi show. Or on overhearing someone else having a demo at the larger dealers. As I pottered about wearing out the carpets with no real desire to spend any money. Rather like a masochistic, ex-alcoholic who visits pubs and bars just to punish himself.

I also gave away my huge stack of hifi magazines to avoid temptation. Peer pressure is an important factor in many hifi purchases. As witnessed by the heavy personal advertising on most audio forums. Reinforced by the constant congratulations as yet another punter succumbs to the persuasive charms of fellow members cajoling. Total expenditure usually far exceeds the original intentions. It's that voluntary "word of mouth" hard selling which the manufacturers must absolutely love! Perhaps A/B/X listening tests should be made compulsory in schools to bring back some balance of objectivity? Articulate and flowery descriptive English may be a little too much to hope for these days.

My ultimate test of system sound quality is a compelling desire to buy CDs, DVDs or vinyl. It works for me every time. It used to be said that leaving a system switched on made the sound quality better. I dismissed this notion as male cow droppings for literally ages. I never left my Naim amps switched on as recommended by anyone who has edver heard of Naim. It went against the grain to waste electricity on sheer hype. Until I broke the habit and left the amps on quite deliberately for a week. Suddenly I wanted to buy music again. Nothing else had changed but the music suddenly sounded like a good wine was flowing from the speakers. Satisfied that I'd completely fallen off my trolley I then switched off the amps again. There's always a good helping of sado-masochism in every hifi buff, including myself. Only the next day I switched back on and the system had lost its magic again. Inexplicable and still no reason to add to the burden of global warming. The sound quality was still pleasant enough to enjoy. I still played just as much music as before. But something was definitely missing.

But I have digressed exceedingly from the plot: There are still masses of CDs about compared with the sudden lack of vinyl when CDs were being over-hyped by the over-eager, obscenely-greedy, record company barons. So I am very unlikely to run out of the shiny little disks before I grow too old and deaf to enjoy music as much as I still do. That said, I must try and find some more organ vinyl. There is something oddly alluring about the sound. One cannot so easily have vinyl playing as background music even to serial blogging and writing endless nonsense on the forums. The music nags at the edges of one's consciousness until one stops typing to actually listen attentively. Vinyl is like a multi-barbed fishing hook and it is all easy to be caught up in the actual performance. Inexplicable, but true. Your mileage may vary.

Vinyl has many physical challenges though. Surface noise, clicks and crackles and sensitivity to footfalls to name a few. There is almost always some record warp or even occasional wow. Is it all really worth it? What about the trials and tribulations of turntable set up? The not knowing if the journey back from the dealers had undone their expensive sorcery. What about the great Sondek dust attraktor itself? Was it worth the effort in struggling against the dark energy physics of it all? They were practically throwing LP12s together at one point despite the high price in comparison with other turntables. I spent many hours sandpapering the grooves in my brand new Afrormosia plinth the day after purchase. Just to remove the hideous tangle of sharp, jutting fibres from the machine slotting. Ugliness which had managed to pass through several stages of construction and quality control without anyone noticing. Nor caring that the woodworking department were using stone axes to carve Linn's favourite (and expensive) product from recycled, industrial, hardwood pallets.

The drive motor has made a racket from birth and continues to do so despite several visits to the dealers for upgrading and set-up. The replacement "improved" inner platter and "better" bearing produced a 1/8" rise and fall at the edge of the original outer platter!! That is until I added packing tape to lift the lower edge of the inner platter step to true the outer platter's wild gyrations to a visually acceptable level. When I had the Ittok 8 fitted the dealer re-used my old Basik base. Producing a misnamed hybrid he probably hoped I wouldn't notice. I was buying Naim pre-power amps and the Denon DL304 cartridge at the same time so respect for the customer at this nationally admired Naim dealership must be inversely proportional to the overall expenditure. Perhaps it was because I didn't buy the Hicap power supply at the same time and upset their entrepreneurial sensibilities? Whatever.

But even such a turntable has its compensations. What about the creaking of the owner's knees at the ceremonial kneeling before the fabled, age-darkened hardwood and pitted, pewter grey relic? The dark offerings to be laid on the sacred, static laden, Linn felt mat. Which lifts without fail with every single record removed from the turntable. The well-rehearsed genuflection of sweeping a line of dust with a vastly overpriced, velvet plush device into a radial bar ready for collection by the naked stylus. The tension-filled moment of dropping the arm, by the silly little lever, suspended on an improbable sculpture of wobbly, Scottish jelly. The cartridge adopting its constant slide to the right as it descends the rubber ski ramp provided by the same lift-lower device no matter however dressed in a "downhill" direction towards the centre of the record. So that the user becomes ever more accustomed to using a virtual 6th finger to lower the stylus effortlessly onto the run-in grooves regardless of one's level of intoxication. The tiny diamond descending from high orbit into a ploughed field full of turnips, boulders and pockets of damp topsoil. Intermingled with puddles from the antistatic, holy water dispenser similarly priced per gallon to designer cologne. All generously mixed with well-felted fluff consisting of dead bits of human anatomy, dog and cat hairs and carpet fibres. This holds true whether you are an alien, own a dog, a cat or even a carpet.

Try doing all this after a few glasses of wine or real ale. Man is truly gifted in the hand-to-eye coordination department when it really matters. He couldn't catch a ball to save his life but can play a priceless collector's LP on a spring suspended, manual turntable with a crappy, lift-lower device. And do so long after exceeding the drink drive limit. Performing chimps don't even come close! Do they even like music? And no, I'm not a drunk. Merely remarking on several decades of experience with the damned thing.

All is vanity! The LP12 after repolishing the platter in the lathe. The lacquer was incredibly tough and far thicker than I had imagined. I began to make progress with 500 emery paper and 100 rpm and then moved onto finer grades backed up with odd bits of wood as the metal became more exposed. I finished off with sponge backed 3M Superfine at 425 rpm half an hour later. The shine is an illusion. The surface is still quite pitted but far more acceptable at arm's length than the hideous, leaden grey, blotchy mess it was before. There are dire warnings from Linn and others not to polish the platter. If, however, the platter metal corrodes over time despite the industrial-strength, Scottish lacquer then something must be done to maintain ones sense of visual decorum.

I really would not recommend hand rubbing with fine emery paper to remove the lacquer. Obtaining an even finish would be very difficult indeed. I used quite a large Colchester lathe with the external jaws of the large 3- jaw chuck expanded very gently just to maintain a grip on the inner edge of the hole in the platter and checked after each run that the platter was still secure. Marking or distortion of the metal is to be avoided at all costs or you might as well throw your platter away! If you botch the fixing on the chuck jaws then the lathe may throw the platter away for you! Approach platter repolishing with extreme caution! Under no circumstances must you try to cut any metal away! A skimming cut might be much quicker than sanding but you may/will end up with a very expensive, very wobbly flywheel on your record playing engine!

I should add here that my vinyl organ music collection grows constantly. I am ruthless in discarding scratched specimens before purchase and seem to have been quite lucky with surface noise. A tiny baroque organ with 8 stops of none greater than 8 feet and no pedal playing Buxtehude can magically capture the period. I wouldn't normally bother to listen to such a thing on CD. It would be stripped of anything worth having. On vinyl the purity, the chiffing of the pipes and the patina of age would be faithfully reproduced. Even if the subwoofer was made temporarily redundant except for action noises. I have been abe to collect some LPs recording very old organs. Many are German or Danish examples. They may lack the thunder of a great British Cathedral organ but still manage their task of distiling the wonder of music into a believable form.

Click on any image for an enlarged version. Back click to return to the blog.
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The experience economy?

2007-04-06T23:30:00.000+02:00

I was listening to the Danish radio as experts discussed the next stage of human "progress" after the present information economy. Many people now spend their entire working lives staring at a computer screen and tapping on a keyboard in air-conditioned comfort. This is completely unlike the behaviour of most normal human beings in the past. Many of these "caged rats" are beginning to seek wildly different or extreme experiences to contrast with the total lack of stimulation in their daily working lives. Rather than lie on a beach sunbathing themselves to achieve second degree burns they jet away to more exotic locations. Hoping to discover maximum contrast with their everyday lives of monotony and lack of physical activity. Some seek ways to experience extreme levels of physical and mental stimulation. White water rafting, paint ball battles, war-gaming, sky diving, bungee jumping, car or motorcycle racing, mountain climbing or other extreme sports are tried to experience very high levels of perceived danger. Hopefully without actually killing themselves. The dangers are obviously psychological rather than real based on the very high survival rates.

Office work is obviously a very poor environment for those needing normal levels of stimulation previously provided by a combination of physical and mental exercise. No doubt there's an element of this in increasingly aggressive driving behaviour. The more comfortable and quiet the car the more distant from reality the driver feels in their cosy, yet indestructible, womb. The owners of such vehicles obviously feel they need to drive aggressively. Or much too fast. Just to lift their stimulation level within their mobile, sensory deprivation tanks.

As I listened to the radio programme I suddenly realised that many of us within the Hifi/AV/HT hobby had been actively seeking extreme experiences for years. We use our equipment to experience an alternative reality as close as possible to the original. By playing a DVD on our equipment we try to experience the full force of the sound and visual effects the director intended. We become one with the characters on the screen. Actively participating in their fights, their wars, their car chases and other violent activities. The more realistic our experiences the more we enjoy the film. It's all a far cry from the tiny 14" B&W TV with a small built-in elliptical speaker which managed to terrify us as kids. I wonder if our natural responses to stimuli have weakened over the years? We do sit rather close to our computer screens, don't we?

Now we aim for the largest screen we can fit, or afford, into our viewing room to increase the sense of reality. This is a comparatively recent phenomenon as plasma and LCD screens became generally available. The CRT was inevitably limited in size by the sheer weight and difficulty of construction of ever larger screens. Many AV fans now push the limits even further. Using projectors which allow huge screens to be enjoyed. Further extending the sense of reality provided the image is sharp enough. We try to spread the image as wide and high as possible to match our normal wrap-around view of real life as seen with our stereoscopic vision. We are obviously trying to emulate our normal field of view just as they do in commercial cinemas. Thankfully our vision works best under these circumstances by flicking between points of concentration. Just as we do in real life. The large field of vision allows us to experience the closest approach to the visual alternative reality being projected there. HD is a recent improvement providing increased sharpness of image with a much greater depth of field. A moving photograph the size of a wall seems like true immersion but sadly we must wait just a little longer for true 3D.

Having fulfilled our visual stimulation needs we add our surround sound systems to immerse ourselves in a realistic sound field. As we try to match what we hear to the events which occur on the screen. Despite our best (and often expensive) efforts by surrounding ourselves with umpteen speakers both sound and vision are still large scale models of the real thing. Which is probably just as well. Or we would be literally deafened within a very short time of being placed in a fully realistic battle scene or gunfight. It is not cowardice which puts ear defenders on those practising weapons training. It is a firm grasp of the dangers of real gunfire to the unprotected ears. Imagine how loud a real explosion is when experienced at close quarters! You wouldn't want to experience this or your ears would be bleeding and you would hear no more!

Despite the obvious dangers of very high sound levels we seem to be pushing the boundaries ever closer to reality. Multiple power amplifiers and huge speaker arrays give us the clean, undistorted headroom which would have been impossible before the age of the affordable power transistor. Particularly when Chinese mass sproduction have made them even more affordable in the last few years. Now, of course, we have our powerful subwoofers to provide a believable foundation of bass to the film's sound track via LFE . (Low Frequency Effects)

Many consider that thunderously loud bass is good enough and cheerfully ignore all sense of reality. It seems that a really loud rumble is all that is needed for many HT fans. Yet bass has many nuances and variations in real life. Even at very low frequencies sounds start and stop quite differently. It is how we can easily differentiate between a bomb and a gunshot. A volcano erupting and a wave crashing on the shore. A car in a violent collision and a passing train. All have their unique qualities of deep sounds even if we have never actually heard them in real life.

Harmonics matching the original sound are vitally important to give a sense of reality to the sound effects. False harmonics give a totally unrealistic sound effect and can become extremely monotonous. Many cheaper subwoofers make every bass effect sound exactly the same as the last. And the next. Attendances at AV shows suggest a cardboard box being beaten with a stick is the rule of the day with many smaller subwoofers. It doesn't matter whether a gun is fired, a karate kick is landed or a car door is slammed on-screen. It all sounds like a box (US carton?) being hit with a stick. Every, single, time.

Then we move onto the more upmarket subwoofers with greater power and usually much greater size. Getting these past one's partner is a trick which not all can accomplish. A partner may not bat an eyelid at a new wardrobe, cupboard, display cabinet, room divider, dining table or sideboard. But will faint at the prospect of a modestly large subwoofer entering their living quarters.

It is here that the True IB Subwoofer can fulfill a range of needs for the embattled HT/AV fan. The IB can be accommodated completely out of sight as a compact manifold behind a small grill of cloth screen in a partition wall. Or fitted under the floor. Or even above the ceiling. Whichever option best suits one's situation and one's partner's domestic sensibilities. Who is allowed to dominate the appearance of the nest when both are out all day at work and usually fast asleep when actually "at home" and not watching TV? Where does a modest grill (to hide an IB) fit into this common scenario? The nest has become a display cabinet instead of a place of rest and shelter.

While perfectly able to play very loudly on demand the True Infinite Baffle subwoofer does so with a greater sense of realism than many (or all?) other subwoofers. The output level can easily be adjusted to match the moment and the mood without loss of impact or detail. I have found that the higher the quality of the bass from a subwoofer the more easily the level can be adjusted up or down relative to the speakers. There is no monotony as the IB reproduces the sound with great fidelity at whatever level is set. There is no sense of overblown bass rumbling away when none is required. Which is a distinct weakness of many smaller subwoofers.

Even at low listening levels almost any subwoofer earns its keep. Not being pushed outside its operating envelope helps as harmonic distortion and compression are very much reduced. The IB suffers from none of these weakness but still manages to sound far more realistic even when playing quietly. There is a noticeably increased depth and realism to the sound denied to most other subwoofers. My big SVS cylinder is easily able to reproduce sounds down to well below my audible cut off point of approximately 22Hz. Below this barrier the infrasonics take over to flex their silent but powerful muscles. At low levels, harmonic distortion from the SVS should be very low indeed. Yet the cylinder still sounds shut in, congested, woolly and monotonous in comparison with the IB. The nice thing about this is that a modest IB will totally outperform the large box or cylinder which your partner detests. There's some leverage here for those who do not command total respect in their own home.
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Good grief! The Firefox civilised English spelling checker is working again! It seems to come and go at random. I'm quite lost for words sometimes. ;-)
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Garbage in = garbage out?

2007-04-05T12:06:00.001+02:00

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Things were stacking up in favour of another source upgrade. I was becoming increasingly irritated by my NAD T533's inability to play hired Sony DVDs. My old Marantz CD63SE also had a question mark hanging over its head as to its musical sound quality in comparison with more modern players. I was quite tempted by the hype surrounding the new Regas. I had also been shocked to be reminded how very much better vinyl sounds compared with CD via my existing disk playing equipment. I had enjoyed music CDs on the NAD DVDP for a while but had drifted back to the Marantz for a little more excitement. The NAD was silky smooth but seemed to flatten the music into a politeness of respectable glances from behind modest fan waving. Whereas the CD63SE was much more obviously grainy but injected a bit more life into all the music it played. Neither box could hold a candle to my Linn LP12 on sound quality. This, despite the obvious difficulties of playing records with my rack sitting on such a flexible floor. Just in case you are wondering I do know how it is meant to be done.

Buying a new CDP and DVDP with Blue Ray beckoning frantically from the wings could have been quite an expensive upgrade if housed in several packages. I'm not against spending money but hate having to judge whether heavy investment in yet more empty packaging makes the boxes they contain worthwhile. The music is the thing. Listening to music is supposed to be simply about enjoying music. The same goes for films. Film watching is supposed to be about enjoying films for their own sake. Not judging blacks, whites and everything else in between. If it looks right, it is right.

Equipment upgrades are terrible time-wasters because they get in the way of continuing enjoyment. Learning the foibles of a new box and in particular a new and ever more complex remote control is never a pleasant experience. It's not that I'm a beginner at this game. Just that I hate the over-engineered solutions to simple tasks. It's as if everything must be made as complicated as possible to maximise perceived VFM. Often this is reflected in a remote control which requires several terms of evening classes just to obtain every possible function without needing the follow-up course of Post Traumatic Stress therapy. If I needed more exercise it would be far cheaper to invest in more weights. Not in more expensive hifi equipment!

Assembly of an AV system requires perfect balance in all things. Upgrade the amplifiers and you are looking for trouble if the source is exposed as a can of worms. Upgrade only the source and the amps probably aren't transparent enough to expose the investment. (or so "they" say) Speakers are the worst thing to change. Everything they play can only ever be the sum of what the poor music signal went through before it finally reached the driver terminals. Are you really feeling that lucky? ;-)

I had already been enjoying a few FTA HDTV channels on my relatively new Fortec Passion HD satellite receiver. I'd heard about upscaling of ordinary DVDs by HD players. I'd looked at Blue Ray players and watched prices fall slowly then more quickly. And then rise again just as rapidly amongst the "heavyweight" [sic] players!

Many new films have dropped rapidly in price only a few months after release onto DVD. Should I go on buying SD DVDs at such low prices (even compared with hiring) if they would soon become as "old fashioned" as vinyl or even 78rpm records? There was no sign of Blue Ray discs arriving any time soon at my local hire shops. Meanwhile there was little evidence of HD films falling in price at the usual chain outlets. It seemed as if my own collection of favourites was simply being regurgitated onto Blue Ray disks. The problem is that they are still selling at brand new "blockbuster" DVD prices. When exactly the same SD DVDs had already dropped to bargain basement prices within a couple of months of release. Blue Ray is going to have to reinvent its pricing if they are to become really popular. If the Chinese start building "supermarket" models of the Blue Ray players then even one older Blue Ray film will be more expensive than their DVD players at current prices!

I had almost convinced myself that I needed a new DVD player. It had better play CDs well and it really it ought to manage Blue Ray discs to warrant the large potential investment involved. I was quite taken by a minimalist Samsung at the bottom of the price scale. I had looked at it and the amazing pictures it produced in various electronics dealer chains.

So there we were sitting in the car in a nearby city. I was so close to talking myself into the Samsung as a Christmas present to us both. . My wife likes films as much as I do provided there is plenty of deep bass and "things blowing up". The dealer was only one hundred yards away and still open for the late Christmas "rush" "Let's try another shop," she suggested. So we did. And another. And another.

In the end we came home with a Pioneer Blue Ray player from yet another dealer. We bought a couple of the latest action films in Blue Ray format too. Ouch!

While my wife cooked dinner I had quickly swapped my high quality Component cables over from my NAD and removed it from the rack to make room for the new player. The size and weight difference were not just my imagination. There was only just enough room for the "double height" LX70A to replace the NAD T533.

Finally we could sit down in great expectation to enjoy a film on our new toy. But! For the umpteenth time I could not even get a brand new, Blue Ray version of "Hancockup" to play! Aaaarrrggghhh! The new player just sat there ignoring all attempts to make it play. I'd twice hired this film as an SD DVD but could not get the NAD to play it either. Not even a menu! Ironically, the hired "Hancockup" DVD would play without a hitch on my computer. Though I never watch films this way. Nor incidentally, do I ever copy films. But don't tell Sony this. I'd hate to spoil their paranoia for their bottom line as they leave their oversized, fossil footprints all over the global consumer electronics market. It gives me some delight to discover that they are in trouble following the global economic nosedive. They might even have to reduce the price of Blue Ray disks to stimulate sales now that they have killed of HD-DVD.

Meanwhile, back at the ranch:

We watched Dark Knight instead of keeping it as a Christmas Day treat. This action film has the most amazing sound track with lots of silent LFE which had the floor moving in slow waves beneath our feet. The rest of the LFE was pretty amazing too. The Joker carried the entire film on his own twisted shoulders despite the supposedly "star" supporting cast. Batman hardly made an impression except for the amazing sound effects accompanying his very unlikely fist fights. There is plenty to enjoy at a second showing so I don't feel too peeved at the high price of this film in Blue Ray. At nearly 2.30 long it still seemed to pack endless action for the viewer to enjoy. Definitely "state of the art" in the bass department. Particularly towards the end of the film.

Just look at all those sockets! The next morning I had downloaded new firmware for the player from Pioneer's excellent support website. A quick check found that "Hancockup" would play perfectly after a very long pause for "Loading". The player had read the latest downloaded 50mB firmware update flawlessly in just a few minutes. I had inserted a DVD-RW which I burnt to the Pioneer support website's instructions. Not quite full marks though because a USB stick software upgrade is much quicker and easier as per my Fortec Passion HD Satellite receiver.

I could have run one of my 7 metre long HDMI cables but decided to stay with my specially imported Mark Grant Component cables running to my JVC LCD screen for the moment. The more innocent amongst us should beware of coppercore, cable porn in the following link:

http://markgrantcables.co.uk/

Sunday morning (much like any other morning) demanded some organ music to test out the CD playing ability of the new Pioneer box. My favourite double CD of Franck's complete organ works started with plenty of clear insight into the great pipes. I tried switching back and forth between the Marantz and the Pioneer and was startled by the clarity and greater weight of the Pioneer. I didn't try matching levels exactly on an SPL meter but I believe the Pioneer is a little louder. The clarity of the pedal organ was stunning from the Pioneer with timbre, complexity, reverberation and weight all beautifully portrayed.

After listening to the Franck CDs all morning I think the dynamic range is also much greater with a more insistent and dynamic sound quality from the Pioneer. Rather like my Linn turntable the Pioneer demands one's attention instead of simply rendering endless muzak. It needs more careful adjustment of the volume control or it can easily become too loud. No real complaints there apart from finding my way around the rather "busy" Pioneer zapper and the slight delay between a button press and a resulting action.

What is remarkable about the Pioneer is the way almost subliminally deep notes can be heard so clearly. It plays tunes well down into the infrasonic with much greater clarity than the Marantz. Sometimes Franck drags a very deep drone anchor along beneath the music. Or plays very quietly in the lowest audible octave. The Pioneer makes these effects work beautifully instead of losing them beneath the 'noise floor' of the rest of the organ. Anything which makes the brain/senses work harder just to be able to hear some detail clearly is bad news in my book. Because it distances the listener from the music by dragging the attention away from the whole work and its performance.

A change of theme and I played Acoustic Guitar's "Arabesque" (Stunt records STUCD 02062) and was shocked at how realistic and musical it all sounded. With the imagery spread naturally across the sound stage there was none of that "singularity" where the player's image has no dimension in space. I am now listening to Garbarek's "Officium" with the Hilliard Ensemble. There is a new and remarkable subtlety to the playing with lots of what I call "micro-dynamics". The small but vital differences in loudness which can make reproduced music seem so exquisitely real. When the music swells it does not merely become louder but is more emotional and insistent. So far I am very pleased with my purchase. Celtic Circle 1 is playing now with superb results. Individual instruments seem to be more exposed in the mix, drawing attention to themselves for a moment. Rather than merely adding themselves to a whole, flat, musical picture. Voices are more realistic, articulate and emotional. The music seems to be giving them more room to express their artistry and feelings about their music. It all seems to make much more sense.

Constant differences in level compared with the Marantz are the most obvious changes. I have listened to the Celtic Circle series so many times yet here I am being repeatedly surprised. There seem to be more vocalists and instrumentalists on many tracks than I remember. Which has got to be good value as I have already paid for them. I keep wanting to stop typing this garbage and go and sit down in the hot seat. Loreena McKennitt is clearly portrayed as the mistress of breath control, purity of tone and exquisite expression. Yet her songs on the Celtic Circle disks aren't a patch on her original CDs. "The Book of Secrets" comes next in my listening order of CDs for today. But first I must shiver my way through the solo female vocal of Clannad's "Theme from Harry's Game."

This machine spends some time checking a disk before it decides it is ready to give its best shot. There are strange whirring, clicking and "computer effects" noises and gentle bleeps before it begins to play. Disks also come out feeling quite warm. Not excessively so and certainly not much warmer than the NAD. The Pioneer weighs a great deal and claims all sorts of technical details to maximise its performance. I think they have succeeded rather well going on today's experience of listening to my favourite music CDs. If I had an HD capable AVR I might wring yet more from its complex innards. Despite having only a hybrid stereo/IB system I feel no loss of sound quality based on the level of performance I am currently enjoying straight out of the box. I have had to turn the bass down on the IB's EP2500 amplifier to get a much better perceived balance compared with the Marantz.

Yeah, right! :-)

(Loud screech of tyres as brakes are applied with a leaden foot!)

Another day and lot's more time to play with my toys in the cold light of day:

I have now listened to three CD versions of Clannad's "Theme from Harry's Game" repeatedly over several hours. Thanks goodness it's a pleasant listen! I tried every volume level up to seriously loud. All the while swapping discs around and listening attentively as I switched back and forth between different sources on the Naim preamp's selector.

I can now conclude that the difference between the CD disks themselves is greater than that between my old Marantz 63SE and the new Pioneer LX70A on analogue stereo. Don't worry dear subjectivists: Both players were using exactly the same quality, hand made cables from the same small British manufacturer. Each time I moved the disks between the players I found I preferred one player over another and so did my wife.

Clannad's 'electric folk' " Harry's Game" track is full of potential as a test subject since it is has a very exposed, very pure, solo female vocal lead, further backing voices and a large and rather brightly recorded (probably multi-tracked) mixed chorus. Plus lots of electronic music, breathing, echo and lip sounds. The "Celtic Circle" version pleased least with an irritating, almost buzzy, emphasis in the treble on the chorus. I preferred a remastered "Best of Clannad" over the original CD album and vice versa. Depending entirely on which player the disks sat in. I am not remotely bold enough to assume that even these findings holds true in a blind A/B/X setting. Though I had switched back and forth so many times it really was a blind test for us both by now.

The overall conclusion is that there is no real sound quality differences between my players on analogue CD replay. A quick listening test using my computer DVD drive as a source on the end of my dirt cheap 10 meter long, twin, analogue REW test cable was also inconclusive. It sounded just fine. Yet, despite this daunting setback for subjectivists with bottomless pockets the world continues to turn. Perhaps I had better drag the NAD T533 DVDP back in for a re-examination of its previously suspect SQ? Do you really think it would help matters now?

I am not greatly disappointed to have a shiny black and Blue Ray player sitting in my rack which seems to perfectly matches my now ageing Marantz on SQ. Though I suppose I ought to have been a bit peeved considering the hype surrounding so-called "high end" players. This was an expensive box at full retail price earlier in the year. I note the replacement models are very much cheaper at around half the price. Though Denon (and others) still have a range of HD players climbing into the stratosphere.

I half expected that there would be little or no difference between CD sources going on previously published blind listening tests. Though I very obviously allowed myself to be completely swayed by New Box Syndrome (or New BS if you prefer) in the diatribe above. I paid a very reasonable ex-demo price for a heavy and impressive box which solved my immediate needs for an HD player and a DVD player which could manage hired Sony films. That it doesn't shoot itself in the foot on CD replay is merely a bonus. There are "advanced" digital audio replay options which might excite those given to studying specification lists but these are denied me by my entirely analogue amplification. Would an expensive, stand-alone CD player have beaten the old Marantz or my shiny new Pioneer? Are you the owner of a pair of the fabled "golden" ears of magazine reviewers? Or just given to making very wild guesses? You pays your money and makes your choice, I'm afraid.

I could have been enjoying music instead of allowing my latest empty packaging to interfere in my usual listening habits. The music is the thing. Not what's in the box. Perhaps owning nice equipment is largely a status symbol? Whether any of it brings real improvements to the music itself seems not to unduly vex a vast global industry. Nor does it trouble its endlessly gullible customers.

Perhaps the psychological pleasure of equipment ownership is all that really matters in the end? The equipment nicely augments the personal pleasure of enjoying reproduced music. Even if it does so almost entirely by proxy. No wonder they can't measure any real difference between the pretty boxes or the silly cables. There (probably) isn't any difference. Except for the name, model number and (oh dear) the price tag. :-)

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A new thin speaker.

2007-04-04T09:39:00.000+02:00

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The BBC website has announced a new ultra-thin speaker consisting of several thin laminations. This idea reminds me of a science fiction short story I wrote back in the 70s. Where I suggested that speakers would be built which worked on the same principle as a thin film capacitor but spread out flat and glued to the wall just like ordinary wallpaper.

With more recent discoveries I was assuming that a sound emitting diode would be invented. The idea in both cases is to provide much larger driver area to move the required volume of air. As driver area increases so excursion drops for the same output at the same frequency. As frequency drops so must excursion increase dramatically. A limit is reached around 10Hz beyond which it becomes very difficult to effectively couple conventional drivers to the air. An impedance mismatch makes ULFs very difficult to reproduce with any degree of efficiency.

The Thigpen rotary 'fan' subwoofer was a brilliant attempt to overcome the difficulties of reproducing anything below 10Hz. It has not achieved widespread adpoption for a number of reasons. Its very high installed price is one of the most obvious problems. Though unwanted noise seems to be an issue requiring heavy damping in a plenum before exposure of its output to the listening room itself. I am surprised that no attempt has been made to provide a cowl to direct the air without turbulence through the panel aperture surrounding the fan itself.

It may be that this new ultra-thin speaker will be just as incapable of reproducing deep bass as the previous NXT panels. Perhaps it is as well that most of us have no capacity for reproducing sub-10Hz frequencies anyway. ULF infrasonics carry well and might cause all sorts of problems for neighbours.

Flat-Panel Stereo Speakers - Popular Mechanics

The new speaker claims a good spread of sound and lack of point source but the wording of the BBC article is extremely confusing. I am not aware of having been "blasted" at any time in my long audio journey. Not even by early horn tweeters.

BBC NEWS | Technology | Thin speaker offers 'crisp sound'

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Music of the wards

2007-04-03T10:44:00.001+02:00

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I was listening to a fascinating account on the radio about the introduction of background music to a large Danish hospital. The specialist/consultant/senior medical doctor described how music was being used in the post-operative recovery rooms. It had been discovered that patients responded well to low level (45dB) music without any obvious rhythms or any strident or dynamic qualities. The music sounded like the sort of thing stressed people would use to help them relax. Soft, meandering electronic sounds allowed the patient to build an oasis of calm in a sea of pain and confusion as they woke from surgery.

Headphones were too irritating and difficult to use in bed so ceiling speakers and pillows with hidden speakers were provided. The patient was offered a choice of music to match their own tastes. Though it was quickly found that most patients would refuse music if it was offered as an open option to having none at all. The were too many unknowns involved for them to make a choice. So the music was left running constantly and always gently and was appreciated by most patients.

Staff were also being trained to consider the patient by reducing noise levels. Nurses were played recordings of normal nursing activities which often involved loud discussions and clashes of noisy objects together! Or trolleys banging carelessly into swing doors. Such noises tended to escalate conversation levels just to be heard clearly above the background racket they themselves were making.

The machines, which are now an essential part of modern medicine, are built to low noise standards. But quickly accumulate to produce a cacophony when multiples of machines were brought together. As in intensive care. Gentle background music helped to mask the often-alien noises of all these machines. The doctor suggested that the difference of providing music could probably be measured in the reduction in the number of pain killers administered.

He suggested that one day an in-ear sound reproduction devices would become the norm. I imagine it would be even more useful than speakers since the devices would damp ambient noise rather like industrial ear plugs. Setting volume levels might be a problem unless these devices had automatic volume controls relying on feedback from the patient's own ears. The patient might often be quite unable to adjust volume levels themselves. Further adding to the torture of recovery from major surgery.

Hospital rooms are themselves often hard-surfaced and highly reflective. With very little natural sound damping. This tended to emphasise impacts and other irritating high frequency noises. Making it hard for patients to relax in such a tiring environment. While hygiene levels had to be maintained some extra sound damping is very desirable. Hard soled footwear is obviously very undesirable in these acoustically undamped environments. All medical staff wear soft, moulded rubber sandals. I know from my own limited experience that it is very difficult to hear the staff moving around normally in Danish hospitals.

Heart patients often have racing pulses and it was found that playing strongly rhythmic music helped to lower pulse rates. Presumably the patient locked onto the slower beat with beneficial results. I presume Metallica is not an option on the hospital sound system!

No doubt the use of music, as yet another tool in the armoury of the medical profession, is still in its infancy. The cost of the equipment to replay gentle soothing sounds at low levels is but a tiny fraction of the medical budget of any hospital. Its benefits are yet to be fully understood nor yet administered in the most efficacious ways to match the needs of all patients. No doubt progress will be made in this field of audio and medicine as in any other.

I rather liked the contrast with our own excitement seeking activities in HT and audio. Where maximum volume and extreme dynamics are the usual requirements. I wonder if there is room for selective infrasonics in medicinal music? Perhaps this is why I play organ music in the background while I'm scribbling online? I am building an oasis of calm on which to float my ideas about our very strange world. If nothing else, it helps to drown out the neighbour's clapped-out, motor-driven, garden machinery! My diagnosis suggests a massive dose of Jeff Beck's Guitar Workshop is indicated. Repeated as necessary! ;-)
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Pass the wrench.

2007-04-02T10:32:00.000+02:00

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While browsing around the countless audio websites I found a recommendation for a track which was considered perfect for matching subwoofer bass to the speakers.

http://www.youtube.com/watch?v=0ohz4x4HhwU

Just out of curiosity I connected my old 16-46 PCI/NSD SVS cylinder sub to the Front speaker output sockets on my Creative SB Live! sound card and switched on: No speakers. Just the sub. Normally the speakers dominate the reproduction but this time I wanted to compare how the SVS and the IB sounded when working alone on the same bassy track.

First I rattled most of the loose objects in the room with the SVS while reading an average of 85dB(C) on the Galaxy 140 SPL meter at the listening position. Then I swapped the cables over to the IB subwoofer via the Naim preamp and EP2500. I didn't bother to turn on the Naim speaker amp. Though I could just as easily have muted the Main speakers on the CX2310.

The difference in presentation from the IB was immediately apparent and really quite shocking. The percussive leading edge of each note played on the bass guitar was suddenly fully exposed by the IB. There was no hint of this from the SVS. At times one could (almost) describe the bass guitar on this track as a drum with multiple tones as the bass player plucked very near the bridge of his instrument. The reproduction was lighter on its feet with seemingly much more upper bass present. Yet the guitar and vocals I could clearly see on the monitor were completely absent. All I could hear was a realistic sounding bass guitar. The SVS had sounded very deep, soft, muffled and rumbly without any speakers to support it. The IB on its own was playing the same track at exactly the same level but seemingly reaching into a higher register.

The strange thing is that the IB has a 24dB/octave active crossover while the SVS uses only 12dB/octave on its low/line level connections. Both were set to 80Hz on their crossover's respective scales. There was no lack of powerful, deep bass from the IB but it sounded far more open and natural despite the absence of speakers. You wouldn't want to listen to the SVS for very long on its own since it sounds almost monotonically boring even when playing only a bass guitar. The IB sounded as it was playing full range but was obviously not. An unexpected result of this straight comparison.

I used to love the SVS when it was my only subwoofer. It sounded so detailed, powerful and expressive on both music and films. I even put a website together to try and explain its amazing performance. (now erased by Orange as it ate up earlier ISPs like a killer shark hunting baby seals) Now the IB leaves the SVS completely for dead on sound quality, accuracy and its incredible ability to reproduce a believable version of reality.

The percussive leading edges were finding some rattles around the IB manifold which were easily cured by re-tightening all the driver fixing screws with an Allen key/wrench. Almost all had loosened slightly and responded to a turn, or two, to snug them enough to grip the rubber driver seal without distortion. The nice thing about furniture screws is that they use a hex socket which is very safe in use. Even a clumsy mechanic would have a real struggle to poke an Allen key through a cone. He would also have to be truly brutal to mar the nice chromate finish on the large diameter heads of these screws. They hold their finish very well and still look like new.

It is odd how one's ears become educated over time as components arrive in one's system only to be replaced by others later on. My old, home-built pair of 6th order series, passive, bandpass subs sounded great to my ears. I used them to support Linn Kans for quite a number of years until I bought the SVS. Then suddenly they sounded woolly, muffled, soft and rumbly compared with the SVS. I would say on this particular test the IB is an order of magnitude better than the SVS. The SVS has no reason to continue its existence in our AV room but my wife likes it and won't let me sell it. This, despite it never being l played from one year to the next these days. It is so eclipsed by the IB that I would have no reason to listen to it except perhaps to supplement my silly little computer speakers. Though only when I can't be bothered to turn on the entire audio system to listen to one track I might find on the internet. Usually the full system is playing anyway while I browse, scribble my blogs and post my nonsense on the forums.

If it is of any interest the Galaxy SPL meter was reading around 100dB(C) at the IB manifold mouth for the 85dB(C) average at the listening position about 9 feet away. Meter settings were C-Slow and the top scale of 80-130dB. The SVS was reading several dB lower at the skirt at around 93-95dB(C) at 8 feet from the listening position for the same level as the IB. Perhaps I should also have read the SPLs coming from the port at the top but it would probably have been outside the bandwidth of the bass guitar.

Interestingly? My wife complained of the loud repetitive bass down in our lean-to greenhouse/ conservatory. Whether this was a result of the SVS playing at frequent intervals I have no idea. Normally she doesn't moan about the IB bass. For some reason the SVS carries more despite having a very similar response curve and shorter LF reach. It rolls off roughly 8Hz Hz higher than the IB. Perhaps being down firing it puts more energy into the rather flexible stage on which it rests? Though it made no audible difference to my ears to place it on a massive, round, concrete slab resting on carpet off-cuts for better isolation. So I got rid of the slab again.

The perceived difference in frequency range really demanded further examination with REW.

Here are the traces of IB and SVS superimposed using (all) Measured: I used the Galaxy 140 SPL meter at my normally seated ear height at the listening position as the test mike: I also ran a few sweeps on both subs to give a firm line to the graphs and to ensure no errors had crept into my testing methods. These were the settings I had lived with for quite a while. In case of doubt the SVS is the green line. The IB is the wider, purple curve.

The difference in upper frequency range is very obviously in favour of the IB. It is also more extended into the deep infrasonics. While the big SVS cylinder rolls off very rapidly below its nominal tuning point. Which is only to be expected from a reflex enclosure. The IB being a (large) sealed box follows a much more gentle slope.

The IB really was playing much higher in frequency than the SVS due to gross inaccuracy in the marked roll-off point on the CX2310 dial. This required the adjusting knob to be reset right down to an indicated 50Hz! The SVS needed to be raised just a little to 90Hz on the crossover dial to match the REW 80Hz roll-off curve. Unfortunately the blue line representing the roll-off curve on the REW graphs doesn't show on (all) Measured. The graph (below) shows the results of careful adjustment of both sub's responses to follow REW's indicated 80Hz roll-off curve as closely as possible. (even if it doesn't look like it!) :-)

In the next REW graph (below) the response levels have been adjusted on the graphs to match each other for a better response comparison over the major power band. There isn't any obvious meeting point between the two curves at higher frequencies. It is difficult to confirm whether there is any difference in the steepness of the upper roll-off curve of each sub. Comparing crossovers of 24dB/octave against 12dB/octave one might have expected the IB to take more of a nosedive with rising frequency.

I think the adjustments required adequately explain the not-inconsiderable, perceived difference in frequency range between the SVS and IB. I really wasn't being fair to the SVS on my listening tests on the bass guitar track. Since it rolled off so much earlier than the IB with the previous settings. Now I will have to do another listening test of that same track. I'll update when I have had a chance to do this properly tomorrow. A quick listen suggests the subs are much better matched now. i.e. both sound very deep, rather soft and rumbly!

BTW: All REW traces have had 1/3 octave smoothing applied. Calibration and test level was 75dB for both subs. With slight adjustments to levels applied later to the graphs shown above.

Update after new listening tests: Rain stopped play! After I managed only a couple of exchanges between the IB and the SVS a terrific thunderstorm passed over. The forecast was for warm sunshine all day! Grrr?

Initial impressions are that the situation has been reversed. With the SVS now having a wider bandwidth. At least that's how it sounds subjectively. The hard attack of the leading edges was almost absent from both subs now that they were similarly rolled off. I could hear strange singing in the background from the SVS but the IB remained totally silent to all but the bass guitar. The SVS port can be made to act as an organ pipe if fed suitable material. It is proving hard to kill the buzzes produced by the SVS in the objects beside which the cylinder stands. The IB excites no such unwanted effects on this track. I had a chance to run a couple of REW sweeps to confirm the rough match in their nominal 80Hz roll-offs. Nothing had changed overnight from the graphs which appear above. I shall investigate these subwoofer comparisons further when time allows.

2nd Update: I have had much more time to listen to the SVS and IB together and separately. I'm now running the SVS from the Subwoofer Out XLR on the back of the CX2310 active crossover. This gives me "remote control" over the roll-off point and gain from the listening position with the SVS' own crossover bypassed.

I have also run a number of REW tests on the effect of adding the SVS to the mix and played about with phase adjustments. I discovered that since the last changes to my IB it now works better when the phase is reversed to the speakers. I have also pushed my speakers back by a foot or so.

The SVS is certainly powerful low down but completely lacks the IB's clarity. On certain organ works (like the Franck double CD mentioned earlier) with the big pipes playing the SVS can be turned up far enough to make the room shake really violently. It is just that the SVS plays only loud tones. Without any indication of timbre or the interplay between the great pipes. There is just no suggestion (at all) of the pipe's character or how it actually speaks. I played the same little piece of a heavy pedal section over and over again. I tried with and without the SVS and on its own and at many different levels. Try as I might I simply cannot get the SVS to sound like an organ. It sounds more like a soulless tone generator. It cannot manage the complex waveforms at the low frequencies at which the IB excels. One particular big pipe had a hard edge on the IB but this went completely unnoticed on the big SVS cylinder. It is probably just as well there is no real comparison or the work involved in building and installing an IB would be wasted. It might be entertaining to try an SVS Ultra to see if it has what it takes to play like a real organ. Other IB Cult member's comments would suggest otherwise.

My latest listening tests have proved yet again that it is not enough to play deep tones very loudly. It must also be done with finesse and clarity if it is to mean anything at all. Those who enjoy organ music need have no doubt that the investment in time and effort to build an IB will be well rewarded.

Now I know that having another subwoofer in the room does not wreck the response curve I may finally go ahead and install four more drivers in two separate manifolds. My earlier tests seemed to suggest that a massive trough would be produced in the frequency response. Depending entirely on the second sub's separation from the IB. Reversing the phase of the SVS relative to the IB solved this problem to perfection resulting in a broad, flat response at the listening position when both subs are working. See: the Port open 180 curve below. This is with the SVS just inside the right speaker.

Click on any image for larger version. Back click to return to the text.

Pottering around amongst the You Tube videos I discovered a huge variation in SQ so it pays to choose tracks carefully. I have discovered videos of many of my favourites by Clannad and Loreena McKennitt. The latter has such perfect breath control and wonderful variations in her voice. I never tire of listening to her CDs. Her version of "Greensleeves" on the "The Visit" CD album is very emotive and powerful. Reminding me rather of Kate Bush's stunning rendition of "Women of Ireland" on the first of the Celtic Circle double CD series (on CD2). Sinéad O'Conner has similar abilities to sound all soft and "girly" one moment and hard edged the next. I am not a great fan of hers but do like some of her work.

Oh good! The Blogspot/Firefox (whatever) spelling checker has just switched itself back on again!

Just a reminder that in some browsers you can hold CTRL down and press + to make the page (and text) bigger. Pressing CTRL and - reduces it. CTRL and 0 returns the page to normal.
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Driver impedance plotted against frequency

2007-04-01T18:28:00.001+02:00

In discussion about IBs on the AVF subwoofer forum it was suggested that manifolds reduce the frequency of the natural (free air) resonance of the drivers. This was something I had not heard of before. The idea was that the mass of air in the manifold would load the drivers sufficiently to reduce Fs (which is measured in Hz). The immediate question was how one would go about measuring this change.

Small signal testing using REW with a series resistance in the speaker leads would provide an impedance curve for the individual drivers. The cable which normally connects the SPL meter to the sound card Line-in socket is connected across the speaker terminals. REW then reads impedance against frequency. The resonant frequency of the drivers should be clearly shown by a peak in the frequency plot.

I used a small audio amplifier to provide the test signal after removing all the heavy cables from the IB's EP2500 power amp to the drivers. The big amp was unnecessarily powerful for testing when a 30 watt audio amp would do the job nicely. Naturally I pulled the mains plug from the socket on the back of the Behringer amp to avoid short circuits between the naked ends of the IB's speaker cables. It easy, after making changes to the wiring, to switch everything back on without thinking. I am as prone as any other to being on auto pilot when it comes to my audio system.

The resulting curves dip downwards at driver resonance due to the polarity of my hasty wiring exercise. The impedance curves of the individual drivers were then superimposed using "Measured" in REW. It seems the new driver's individual Fs hovers around 13Hz. A trifle under the expected 15Hz but I am certainly not complaining.

Now onto the problem of measuring the reduction of Fs due to the manifold loading all of the drivers simultaneously. My fuzzy logic suggests that, individually, the drivers would not be loaded enough by air mass to notice my very large manifold even existed. So I connected all four drivers in parallel using lighting flex for a 1.8 Ohm (DC resistance) load. I wanted to see if there was any reduction of the combined Fs on a small signal. It seemed unlikely given the tiny driver excursions involved. After checking driver polarity with a 9 volt battery I used a 100 Ohms variable resistor in the positive leg of the cable from the speaker terminals of the audio amp. This was to ensure a clear impedance peak at the combined driver frequency resonance.

Another REW sweep and I had the answer. No obvious change in Fs due to all drivers being driven on a small signal.

The question is how any manifold air mass loading might take effect. Would it be linear and based on driver excursion? Does manifold size and opening affect the driver's Fs?

The curves above are interesting. Blue is the impedance curve of all drivers driven in parallel with the door out to the IB enclosure open. Green is with the door closed. The door is right beside the manifold so it short circuits the intended infinite baffle dimensions when open. The total distance from front to rear of the divers is no more than a few inches via the hinged side. These old glazed doors hang on the original antique hinges which makes for a 3/4" wide gap at the hinge when the door is open. When the opposing pressure waves and rarefactions meet they should cancel each other out below a frequency which depends on the distance they have had to travel. This roughly equates to the diameter of an open baffle of similar dimensions to the short circuit between front and rear of the driver cones.

Just out of curiosity I closed off the mouth of the manifold with a piece of plywood to see the effect on impedance. The remaining open slot of about one inch width seemed to do nothing serious to the shape of the (closed door) curve but slightly reduced its level. I had made no changes to the REW settings or the test amplifier during these last three sweeps. I have raised the curve by a couple of dB using REW's trace adjustment to match the level of the earlier curves to aid comparison. The new trace is the red curve.

I am no nearer to proving a reduction in Fs due to manifold loading but have learned a few more things about my IB system with these REW impedance tests. Not least where that 12Hz frequency response peak probably comes from. I have been trying to discover a dimension in my home which would boost 12hz by boundary gain. I even wondered whether the living room downstairs was acting as Helmholtz resonator using the open stairwell as a port. Imagining the nearby IB manifold was driving the system like a huge reflex enclosure. Rather like blowing over the open top of an empty bottle. It now seems there is a much more likely connection between the driver's natural resonance and the frequency peak at 12Hz. Suggesting that the drivers are not quite critically damped by the EP2500 at system resonance.

BUT! Let's not jump to any false conclusions. Having just re-read my earlier posts I realise that my 32Hz drivers also exhibited a 12Hz peak when boosted at 20hz with the BFD. So it must be a room effect after all. I shall have to close off the stairwell with large piece of ply to see if I can kill that peak if only to satisfy my curiosity. I do know that my AV room is rather prone to exaggerate diesel rumbles from the distant road and delivery vehicles in the area.

My gratitude goes to aktiondan (a member of the HT Shack) for the clever suggestion of impedance measurement using REW and a series resistance. The original HT Shack thread was back in June 17 '07. Dan's original posts are well worthy of careful study. A direct link appears below:

http://www.hometheatershack.com/forums/rew-forum/5216-room-eq-wizard-doubles-crossover-design-measurement-tool.html?highlight=impedance+measurement

If clicking on the graphs doesn't provide a large enough image to show the detail then try the CTRL+ trick to magnify the whole page. CTRL0 will return the page to normal. CTRL- will shrink the page. Back click to return to the text.
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Throttling the beast

2007-03-30T19:39:00.006+02:00

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I remain determined to find a way to narrow down the optimum size and opening of an IB manifold. Yet IBs seem so forgiving.

Here is an REW graph of multiple sweeps running up from 70 to 85dB(C) measured at the listening position. The manifold mouth was also progressively closed off to almost nothing with a piece of plywood:

I have moved the curve levels up and down using REW Trace Adjustment to match them all at the 12Hz peak. The variation of the curves is hardly worthy of discussion. There is no obvious clue in the frequency response to any of the changes in level or the size of the mouth opening. Which leaves testing for compression effects as the next step. To do this properly I shall need a far more sturdy shutter. I am already producing a loud rattle in the plywood mask at higher levels and would like to go to much higher SPLs.

More on this subject when I have the house to myself again tomorrow. I am presently enjoying my favourite Franck organ CD while dinner is prepared.

I thought I'd try again to narrow down where this 12Hz peak was coming from.

So I set up the Galaxy 140 and REW and took some more sweeps. I tried opening and closing the entrance hall door which is just below the stairwell. I tried blocking off the stairwell with some large pieces of thin plywood. Not a perfect seal but it should have killed any sharp port tuning effect. Finally I did a nearfield plot with the SPL meter resting on thick cushions. No changes were made to any settings during these tests.

I think it is obvious that the 12Hz peak is a room induced effect. The red nearfield curve shows only a hint of a wiggle at 10-11Hz. Look at that nice straight line of the nearfield plot! Amazing linearity before the 80Hz crossover starts to drag down the curve.

I didn't have time to prepare a really solid shutter to mask off the manifold for high SPL testing of possible compression effects.
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Gilding the lily

2007-03-29T14:32:00.040+02:00

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After months of procrastination and searching for new, potential IB sites in and around my AV room I have finally decided to try a tall, 8 x 15" driver manifold. This will be placed in the original IB enclosure. Four new and four old AEIB15s will be arranged in mechanical opposition to each other. The driver magnets will be brought inside the box to avoid the metalwork sweating in the rapidly rising temperatures which my unheated IB enclosure is subject to. The drivers will be wired to ensure all cones move towards the manifold centre simultaneously. This will make the open mouth of the manifold into one huge compression driver equivalent in power and area to all 8 x 15" drivers. The mouth of the manifold will closely match the total Sd for the eight drivers. (Sd = effective cone area for a single driver) Regardless of magnet position, opposing the drivers on each side of the box will kill the driver reaction forces which can otherwise cause severe structural vibration. All these features are standard design principles for a successful IB as recommended on the IB Cult forum. (and as per the truly encyclopaedic IB Cult FAQs)

The reason I haven't gone ahead with this rather obvious driver arrangement before now is the limited size of my IB enclosure. It started off at 600 cu.ft. for a total Vas multiplier of between 9 and 10 but the enclosure has become a rather handy storage area. Adding four more drivers means I am completely ignoring the total Vas multiplier rules. Even if the enclosure space were completely empty I would be down to somewhere around 4.5 as the total Vas multiplier. It is difficult to calculate the loss of volume to storage but it must have some effect. The upside is that the enclosure is very leaky and lined with at least 12" of Rockwool on the sloping surfaces. It will be fascinating to see whether the Fs of the drivers is raised by cramping their style in this way. Somehow I doubt it, but I am about to find out. IBs seem to be very forgiving in other aspects of their design so I am now probing how they cope with undersized enclosures.

The new and old AEIB15 drivers are very different from each other. The earlier drivers have an Fs of 32 Hz. Double that of the newer drivers. The old drivers have conical poly cones. While the new drivers have heavily flared paper cones The intention is to drive each set of four drivers with one channel of the EP2500. Using series-parallel wiring the old set will provide a 4 Ohm combined load. The newer drivers are 8 Ohms each and there is no way to wire four of them to achieve a 4 Ohm combined load. So they are very likely to end up as a nominal 8 Ohms. It will be interesting to see if their outputs can be matched using the crossover gain settings and/or the control knobs on the EP2500. If balancing the sets proves difficult it might be worthwhile wiring the newer drivers in parallel for a 2 Ohms load and greater output. This is a lower impedance than is desirable but there are members of the IB cult running their systems at 2 Ohms without obvious problems.

I would like to achieve a smooth response from 10Hz upwards. Hopefully the older drivers will provide just a little more oomph between 30 and 80Hz. Whether this is possible has yet to be discovered. It is usually a very bad idea to mix drivers in the same IB system. However I am pinning my hopes on isolating them on separate amplifier channels to minimise any problems. Normally it is difficult to share the loads equally between different kinds of drivers. So one driver might take more than its fair share of the load and be over-driven. While others just loaf along providing little output.

The new manifold will be approximately 6 feet high x 23" deep x 17" wide externally. This is the minimum box size which will hold all eight 15" drivers with as little space around them as possible but leaving room for "window" shelf braces between each pair. These will provide extra stiffness across the box where the major reactive loads are concentrated. Three inches of the 23" depth will be lost between the existing wall studs. The inner layer of the box is extended forwards for this purpose. The outer layer will abut against the wall surface on the enclosure side.

I have quite severe clearance problems out in the enclosure with the sloping ceiling and the adjacent double doors. This limits me to the space between the two wall studs where the original array and the present four driver manifold have previously been positioned. I will want to raise the mouth of the manifold as much as possible to match the 10" difference in height between the enclosure floor level and the stage indoors. This will help to maximise the area of the manifold opening. Naturally the box can only be raised until it touches the sloping ceiling and no more.

The manifold will consist of two layers of good quality 3/4" multi-layer plywood glued and screwed together for strength and stiffness. The open side of the box will face into the AV room as is usual with an IB.

Construction is well under way using some 3/4" flooring plywood which I just happened to have left over from building my workshop. I also recycled the original array baffle by trimming it slightly. Then I used it as the pattern to mark out the other side. It seemed a shame not to re-use the high quality plywood of the array baffle as it formed a perfect right hand side for the intended double height manifold. I had designed the array baffle to bring the drivers close enough together to avoid the top of the baffle hitting the sloping ceiling. I also used this panel to check clearances out in the IB enclosure. The missing triangle at the top (in the images) is where the original array baffle met the sloping ceiling. I shall have to glue on a triangle of similar plywood to make up the missing section.

Here's a quick mock up of two sides for scale. It's actually slightly taller than I am! Now I need more plywood to finish the job properly... The single thickness new side is in the foreground while the double side from the original, line array baffle is in the background. The visible rebate on the edge of the old baffle will have to be filled with more plywood to avoid having the back, top and bottom panels all of different widths.

At the timber merchant I was offered a choice between two qualities of similar plywood. One weighed probably twice as much as the other and the surfaces were of higher quality. The price was nearly the same so I took an 8' x 4' sheet of the heavier board. A quick prior check on paper proved that I could cut out two backs, the second layer of one side and three tops and bottoms from one sheet. I already had enough material for another top or bottom so I had just enough material for my full complement of pieces to make the double thickness, manifold box. Half an hour later I was back at home and had them all cut out with my cheapo circular saw. I don't do enough woodwork these days to warrant the investment in anything better. It gets the job done but the blade usually rusts between bouts of use. Quite honestly, I prefer a carpenter's, hard tooth, hand saw to the infernal racket of a circular saw!

Rain intervened before I could cut out the four driver circles for the last side panel. I am using an electric jigsaw for the circles instead of a router since I have no need to countersink the drivers. In fact I want to separate them as much as possible where the magnets come together. I have taken my own advice and narrowed the manifold very slightly to avoid struggling to fit this new box between the old and twisted wall studs. Doubling the plywood will also add enough extra width to afford extra clearance between the magnets. As this manifold is almost twice the height of the present one the risks of a tight fit are even greater.

I also used the original baffle array panel as a pattern to drill out the driver fixing holes in the other panels after clamping them together. I'll wait until tomorrow now to finish the cutting and sanding to avoid messing up my workshop with fine sawdust. The forecast is full sun tomorrow so I can work in comfort outside with plenty of room to manoeuvre.

Making progress. Another hasty mock-up clamped together and posed for the camera. The single thickness strip at the front of the side boards will fit between the wall studs. The double thickness edge will then seal against the wall surface leaving a neat edge.

All the parts are cut out but nothing is glued together as yet. However tempting it might be to glue it all together I'd have the problem of getting it upstairs into the IB enclosure. Being double the thickness and much taller than the last manifold the new box weighs considerably more. Without bracing shelves the bare box weighs 10 UK stone or 140lbs! That is much too heavy for me to lift comfortably. The eight 15" drivers will add a further 150lbs to the finished structure. Welcome to the 300lb IB gorilla!

Here's the new, nearside manifold panel being clamped up after gluing with PVA. You can never have enough clamps. Nor find your missing clamps when you need them! The cheapo sliding clamps with the red handles are actually very useful thanks of their deep jaws. G-cramps or C-clamps (depending on your geography) are rarely deep enough in the gape to clamp large panels effectively. Fortunately the driver cut-out holes allow greater flexibility in clamp positioning. The eagle-eyed might notice the work stands have been brought quite close together. This was to compensate for the bow in the new sheet of plywood. Sighting repeatedly along the edges of the panel as I clamped up ensured they were dead flat as the glue dried. The panels relaxed across the work benches until they were flat. I suppose I could have deliberately overdone it to ensure they dried dead straight but they look good enough. In theory one should laminate three pieces of ply together to ensure an odd number of laminations. Even numbers may tend to warp over time. Though I'm not paranoid enough to lose sleep over this.

Yet another mock-up. I wasn't certain how I wanted the box to appear from the outside. So I made the back, top and bottom panels over long. Now I think about it the box is so tall I shan't ever see the top. So the back panel should be full height for neatness and the top and bottom inset.

I still can't make my mind up whether I want the finished box to slide up a ladder and in through the window like the last one. It would be so much easier to finish the box outside, sand it and (perhaps) even give it a coat of paint. If I finish building it indoors I'm stuck with the finish I get. I can't tidy up the edges of the panels where they meet without making a terrible mess in the enclosure. I can use a block and tackle if need be to help the lift up the ladder. I just need to align a strong beam across the AV room doorway at the correct height for a nice, straight pull through the open window. The windows are nearly 4' square so there's plenty of room to get the box through the opening.

I should make it clear that I don't need any more drivers in my IB. The IB is already everything one could ask. It is just that the older drivers are sitting in their boxes and going to waste. Which seems like a perfect opportunity to test an IB's real life performance under abnormal enclosure volume limitations. The old AE drivers sounded fine when they were in use in the manifold. That is until the new drivers arrived and offered more bottom end output without EQ boost. My hope is that the extra cone area and displacement will add more of what I have already. The goal is increased dynamic range with even better realism. Not much to ask for the work involved in replacing the manifold. Cost to date is just a single extra sheet of 3/4" plywood. If the new manifold isn't an improvement then the old manifold only needs to be slid back into place and the newer drivers refitted.

Well, the old manifold has now been sacrificed to recycling though the extra panels do not appear in this image of the glued and screwed manifold. I intend to paint the new box but haven't a clue which colour to choose. The manifold is visible through the glazed doors to the enclosure and I became rather tired of the tatty appearance of the unpainted plywood of the last one. Black would be most appropriate in best "2001 monolith" style. Almost everything in my system is black already but I'm not sure I want this thing black.

Black it is. Hardwood kitchen worktop lacquer in black with a low sheen. Two coats applied by SWMBO. It looks quite like "Black Ash" speakers in the flesh. The picture really doesn't do it justice. It must be the odd, late evening sunlight. I might try photographing it again tomorrow before it is too late. The bracing shelves have been removed for lightness while I try to get get the box upstairs.

The "hole in the wall" has been enlarged to its former [vertical array] glory but I doubt I can manage a full six feet of mouth opening before the box hits the ceiling. The door frame has been beefed up with a new 4"x 2" solid timber stud thanks to the slightly slimmer manifold. It must have been 100 degrees out there in the enclosure in late afternoon sunshine! The fresh paint still smells a bit strong so the manifold is having a rest outside overnight. Tomorrow the big lift begins. The ladder route has been placed off-limits by SWMBO. So two triple pulleys and miles of rope will assist the manifold on its way upstairs. But first I must get the huge and heavy box to the bottom of the stairs!

This chapter is likely to be heavily modified and corrected as I progress. So you may like to reload the page occasionally to ensure you see the latest version.

Click on any image for a larger version. Back click to return to the text.
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The big lift

2007-03-28T19:34:00.016+02:00

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Getting the 140 lb manifold indoors and up the 60 degree stairs was interesting. I had a block and tackle set up for the lift but it had eaten up all the distance between the triple pulley blocks before the tension was taken up. So I grabbed the loop of rope which I had passed through the top driver cut-outs and lifted it up the stairs bodily with one hand. My wife was down below pushing for all she was worth despite her 5'1" and seven stone stature. We are both past our sixtieth birthday but remain fit thanks to constant physical activity and a healthy diet.

Here's the manifold waiting to be pushed on a sack truck to the house steps. After that it was a matter of walking it up some 2 x 4s into the entrance hall and along to the bottom of the stairs. This bit was easy enough but then it required a 90 degree roll to get it on its back up the steep and narrow stairs of our old cottage. Some large cardboard packaging was quickly tied to the huge box to protect the black ash finish.

Once upstairs I walked the manifold into the IB enclosure and pushed it straight into the empty space between the wall studs. Then I spent some time lifting and lowering it on various wooden blocks using a batten lever as I searched for the optimum height. In the end the sloping wall would not allow a full exposure of the manifold mouth and I lost 4" in all over the full potential height. A couple of hours later the manifold had been screwed to the wall studs and I had closed off the various empty spaces above and below the manifold with large bits of solid timber. Still some work to do tidying up the wall on both sides.

Manifold in place but no drivers yet.

The view of the manifold, with the drivers in place, seen through the old glazed doors from the hot seat

The view straight into the manifold with all drivers in place.

Here's comparison between the old (right) and new AEIB15 drivers.

Each vertical set of four drivers was wired in series-parallel using lighting flex to see if it had been worth all the effort. I checked the DC resistance of each set with a DMM to ensure I had wired them correctly. Then used a 9 Volt battery to check all the cones jumped inwards on a positive signal. Then connected each set to separate channel of the EP2500 using more lighting flex. Finally, I could play Franck's organ works.

The result was very satisfying indeed . Particularly considering that no attempt had been made to balance the outputs of the two sets of drivers to ensure a smooth response. In fact the bass was a little too heavy at the previous gain settings and using the BFD filters set up for the new drivers. The detail and timbre were at least on a par with the 4 x 15" manifold and occasional sections of big pipes were incredibly heavy.

I have been trying all afternoon to capture the view inside the manifold and out through the old glazed doors simultaneously. The view through the glass is overexposed while the manifold is in almost complete darkness. I wish my skills at photography were as impressive as my new subwoofer.
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Polishing the gilding.

2007-03-26T11:10:00.023+02:00

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Having had a listen to the beast on organ music I tried REW to see what was happening to the response. With the two completely different sets of drivers at work one might expect all sorts of problems. Would the old and new drivers compliment each other?

My first mistake was trying to match the drivers at the usual peak of 12-13Hz. With an Fs of 32Hz the old drivers were never going to match the bottom end power of the new ones. So using 13Hz as a hinge point forced the old drivers to do all the work. I ran the REW Calibration subwoofer pink noise signal and then touched the driver surrounds on each side of the box with my finger tips. It was obvious that the older drivers were quite busy but the new ones felt almost inactive. First things first: I bypassed the BFD to ensure neither driver was getting a free ride:

It seemed my attempts to lift the old drivers by 8dB on the BFD was removing the newer drivers from the equation. They might as well not be there. Which was not the idea at all.

So I returned the gain settings to normal on the CX2310 and tried again:

That's better! Now the drivers are working equally hard and the meeting point has been lifted to about 23Hz or almost a whole octave above 12-13hz. The combined response is showing a saddle between the VLF peak and a peak at around 27Hz. I'd like to flatten this result a little with the BFD.

Here is the improvement after 15 minutes of playing with different BFD filters and the resulting curve given 1/3 octave smoothing. I have since reduced the output at the EP2500 to 26 on the dials as the bass was beautifully expressive [and impressive] but just too heavy. (loud!) This resulted in rather too much distracting LF ambient noise from the cathedral. Now the bass is not so much in my face but amazingly detailed and muscular. The pedal organ has taken on a superb new realism with a more descriptive rendering of the timbre and interplay of the great pipes. The character of each pipe and rank now seems more obvious than before.

This subwoofer plus speakers trace above shows there is a serious trough between them despite an attempt to reverse their relative polarity. (and then returning it to normal) Without any means to alter relative phase or delay I'm rather stuck with this trough unless I start moving the speakers around. The other active crossovers in the Behringer stable offer more facilities than the basic CX2310. I might have a look at the potential of a better crossover.

The exterior of the manifold seen from inside the enclosure. The picture is distorted due to the closeness of the camera to the box. It is difficult to get far enough away to take a decent picture. On its supporting timber block the manifold stands over 6'4" high. The large, galvanised, roofing washers and nuts from the screwed rod braces are visible.

Out of curiosity I checked the sound levels out in the enclosure with my Galaxy 140 SPL meter and found the SPLs were about 2dB lower than in the room. The room levels drop rapidly with distance from the manifold. Out in the enclosure the levels were fairly stable regardless of distance from the box.

The doubled manifold thickness certainly feels and sounds more dead to a knuckle rap than my old manifold which had only one thickness of 3/4" plywood. Yet pressing my ear against the box proves it to be quite transparent to higher frequencies! Perhaps I should have used MDF and flake board? (OSB)

I've tried a few film scenes. The difference is not so much of quantity but of kind. Brutal realism without warmth or hangover. The battering ram on LOTR Two Towers had a real wood splintering malevolence despite hitting only a 108dB(C) peak. The walking trees shook the room but there was no overlying warmth. The collapsing underground bridge scene was also nicely realistic.

Xmen: Last Stand bridge drop scene hit 111dB(C). There seems to be less rumble for effect and much more violent content. Cone movement hasn't exceeded a few millimetres so far. Now I'm enjoying Blue Man Group's Audio at 85-90dB(C). Very tight with clear and clean vibrations from the VLF instruments. No visible cone movement. Same with Bass Outlaws' Extreme Woofer Test.

Mike Oldfield's Millennium Bell is another favourite bass test. There are great quantities of deep drums and beats. Try track four: Sunlight Shining Through Clouds at 85dB(C) average for an excellent example of what I'm talking about. The beat is almost nauseatingly hard hitting with a deep underlying pulse. Track 7: Mastermind also has plenty of deep bass. The last track played at a steady 95dB(C) is good fun too.

Screwing the controls on EP2500 back up to full results in fairground levels of bass but it is too exhausting to experience for very long. After several hours of listening to music and films at robust levels the IB enclosure is now at 84F. With a reading of 92F on a digital thermometer with its sensor resting on the amp top plate.

A remarkable recording of Messiaen's Les Corps Glorieux is played by Vibeke Astner on the Maribo Cathedral organ on the Helikon label. Track 4 "The battle between life and death" has the most subtle, deep breathy bass at times. At the absolute limit of audibility this is probably a quiet theme on one of the 32' stops of which there are two: The Bordun and the Tuba.

It is shame that Messiaen is not very accessible to the average listener. The quiet, wistful upper theme is hauntingly beautiful but full of pain and anguish. The real magic to this piece lies in the subtlety of the, sometimes almost infrasonic, underpinning. The great pipes restlessly rise and fall in pitch with changing character and strength. Like an oily swell lapping menacingly on the seaweed-encrusted steps of an ancient harbour amidst blackened rocks. A 22 minute masterpiece which will probably pass without much notice to most owners of ordinary subwoofers. They may hear an occasional, diesel-like rumble but nothing more. The real truth about the subterranean depths beyond the visible surface is clearly depicted by the IB. The counter to the beauty of light and life lies in the suggestion of unspoken fears below in the inky darkness. Always just out of reach. Hidden from view, but a constant presence, behind the barrier of in/audibility. Death is always just beyond our visible gaze. The unknown and the unknowable beyond yet another barrier to our ordinary senses.

All chapters in my IB blog are regularly corrected and sometimes sections are rewritten. Try reloading the page to ensure you are reading the latest edition.
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If it aint broke..

2007-03-25T21:41:00.041+02:00

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Today I rewired the 8 drivers with single runs of 2.5mm^2 stranded speaker cable to replace the 0.75mm^2 flex. I could have used the larger size of 4mm^2 but had found the terminals on the EP2500 (and both sets of drivers) unable to take anything larger than 2.5mm^2 when doubled.

After checking driver polarity with a 9 Volt battery on the left and right channels I played Track 4 of Millennium Bell by Mike Oldfield. Switching polarity on one bass channel of the CX2310 crossover quickly confirmed that I had all eight drivers firing in unison. What was rather worrying was how little difference there was between correct polarity and reversed. Muting the stereo speakers and winding up the volume on the IB to a reading well over 100dB(C) on the Galaxy 140 SPL meter found more rattles around the room than was at all desirable. It seemed the lack of bracing was causing each set of drivers to act as an isolated vertical array. The output was stunningly loud and brutal but the manifold was obviously flexing badly. I had underestimated the reaction forces of the eight drivers in comparison with just four. The much greater height of the manifold was also compounding the problem.

It would be tempting to fix the three plywood bracing shelves which I have already cut to size. However, unless the shelves are very firmly bonded to the manifold sides, there will be very little bracing against reaction forces in the reverse direction. So I am rather tempted to duplicate the screwed rod brace of the last manifold but this time in triplicate. The use of sturdy 10mm studding with tightly fastened nuts and large, load-spreading, roofing washers inside and out should ensure the sides of the manifold remain quite still. Both inward reaction forces, and those which would tend to spread the sides of the manifold apart, should be safely restrained. I could even use up to to six or more such braces arranged symmetrically between the drivers if it proved necessary or beneficial.

Here's another image of the 10mm studding brace I used on the former 4 x 15" manifold. There are washers and nuts both inside and outside the manifold sides. The nuts are carefully adjusted so there is no inwards or outward pressure on the plywood before tightening well with large spanners. Such a rod is quite capable of accepting a load of several tons in tension. When in compression, using tightened nuts and large washers, I imagine it is just as capable of resisting the cyclic reaction loads in the plywood manifold. The reaction loads cannot be very high or the box would be crushed or ripped apart at the joints.

Luckily I had enough 12mm (1/2") studding handy to make three bracing rods. I can report a considerable reduction in structural vibration above 100db(C) [sub only] on Track 4 of Millennium Bell. It is extremely unlikely anyone would ever want to listen to full range music at this level so I can relax a little on that particular problem.

Photographing the manifold is proving extraordinarily difficult even with flash. This is the best of 40 odd images! I can't get a straight shot because of the LCD TV in the foreground. The slightly odd angle makes the rods appear non-parallel despite their hole positions being measured to the millimetre.

Here's a video of the new IB playing 10hz sinewaves at a reading of 90dB(C) on the Galaxy SPL meter. With a 10Hz conversion factor of 13.45 dB this equates to 103dB. The drivers are well within their capability. I had to open the door slightly to stop it rattling like mad! The draught through the narrow gap was incredibly fierce! Interestingly, the cone excursions dropped with the door open and increased when I closed it.

http://www.youtube.com/watch?v=C_2C9M3_HLo

The video is also embedded just below:

If you decide to watch the You-Tube video in HQ full screen to get some idea of scale then click on Exit to return to standard blog size. If you have your computer speakers on you will hear the whistling buzz of the replacement (quieter) EP1500 cooling fan and me trying to be very quiet rather unsuccessfully. The 10Hz sinewave tone was absolutely silent to my ears. Nothing tickled my senses at all at this output level. Only the door shaking violently gave away that anything was happening.

Later I reached 106dB(C) @ 1 meter on the Galaxy 140 SPL meter with the door open. Add the 13.45 dB correction factor for the Galaxy @ 10 Hz. A totally silent 120db? Why can't I hear anything of the fundamental or any of the harmonics at this level?

Having played for some hours I have come to the conclusion that the different sets of drive are not well balanced with regards to reaction forces at 10Hz. The greater extension of the new drivers means that they take the brunt of the load in reproducing 10Hz. The older (more emasculated) drivers are hardly moving so contribute little or no reaction forces of their own. Further up the frequency scale they are much better balanced and both sets contribute more equally to output and thus help to cancel each other's reaction forces.

The basic advice to use all the same models of drivers still holds good. Imbalances of drivers will suffer much heavier vibration than identical sets as I have just discovered. The drivers on each side of the manifold are acting more like independent vertical arrays in my case. With all the disadvantages this entails. I owe it to myself to order four new AE IB15s to kill the vibration and probably extend the VLF performance of my IB. Away from sinewaves though the new eight driver manifold seems to be a considerable improvement on the last. Particularly with regards to sheer realism and shock value on both films and music.
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Just in case you thought 8 x 15" drivers was slightly over the top: Here's a 12 x 15" driver IB depicted on a really great website:

http://www.subwoofer-builder.com/ib.htm

Have a good look around at the endless other projects and inspirational construction features. This website offers a great deal for the speaker and sub builder! I wish I had studied it more closely in order to do a better job on both of my IB manifolds. This website makes my blog look like childish scribble!
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Mended

2007-03-24T12:45:00.011+01:00

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The cosmetic choice of having all new drivers on the visible face of the manifold was a mistake. There was far too much vibration through the lack of reaction force cancellation. I have now fitted the older drivers in opposition in the lower part of the manifold. The upper four places are now taken up by the newer drivers. It proved almost impossible to tell them apart at a glance anyway.

A quick fire up proved that my car audio bass tracks CD could be played at 115dB(C) + read from the Galaxy SPL meter without any sign of cone movement or rattling of the enclosure door.

I had used the opportunity to remove all the drivers and open up the driver fixing holes slightly to improve T-nut grip. Then caulked the inside edges of the box with black acrylic stuff before refitting the drivers. The latter exercise was great fun as the release trigger on the cartridge gun kept sticking so hard I could not release it! The stuff was everywhere before I had finished! Fortunately it was water washable and invisible against the black finish of the box.

It didn't take long to put everything back together and rewire in series-parallel sets per channel again. A few minutes adjusting filters slightly on the BFD smoothed everything out nicely. The violet line is the new (provisional) 4 + 4 + BFD curve.

Bass is prodigious on Franck's organ works despite backing off the EP2500 even further to 12 o'clock. ('24' on the big Behringer's twin control dials) Bass quality is stunning. Timbre is even further exposed with a harder edge, more character and colossal weight. The great pipes seem even taller.

Felix Hell's "Organ sensation" Track 6: Liszt Prelude on Bach is great fun but lacks the unique character of the French organ on the Franck CD. There is weight aplenty but the pipes do not speak with the nasal authority and sometimes malevolent roar of the St Eustache organ in Paris. The Franck work seems to have as many different voices as it has great pipes. Increasing the volume on The Felix Hell CD works wonders though.

I have been listening a great deal to the new IB in between tidying up around the large hole in the wall. I lined the triangular inner face of the wall with 3/4" plywood to beef it up a little. Hopefully the plywood will help to carry the remaining reaction loads into the roof structure where the great structural mass can resist vibration. I have been looking at potential wooden mouldings to finish off the edge of the manifold mouth. There is never enough air movement to warrant a neat radius but it still needs something to fill the rebate where the manifold stops and the wall surface begins.

It is surprising how the bass varies as I wander around the room. This is to be expected as one moves from node to anti-node along the considerable wavelengths involved. My computer desk was always a bass hotspot before but this seems to have reduced in favour of the stereo hot seat. The detail in the deep bass is much more obvious when I sit there and listen attentively.

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Operator Error!

2007-03-23T23:26:00.004+01:00

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I am still concerned about the frequency response imbalance between the two sets of drivers. Unless I am being particularly dense I cannot find a way to programme separate BFD filters for each channel feeding each set of drivers. The old drivers are quite flat but roll off earlier than the new drivers. While the new drivers are stronger low down but then roll off much more quickly at higher frequencies. While they compliment each other perfectly I wonder whether I am really maximising their combined potential. I am presently limited to a single filter set applied to both channels simultaneously. Would a second DSP1124 BFD (or FBQ2496) produce a better frequency match between the two sets to ensure all drivers are working equally hard at all frequencies? Should I opt for another, cheaper BFD? Or lash out a little extra for the FBQ to have a new toy to play with? Is either likely to provide an improvement to the IB's SQ?

By sheer coincidence a question arose on an IB Cult build thread regarding the twin channels of the BFD. They say that fools rush in where angels fear to tread. I dived straight in head first without a lifesaver. Or even arm bands. Fortunately the famed and ever-patient ThomasW of the IB Cult has finally dented by unshakable belief that both BFD engine lights should be lit simultaneously. Now, of course, it seems so obvious. Whichever channel light is lit is merely a coincidence due to the last filter being set on that particular channel. (or engines as Behringer chooses to call the channels)

When both lights are lit, by pressing both buttons together, both BFD channels are compounded. Both channels receive exactly the same filtration. This is where my error of logic lay. I could not accept that a commercial product would leave a single light on one channel when both channels were active. The idea seemed so ludicrous that I could not bring myself to even consider the idea. So I had repeatedly applied separate filters to each set of drivers to satisfy my REW testing. Then had instantly undone all my careful work by compounding the channels. The idea that a channel was active despite its light not being lit never occurred to me. So I never bothered to check whether the unlit channel was active. I had searched online for hours but could find no reference to setting independent filtration of channels on the humble BFD.

Having, very publicly, discovered the error of my ways I have begun all over again to try and match the frequency responses of the two sets of drivers. Even this was fraught with difficulty because the CX called one set of drivers Right channel and the BFD called them Left. So there was some swapping of XLR plugs before I could get the boxes to agree which channel was which. By the time this was sorted out I had little time to start building new filter sets before being dragged screaming and kicking to the car to fulfill chauffeur duties for the rest of the day on a 50 mile distant, shopping trip. A small price to pay for my wife's infinite tolerance of my crazy audio activities.

It may be that simply trying to match the individual responses of each set of drivers will not match the driver's acoustic loads exactly. I may be approaching the problem entirely from the wrong angle. We shall see.

Now I have discovered a European distributor for Fi drivers (Blade ICE of the UK) I am increasingly tempted to dump my eight AEIB15s onto the European forums small ads and start from scratch with four Fi IB318s. The problem is convincing myself (and my wife) that I can break fairly close to even on the change of drivers and that it would be a certain SQ upgrade. I would gain only slightly on total displacement and lose out on total cone area. Though the manifold/box would be considerably lower but deeper and the total Vas multiplier improved. I can still remember a Fane 18" PA driver in a shop window when I was still in my teens. The sheer size of that dustbin lid of a driver has left a lasting impression to this day and is still fresh all these many, long decades later. Now there is talk of even larger drivers for IB use! Where will it all end?

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Game, set and match

2007-03-22T11:07:00.016+01:00

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The imbalance between the two sets of drivers had bothered me. So having overcome my tunnel vision regarding bonding the channels of the BFD I set to work to match the two sets of very different drivers.

The results of half an hour wearing out the carpet between the computer and the BFD resulted in the following graphs:

1/3 octave smoothing applied. I have boosted the input to the newer drivers to lift the curve bodily to better match the older drivers. The newer drivers have some cut at 20hz and the older driver are boosted at 20Hz. From long experience I know the older drivers can take + 16dB boost at 20Hz using 120 B/W.

The idea was also to flatten the overall curve with less emphasis on the infrasonic as well as ensuring both sets were working equally hard at all frequencies. This would hopefully increase dynamic range and lower distortion from the doubling of the active cone area. To have maintained the frequency response imbalance would have meant a lot of the potential gain could be have been lost. Only a single set of drivers would be contributing in full at any particular moment. The downside of employing two sets of very different drivers has hopefully now been minimised. Of course I cannot overcome the impedance imbalance between the sets but the 2dB boost to the newer drivers will help to overcome their differences.

I went out into the enclosure and touched the driver surrounds lightly while some "heavy" organ music was playing. Both sets now seem to be working together. I was unable to differentiate between the vibrations of each set on loud organ bass. Cone excursion on organ bass is so low that only the surrounds registered any sense of cone vibration. None at all was visible. Bass levels out in the enclosure seemed very low indeed compared with the room itself.

Then I put on Bass Outlaw's Extreme Woofer Test/Stereo Bass/Track 12 from the Illegal Bass CD to confirm my previous findings on organ music.

http://www.youtube.com/watch?v=WJTDLxdfY7o

Unfortunately this YouTube video is not a good example as it plays at rather a low level and has a number of HF artefacts as well as seeming to lack the bottom end power of the original.

I was averaging only 98dB(C)-Slow on the galaxy SPL meter inside the enclosure but peaking around 118dB(C)-(Max hold) at the manifold mouth. Zero visible cone movement at these levels but some in-room rattles around the manifold were a bit annoying. The decorative plywood cladding really needs to be bonded to the new 3/4" plywood backing. Or replaced with yet more thick plywood! Even 95dB(C) while sitting at the computer, probably 12 feet away, is on the verge of being unpleasant to experience with this track. I have never found another track like it for massive low frequency content. A complete absence of inane vocals or screeching high frequencies just adds to the fun.

BTW: For the curious: 8 x 15" drivers is roughly equivalent to a single 36" cone but without serious cone flexure and all the other annoying little problems of housing such a huge thing. Not to mention resisting the vicious reaction loads of single, large driver. Interstingly 4 x 18" drivers have a similar cone area to 8 x 15". Though displacement may be different depending on the Xmax of the individual drivers.

My IB's displacement is now 22.6 litres (4 x 2.65) + (4 x 3) litres = 10.6 + 12 litres. Hardly worthy of dicussion when cone excursion is now limited to invisibility. The vital thing is the huge cone area. Not how far the individual cones move in an emergency before they all hit the end stops! I have the immediacy of a large electrostatic panel moving a fraction of a millimetre with the potential stroke of a huge dynamic driver to ensure massive infrasonic bass levels when called for.

Power per driver has halved despite each set being wired in series-parallel to keep total impedances an easy load for the EP2500.

The 4 Ohms set 650w/4 = 162.5w per driver.
The 8 Ohms set 450w/4 = 112.5w per driver.

With the previous manifold housing 4 x 8 Ohm drivers in parallel pairs per channel they were seeing 325 watts each. This may explain why I feel the dynamic range has dropped on films. The IB will go very loud indeed on sinewaves but seems to have lost some SPL impact on films. Despite having such a huge subwoofer in the side wall there is still absolutely no sense of the bass being lop-sided. The speakers do all the steering of LF Effects in the soundfield. I am quite tempted to rewire all the newer 8 Ohm drivers in parallel for a 2 Ohms load on one channel. The newer 8 Ohm drivers need a little more input to match the older 4 Ohm drivers. This could be the answer to providing more dynamic range since I can't safely bridge the amp across two sets of very different drivers. It would be very difficult to predict how each driver type would react to being driven together in the same circuit.

Okay, the newer driver set has been wired in parallel for 2 Ohms load. I'm seeing a considerable gain but have backed off the EP2500 control on that channel to match perceived cone excursion. There is a new, effortless quality about the bass on the car-audio tracks.

The odd thing is the massive difference between my SPL readings out in the enclosure compared with the AV room. The difference is now averaging around 10dB(C) @ 1 metre from the manifold! I have repeated the exercise on various tracks and the 10dB holds true on all these bassy tracks. It hardly feels as if the manifold is working out there with the door closed. When I come back into the room and close the door again I immediately gain 10dB on the SPL meter. In the room the bass is thundering and pounding at 95dB(C)+. Out in the enclosure it's a complete non-event. This can only be the effect of reducing the Total Vas multiplier. The amplifier seems quite happy. The orange lights are flickering on and off with around 4 bars showing on the BFD on bass hits. No sign of clipping at all. This is great fun!

I'll have to check relative levels with REW but I'm enjoying the sheer energy in the music too much for testing. :-)

Satriani's "Engines of Creation" CD went down well at a steady 95-100dB(C) on the meter. Now I'm back to organ music with Bowyer's "For Weddings" on the organ at Chichester Cathedral for a bit of a rest. The end of the Vidor Toccata: Track 23 from the 5th Symphony was magnificent at only 85dB(C). By comparison the Boêllmann Toccata from Suite Gothique by Bertalan Hock on "Wedding Music" almost ponderously superb. I'm a complete sucker for quiet organ pieces underpinned by deep bass. It would take a real Philistine to screw up the volume but I can always pretend I am profoundly deaf. Besides, I alone am listening.

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Bass, I love you.

2007-03-21T19:00:00.009+01:00

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I thought this was interesting:

http://www.youtube.com/watch?v=IN5cuRzvTGc

No, not my crappy video with the cameraman falling over the junk on the floor out in the IB enclosure. I was completely sober. I swear! :-)

I was playing a YouTube video through my system and thought I'd record it from out in the IB enclosure. The bass was terrific in the enclosure while playing the video at 95-96dB(C) average measured at the listening position out in the AV room. Luckily I managed to record most of the bass with my Canon camera. But what was really interesting was the lack of extreme cone excursions when played back through my system but which the original YouTube video had produced in my (newer) IB drivers. The camera has failed to capture the extremely deep bass. It is known that there is considerable 5 & 8 Hz content (and above of course) at quite high levels on this track.

If you play the video on ordinary computer speakers all you hear is the simple, plinketty plink of the repetitive theme. But when I play my own video through my system the bass is almost nauseatingly heavy with little else in the background. As is to be expected, of course, when I was recording it out in the enclosure where the output from the speakers could not easily reach. You may like to play the video with your own IB subwoofer to see what it makes of it. I find it almost unpleasant at quite modest levels so heavy is the bass.

The newer drivers with their low Fs (16Hz) take sole responsibility for the extremely low bass while the older drivers take over upper bass duties . Not what I really wanted, after all my efforts to match their responses, but so be it.

It seems that YouTube is well able to reproduce and "rebroadcast" 5hz at high levels without any problem. Which is quite amusing when emasculated sub owners complain about the lack of bass on some tracks on YouTube. Including the one I played through my IB above.

Here is the original YouTube video music track for you to try playing through your system. Do *not* wind up the volume until you check out the few extreme cone excursions on the occasional off-beat! It doesn't happen frequently in the duration of this track but it might just catch you out. So beware! The video title sequence is not over hyped, for once.

http://www.youtube.com/watch?v=u-BXKtpO-rc

These wild cone movements put me off playing the track any louder than I did. They didn't produce any sense of pressure or any other stimulation (at all) in my own senses. So presumably the composers (Bassotronics) own a Thigpen rotary sub. Why else would they put content on a track which cannot be reproduced by any but the largest subwoofers on the planet? I wonder if an extreme car audio subwoofer installation could harness cabin gain to reproduce something which might just be sensed at 5-8Hz by the vehicle occupants?
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Recovery

2007-03-20T21:26:00.017+01:00

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I find that if things are too easy they aren't worth doing. Mistakes are priceless learning experiences and best enjoyed in one's youth. This leaves enough time for real successes to kindle fond memories in old age. Easy successes are the cream on the cake which always make you feel too full. One can never be sure what one learns will ever be useful to one's survival. So it pays to be flexible and attempt everything which presents itself on the journey. Regret can only come from not having measured yourself against each hurdle which spikes your interest. Failure is perfectly acceptable if you have tried your best but failed. There can be no shame in coming second or third to your own ambition. Humility has its place but should not be used as a crutch to earn sympathy. Smile often. Somebody might need one desperately more than you do.

The new 8 x 15" had failed to please. My unconscious emotional needs had not been met and I had been distracted by other projects rather than enjoying music and watching films. Bigger had proved not to be better and I was always a mug for the underdog. SQ was absolutely stunning. On films the new manifold had been fast and brutally real. It was just that it lacked something I could not easily define. I called it "a lack of dynamics" but it was more about missing excitement. Perhaps a painting can better conjure up emotional responses than a photograph? Is this why advertisers "tidy up" all the human weaknesses of their already perfect models before publishing unblemished perfection? Yet leave the viewer completely uninvolved.

I felt uninvolved. Unmoved by the new accuracy. With all the fine nuances laid bare and exposed to blatant aural scrutiny. My emotional connection to the music had been severed. It was too clinical. All stainless steel and white porcelain but no warmth. I had turned my back on it after all the fuss over construction and the final assembly of the larger manifold in its place. It was certainly impressive, but so what? Like an expensive Hifi system which cannot play music to be worthy of existence. Many systems are like that. They have no rhythm. No powers of intoxication. No sensuality. No warmth. No... nothing. Loud? Impressive? So are farm tractors and pneumatic drills

So I left it to its own devices. Standing tall and black. Sulking out there beyond the old glazed doors with the clumsily scratched glass and old, flaking paint. Some call it shabby chic. I just feel that some old things can never be improved by a new coat of paint. Buy an old cottage and "do it up" and you strangle the life out of it no matter how sympathetic your restoration. No matter how many wagon wheels, old scythes and modest, horse drawn ploughs you may hang from the ancient but freshly re-rendered walls. Its sad dilapidation and untouched patina of age are what get you emotionally involved in the first place. Why soil the perfection you loved with a shiny new roof and care-free plastic windows? You are merely the caretaker for the duration of your stay. Not the unthinking destroyer of historical interest for all who pass by. Buy a modern house and leave the tatty old housing stock to the locals. Who won't be able to afford to gut the place, install a new Formica kitchen and "Arrger" stove and bleached, recycled pine, panelled doors throughout.

Meanwhile, the black ash monolith remained unmoved and unmoving for all my feigned indifference. Should I buy another amp? Buy four new IB15 speakers? Sell the lot and move the smart money into four Fi IB318s? It was all so tempting and all so futile. Which option should I plump for as the least of all evils? All routes involved considerable expenditure and effort. None guaranteed success or (far more importantly) satisfaction with the results. How could I possibly know in advance?

I half heartedly measured dormers, help up cardboard circles against IB-unsullied walls and ceilings. Even probed with fine carbon fibre rods through tiny holes drilled in my wife's absence. Seeking usable open spaces within an unforgiving roof structure but finding none. I secretly found the old pre-digital prints I had taken which I took while I was replacing the roof.

There were absolutely no unknown volumes for an IB enclosure which did not involve making large holes in the boarded ceiling. The ceiling is sacrosanct. No man's land. Least of all for a foreign IB invader. That way lay financial ruin from trying to heat a house in a -18C climate using only a cool running EP2500 for warmth. What would happen when the last of the furniture had been sacrificed to the wood burning stove downstairs?

Turning the ceiling and its rockwool winter coat into Swiss cheese for the sake of audio was too unthinkable. Even I could see the logic as I paced frantically- Seeking inspiration like a caged animal in my AV den. Meanwhile the Head Gardener pottered innocently at her gentle horticultural tasks out in the warm summer sunshine. It was all slash and burn, out there, but without the burn. She detests those who have garden bonfires. It's an endless war against nature and modern farming practices out there!

In the end I sought the advice of the IB Cult members. Whose combined intelligence, resourcefulness and experience dwarfs that of any individual. The answer was obvious but had not even remotely crossed my mind until it was actually suggested: Raise the crossover frequency!

It worked like an elixir on the great black tower. The warmth came back. The power and the fury. It was a miracle. I had been cured! Organ sounded lovely again. The first hired film I played was stunning.

I even returned here to scribble my blog. Unloading all the built-up frustration in weaving a new tapestry of words. As my wife calls the strange mixture of imagination and wishful thinking. Which I call embroidering the simple truth for greater effect. And which she calls lies. The truth lies somewhere in between. Where are the damned smilies when you need them? ;)

For those curious as to the position of my speakers relative to the IB here's a clue: A 1/4 second hand held shot. The speakers are sitting 5 feet apart now instead of the former 7' apart as they had been for some years:

Shabby chic but without the chic! A pair of Mission 753 Freedoms, a 37" 100Hz JVC LCD with a Mission 75C below and a cavernous manifold containing 8 x 15" drivers over on the left. My ever-expanding collection of organic vinyl is just visible in the box to the left of the welded steel TV stand. The tall 16-46 SVS cylinder looms off to the extreme right. In the foreground the old, black gloss handrail we look over to see the TV. Not that it is ever noticed. No more than the huge, vintage, white opal, café globe light above the open stairwell in front of us. Which is only occasionally lit. Usually when we have lost something.

Some time ago we stapled some nice red cloth over the hideous decorative ply on the 45 degree sloping wall behind and above the TV. The intention once was to line the whole attic with plasterboard. Decorative t&g boarding would have been much easier but my wife just will not accept it again. She calls it, "Like living in an upturned boat!" Unfortunately the decorative plywood is fixed to a random frame of long planks nailed onto the undersides of the rafters. Removal of the ply and the planks would expose the underside of the rockwool. Probably causing a rock(wool) fall.

I have really no desire to get involved in this work if it can be avoided. Fixing plasterboard at 45 degrees to overhanging rafters is probably well beyond my ability these days even with the help of a hired board lift. Though no doubt the sound quality would improve considerably with a more solid lining to the attic. The present, rather flexible, decorative plywood must absorb quite a lot of sound waves. Particularly as it is backed by thick rockwool. All over bass absorption panels!
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The + 3dB shelf?

2007-03-19T13:38:00.010+01:00

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The changes to the IB had wrought doubt regarding the overall frequency response. Testing my speakers was not an area with which I had exercised myself too much. Subwoofers are far more fun and don't leave jagged spikes all over the REW graph box. Smoothing is beneficial to improve both the response curve and one's sense of well being. Audio paranoia is never far from the surface.

With 1/3 octave smoothing the results were far from pretty! Several hours passed while every possible variation in microphone and speaker position was run through REW. In phase, out of phase and changing the roll-off point were all tried in combination with the above. Even with the shortest sweep fatigue quickly sets in when testing speakers. One can test subwoofers all day long with much longer sweeps without growing tired of the resulting, deep, soft whoop. Speakers are more audible and whistle into the bargain. Each new graph had 1 octave smoothing applied to follow the trend as I raised the speakers on 4" wooden blocks one moment and then removed the concrete slabs on which they sit the next. Nothing helped. Nor did moving the test microphone back and forth and up and down. (I used my Galaxy 140 SPL meter) Eventually the speakers ended up 5' apart instead of the former 7'.

Then Jim on the IB Cult suggested I try fitting a shelf. I found the shelves I had made and even painted but had never used. One slotted into the middle and I snugged up the nuts on the studding to pinch it tight. The result was absolutely remarkable on both the REW graph and sound quality: Here one octave smoothing is applied.

The magenta curve is with the shelf in place. The IB's bass plateau is more extended from 50Hz upwards to well beyond 100Hz. The trough just above is filled in a little more and then the response is generally smoother beyond that point until the upper roll off point. Far, far beyond my own ears' ability to hear anything. The black curve is without the shelf and is shown for comparison.

Organ bass suddenly sounded much harder edged. With much more character and timbre. I was surprised how much louder it sounded too. There seemed to be bass growling along where I had never noticed bass before. We are not talking nuances here!

The real proof of the pudding? My wife commented immediately we started watching "Hot Fuzz" (for the third time) how the sound quality had improved dramatically. The bass is now absolutely stunning! Effortless, brutal and real! Oh, and loud! None of your shapeless, deep rumbles. This is a whole new experience! Whoopee! Is that enough exclamation marks for one paragraph? Exaggeration? Not a bit of it! The sound quality from top to bottom just seemed to be improved. BTW:

"Hot Fuzz" is one of the funniest, most wicked films ever made. Such a shame that you have to be British to understand all the inside jokes. No stone goes unturned in the search for English humour. Never mind the language. Enjoy the carnage! Bring the noise!

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Resurrection?

2007-03-18T18:58:00.014+01:00

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I finally had to do something about my lack of enthusiasm for the new 8 driver manifold. I had managed to match the outputs of both old and new sets of drivers despite their differences. However I had neutered the newer drivers ability to play low and loud. The result was amazingly fast but lacked the tactile effects through the floor and air. The weight (or authority) which I had become so attached to since building my first IB was absent. There were no more asteroids in collision. No jumbling icebergs of infrasonics underpinning my favourite organ music pieces.

I had plenty of time to think about the problem subconsciously while I became engrossed in another project completely unrelated to audio. We had watched a couple of films in the interim but I had hardly listened to any music to speak of. It was the bottom end which was missing of course. It suddenly struck me that I had no need to protect the drivers since I had lots of experience using them as individual sets. I had used +16dB boost at 20Hz on the old drivers to get anything out of them below 30hz. That was the price of having over-stiff suspensions and an Fs of 32Hz. Why shouldn't I boost them again? The newer drivers with their 13Hz Fs could take care of themselves but had rather weak shoulders in comparison with the more muscular, older drivers. As individual sets both lots of drivers had seemed almost (LFE) bombproof. Together they could rule the world!

A spare hour or two presented themselves and I went to work with the BFD. Raising the older drivers back to +16dB boost @ 20Hz was the starting point. I wore the carpet out for over an hour running backwards and forwards between my computer and the BFD in the rack. A ten inch high step and sharp right angle turn intervenes in every journey but I survived without mishap. Finally I had two sets of drivers with closely matching response curves except for the extreme bottom end. The combined curve didn't look too bad either and can be smoothed further if necessary. A quick check suggested that the speakers want to be out of phase with the subwoofer and I needed to raise the crossover frequency a little to get the combined response shoulder to match the nominal REW roll off curve with the main speakers playing.

Close enough? In case there is any doubt the overlapping (lower) curves represent the two different sets of drivers measured at the listening position. I no longer have the previous rising ramp into the infrasonics but that may not be a bad thing. The high energy area between 25 and 60Hz is well to the fore for real impact on films. The sub-20hz infrasonics are not too shy of the rest of the curve so should be able to get the floor moving on films. Organ already sounds much more interesting. The weight is back even on the most fleeting of pedal touches. The timbre of the great pipes has returned with a vengeance. Time will tell whether I need to fine tune what I have obtained so far.

Interestingly, I have gained considerably in output. Before I rolled back the IB's gain on the Behringer crossover by -6dB I was almost 15dB above average speaker level with flat settings on the active crossover! I believe low crossover points rob the IB of a great deal of output. The balance sounds about right at the moment on organ at fairly modest levels. I have now backed off the EP2500 controls by 6dB each to tame the IB on music. On films I shall watch carefully for clipping and excessive cone excursion as I gently raise the subwoofer output to produce suitably "exciting" levels. I seriously doubt it will need much more than present levels. :-)

Next day: Well I put 4dB back on the IB at the crossover and enjoyed the Matrix Trilogy on DVDs back to back. There were some nice moments where the floor and even the room seemed to pulse silently but I doubt I was even matching the performance of my old SVS 16-46 cylinder. In fact I hit only 105dB(C) max hold on the Galaxy SPL meter when I turned it on out of curiosity towards the end of Revolutions. Only the newer drivers showed any excursion (at all) and then only about 1/2" total movement. (12mm). There is obviously lots more room to run the IB hotter for action films. The EP2500 controls still have plenty of reserve with the CX2310 level controls now set flat.

Given the massive 20Hz boost on the older drivers they really ought to be trying harder by now. The obvious thing would be to remove them and fit closer-matching, newer drivers. That just isn't going to happen. Not unless I win the lottery. So it's really a matter of maximising what I have already.

It might be thought that I am not really enjoying my present IB set-up. Then you haven't felt the incredible vibrations when I played some organ music without turning down the EP2500 controls.
I am the victim of my own ambition to use the spare drivers. Nobody made me do it. The 8 x 15" is better than 4 x 15" of either vintage. 8 x 15" has the speed and effortlessly brutality which 4 x 15" simply cannot manage.

At my first attempt I ran into trouble while trying to match the very different sets by strangling the drivers which really could play low and loud without the help of the BFD. Lowering the crossover point was also bad news because it robbed the drivers of their best bandwidth. Just because the curves match closely does not mean that the drivers are physically matched at different frequencies. They still want to do their own thing. The surprise is that they do it so well together despite being in a grossly undersized enclosure! I have lost at least 5hz of infrasonic extension by insisting on using all 8 drivers. IB drivers aren't supposed to be used in undersized boxes because they don't have the motor power to overcome self-induced back pressure. I broke almost all of the IB rules and had to work to rescue the situation. I'm still working at it when the mood takes me. I'm still learning (slowly). Most people would be happy with four drivers. Most would be very happy indeed with my grossly mismatched 2 x 4 driver IB. If it ain't broke don't mend it. My 4 x 15" was a superb subwoofer.

Why would anyone be daft enough to copy my example? Anybody sensible would use eight identical drivers and reap the rewards straight out of the box. Low Fs drivers would ensure that lower bass is truly prodigious without any boost. Cone excursion would remain modest as all drivers summed their perfectly matching responses at all frequencies. The enclosure should match the Total Vas x 10 minimum requirements. Thereby ensuring the drivers enjoy a suitable environment for best performance without raising their natural resonant frequency with unwanted back pressure. So no early roll-off. The walls of the enclosure would not be subject to flexure because the pressure would remain low at all frequencies and at all output levels. So no pumping losses either.

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More Calibration

2007-03-17T18:58:00.013+01:00

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Nagging doubts still existed about the matching of the old and new sets of drivers. It all looked fine on the REW graphs but there was an asymmetry in the EP2500 channel settings. The older drivers were still not matching the newer ones below 20Hz. There was also a deep trough at the crossover point and above. More obviously needed to be done before I could be completely satisfied that I had plumbed the full potential of the 8 x 15".

The first thing which needed doing was to extend the boost to the old drivers to the full 120B/W on the BFD. The other settings, for the older drivers alone, remained at +16dB boost @ 20Hz. I ran some new sweeps and noted the old drivers response had lifted nicely to match the newer drivers around the 13.8Hz room induced peak. The graph hinged from this point to run just above the newer drivers.

By a combination of adjusting the level controls on the EP2500 and fine tuning with BFD filters I was finally able to match the response levels of both sets across a wide frequency band. This still left the combined response overpowering the speakers. Now that the asymmetry was gone on the power amp control knobs so I could safely reduce the combined levels to better match the speakers. This occurred at 20 on the EP2500 control scales. At the 26 setting I had the IB good and hot for action films. Now I no longer needed to remember the differential between the control knobs. Then I added some gentle boost filters around 90Hz to try and fill the crossover trough. This required further filter adjustment on each driver channel to keep the combined response from humping at 100hz.

Note the closer matching between the two sets of drivers over a much wider power band than before:

The film setting runs rather hot compared with the main speakers:

And the graph showing both the film and music settings including the greater subwoofer output for film watching. The (third from the bottom) blue line arrowed is the music response curve. The two lower curves are each driver set alone. The smaller difference in level for music does not rob the music of bass. Far from it. Though there is considerable variation in bass levels between different CDs I can now adjust bass levels far more easily.

There is still room for improvements. A house curve would be nice to overcome the natural roll-off of my rapidly ageing ears. it started with target shooting in the ATC at school without ear protection. Then weekly rock concerts at a local dance hall in the SPL unlimited 60s and regular attendance at motorsports have all damaged my HF hearing beyond repair. Many a week passed with tinnitus in my youth and, more irritatingly, much later.

Ear syringing can return me a few more kHz but the procedure is not completely without risk. All of this this does not mean I cannot judge the SQ of my system. I seem to be just as sensitive to bright speakers at the dealers and AV shows as I ever was. I can still hear very nasty resonances in very expensive, electrostatic, panel speakers despite the protestations of their owners on the more upmarket forums. :-)

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Insulating an IB

2007-03-16T18:47:00.010+01:00

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I am struggling with my environmental conscience in having a large area of uninsulated, paper-thin driver cones and an unheated and poorly insulated IB enclosure. More than that I'd like to perforate the ceiling to use the remaining roof space for an enclosure. This would allow me to reduce the number of drivers in the present manifold. Which would solve the problem of having an undersized enclosure. The enclosure is ideal for a 4 x 15" (or two x 18") but not 8 x 15". My previous house curve has evaporated with the addition of four more 15" drivers. Using the roof space would require that I make large holes in my 12"-16" of rockwool. I carefully placed every single sheet of the multiple layers of insulation and snuggled them together when I rebuilt the roof. This was following a severe storm some ten years ago. It would be thermal suicide to make holes in my lovely insulating blanket. Which would probably result in massive condensation and heat loss in winter.

So what to do? The older (4 x 15") 3/4" manifold had already sweated inside in winter as the warm, moist air from the house condensed on the cold plywood. I have already had to reverse the magnets-outward drivers to bring the massive lumps of freezing cold metal back into the warmth. As they lagged behind the temperature curve of the enclosure they literally dripped onto the enclosure floor! Now my manifold is made up of two layers of 3/4" plywood so it is better insulated than before. I can't easily cover the drivers in anything without stopping the vital air movement. On the other hand the inside of the manifold acts as a giant compression driver. Its power to move lots of air is concentrated in the manifold opening. Could I use this pumping capacity without spoiling the IB? A DPM (damp proof membrane) would also be nice to stop the damp indoor air from ever reaching the drivers and the insides of the box...

I'd made a large ABR in my younger days but lacked a suitable driver to get it moving at the time. This had consisted of a piece of polystyrene foam ceiling tile with a soft rubber suspension cut from a baby's cot, rubber underblanket. The ABR had moved nicely on its suspension provided it was vertical. Could I make a large ABR to fit in the mouth of my manifold? What would be its effects on air movement? It would have an unknown mass and compliance leaving considerable doubt over its natural resonant frequency. Excursion might be dramatic! The foam would certainly insulate and the area of the suspension would be very much lower than 8 x 15" naked cones. The problem would be to make it damp proof as well. If the insulation reduced the temperature inside the manifold it would be even more prone to damp as the ABR would let moisture through unhindered. Covering the foam to make it damp proof would considerably increase the mass lowering the driver's resonant frequency.

Given a big enough wall it might be fun to build a huge ABR to sit on a frame in front of the IB to be driven by air pressure. Who knows what the effect would be? My triangular baffle wall offers no space for such a device as the tall manifold already reaches the 45 degree sloping ceiling/wall.

Back to a stretched sheet DPM over the manifold opening idea then. At first I saw a choice between a simple sheet of sponge, polythene or thin rubber permanently covering the manifold mouth. The sheet materials would be fixed to an airtight frame with just enough tension to keep them taught to avoid sagging or actual rattles. The sponge would allow moisture through. Being able to breath would reduce the risk of sound quality effects at the cost of likely increased condensation problems.

A thin sheet of rubber would be airtight and moisture proof to a much higher degree compared with sponge. Its mass and tensile strength would be considerably higher than sponge too. But would it flex/stretch enough to make it disappear as far as the drivers were concerned? Too high an internal manifold pressure would raise the drivers free air resonance (Fs) considerably. Probably resulting in a serious loss of lower bass and introducing compression effects. There were a lot of unknowns whatever I tried but at least I could monitor the effects with REW. Would a single sheet of thin rubber insulate enough (by trapping still air inside the box) when not in use to make the exercise worthwhile? What if I made the rubber sheet considerably larger than the manifold opening to reduce internal pressure effects? Polythene sheet would be easy and cheap to do but it would be likely to be very noisy and inflexible unless very thin. Its insulation qualities would be minimal.

I had already discounted a well insulated cover for the manifold opening. The removal of a cover in cold weather would cause a rush of warm, most air onto the metalwork resulting in massive condensation. I'd need a fitted drip tray in the bottom of the manifold with a drain! Heating the manifold artificially is out of the question too. Nothing sits on standby in our house.

A much more practical alternative might be insulated covers for the driver panels of the manifold. The metalwork of the drivers would stay warmer from being indoors. So it shouldn't affect anything provided the insulation is removed when the IB is active. The rest of the manifold could also be lightly insulated to further reduce the risk of condensation in winter.

It might be possible to have a frame covered in open cell foam fitted permanently over the drivers. A mounting frame around the outer edges of the box would spread the compression effects over a larger area than the manifold mouth but result in a rather small enclosed volume. Deepening the frame might help but open cell foam can breathe rather well anyway. There should be no condensation problems as the drivers and box would seal moisture indoors. It remains to be seen whether the foam can breathe well enough not to affect the driver parameters. Only practical experimentation will prove whether any of this is really worth the effort. I can even monitor the temperature inside the insulating covers to see if this would make a roof manifold practical. Though I doubt 1/2" of foam can compete with 12-16" of rockwool. Perhaps several layers of thin sponge sheet would help to trap warm air between them without reducing the IB's airflow too dramatically?

Whoah! I'm looking at this problem completely backwards! All I really need to do is to keep the drivers on the warm side of the insulation. So simple I wonder how I missed it! An "outie" manifold (with baskets and magnets exposed) on the ceiling. It can have a well insulated, plywood box trapdoor on top to close the mouth off from the roof space when not in use. Normally closed, hinged, weighted, foam sealing strip, operating string through an eyelet in the ceiling ring over a screw hook in the dormer wall? The driver's metalwork always remains in the warmth of the room. Only the extended manifold mouth peeks above the roof insulation. Two drivers at 45 degree in two dormers? One pair up front right? The other pair rear right? Or 4 in a downward pointing pyramid up high at front right? So many choices. :-)

So near yet so far. Winter is already upon us! I can't open up a 4' x 2.5' hole in the ceiling to replace the dormer ceiling with a plywood panel + finished manifold. I'd have to work from above on a roof ladder. Remove the dormer roof, remove the rockwool insulation and clean up well. Only then can the T&G boarded dormer ceiling panel be removed downwards in the AV room. The Head Gardener would have a complete fit! This requires careful planning...
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IB Distortion

2007-03-15T14:49:00.041+01:00

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I was browsing around the various audio forums when I saw a thread about a Tuba HT, folded horn subwoofer. The builder showed what looked remarkably like an REW screen measuring distortion "live" on his YouTube video. If memory serves he was measuring 22Hz at 122dB! His house and courage must be far stronger than mine!

I wondered why I hadn't tried measuring IB distortion myself. It couldn't be that difficult to produce some meaningful distortion figures of my own for my 8 x 15" IB. I certainly remembered that REW could produce distortion graphs and figures for up to 8 harmonics but the actual method had become vague from lack of practice. Thankfully John Mulcahy's REW help files covered the subject in sufficient detail to make me dangerous. It seemed easy enough to make it work at a basic level even if my understanding of the purpose of the variable controls were still a bit fuzzy.

I have used italics in the paragraph below to indicate REW controls and settings: This may help those unfamiliar with REW but bent on trying to replicate my measurements technique on their own subwoofers or even their speakers:

Having already Calibrated REW using my Galaxy 140 SPL meter and my IB on pink noise (with the SPL meter at the listening position ) I knew I had a safe level to work from. The REW Generator is opened and set to produce a sine wave with Frequency follows cursor for ease of use. Spectrum is selected from the test options just above the graph box. Then the Distortion measurement box down below the graph box is ticked. Once the chosen test tone emerges from the subwoofer the red Record button is pressed. Now a dancing, "boxy" curve is produced in the REW graph space. I played around with the settings then chose RTA 1/6 octave to provide the best looking graph. (IMO of course) The number of sample Averages is also available as an option. This steadies the constantly shifting curve until (eventually) it becomes fairly fixed and "snapshot" can be taken and saved to Windows Documents.

As it is a horribly rainy day my wife (The Head Gardener) is stuck indoors. So no window breaking SP levels were attempted for the moment. Still, I think the results are interesting enough to share even though they only peaked at around 100dB(C). If this figure changes I may have summed up the courage to try even higher levels.

I was actually quite disciplined, for once, and saved the graphs and images at each test frequency. It is quite normal for me to omit recording the blindingly obvious. So being able to resuscitate REW graphs and measurements, at will, is a vital characteristic of this amazing program. I haven't quite grasped how to Copy and Paste the numerical values for instant regurgitation onto a blog so it will mean typing them out longhand. The numerical distortion figures and test conditions all appear just to the left of the REW graph window. So they aren't lost provided the individual graph is saved. I had hoped I could copy and paste the figures for all distortion harmonics straight into a useful format here but it seems not. The time it takes isn't the problem just that I fear inaccuracy creeping in as I carry them across. (this involves maximising the REW page, noting the figures, then maximising the blog page again to enter the details in the table) For the moment I have simply listed frequency, Total Harmonic Distortion and SP Level. I may list the separate harmonics later if I have sudden burst of conscience.

First, an image of the (All) Measured plots for 10Hz upwards @ 75dB until lunch intervened at 50Hz precisely. This is simply all the curves for each test superimposed on one graph to give you an idea of what I was seeing on the individual tests.

The vertical pillars in the graphs indicate the level of distortion harmonics around each fundamental from 10Hz up to 50Hz. The shoulders of all test frequencies are remarkably "clean". As is witnessed by the very low numerical figures for harmonic distortion listed in the table below. No change was made to volume levels throughout the nominal 75dB range. The variable SPL readings are the natural result of output at each frequency depending on the IB's own frequency response. Later I wound up the volume on my preamp to achieve much higher levels. I also stepped up the Galaxy 140 to the next higher range. (50-100dB) Switching ranges on the Galaxy affects the meter output voltage which REW reads. So it is best to match the range to levels actually being measured. Or the measured curves fall below the bottom of the graph window.

Thanks to my flimsy baffle wall vibrating there are some serious structural rattles at higher SPLs which may be contributing to the measured harmonics! I shall snatch higher SPL readings when my wife is absent so do remember to reload the page to ensure you get the latest version. (assuming you are a regular visitor here) The 10Hz measurements were taken last.

10Hz THD = 4.4% @ 75dB _ 10.33% @ 86dB_33% @ 88dB

15Hz " = 2.75% @ 77db _2.75% @ 90 dB_3.65% @ 95dB

20Hz " = 2.05% @ 77dB _2.34% @ 95db_2.65% @ 100dB

25Hz " = 1.34% @ 80dB _0.81% @ 93dB _1.08% @ 98dB

30Hz " = 0.3% @ 81dB _1.96% @ 90dB_ 0.67%@ 95dB

35Hz " = 0.85% @ 82dB _1.03% @ 92dB_1.95% @ 95dB

40Hz " = 1.40% @ 84dB_1.38% @ 92dB_1.46% @ 96dB

50Hz " = 0.81% @ 85dB_0.95% @ 90dB_1.22% @ 95dB

I hope this table comes out of the wash in good shape. There is no telling what others see on their screens. I have glanced at my blogs on other computers and it is surprising how such tables are folded to a newline on some machines. I have deliberately made the table narrower than the standard text width. Guess what? The editing text width is much wider than the published blog! If you'd like the text and images larger just click on CTRL+ few times. CTRL&0 will return everything to normal size.

BTW: 10Hz is the minimum frequency available for these distortion measurements. It is also the barrier beyond which the cursor's control of frequency will not go. The IB doesn't like it either as distortion goes silly with rising output. A few dBs and distortion doubled.

A disadvantage of using prolonged test tones is rapid fatigue at higher SP levels. Using fewer samples for averaging may help here. As does clicking the red button to freeze the graph followed by a click on the generator button to stop the tone. I can still hear some of the higher level tones long after they have ceased!

Some representative harmonic distortion graphs at different frequencies appear below. Left click for much enlarged images. (only 55kB)

Here you can see a worst case scenario as all 8 drivers reproduce 10Hz at 86dB at the listening position. The door to the IB enclosure is moving about half an inch and output rising only very slowly as I crank the volume control on the preamp beyond half way. The baffle wall and door are obviously limiting output. Probably due to being out of phase with the manifold. At 88dB REW was measuring 33% THD!

10Hz again but much better behaved at 75dB.

My wife returned from a quick slosh around the waterlogged garden to report she could clearly hear the test tones rattling the house while she was outside. I have a headache now after several hours of running back and forth between the computer and the volume control. The odd thing is how different objects and surfaces vibrate in sympathy with particular frequencies. My glass-topped computer desk shakes like a big diesel engine at 25Hz. Tones below 20Hz are silent to my ears so don't bother me. As the frequency and levels rise the noise becomes very uncomfortable indeed. I could wear ear defenders but would worry about not being able to closely monitor structural vibration and loud rattles!

My summary is that distortion is low above 20hz but rises rapidly below 15Hz with increasing output. This is probably structural and exacerbated by the flimsy construction of the enclosure area. I inherited the structure and doors from the previous owner. Only adding a glazed gable end to close off the roof against prevailing winds. This had once been a completely open balcony with a view over the garden below.

These results make me even more determined to distribute smaller manifolds elsewhere to reduce the pressure (sic) on the existing IB enclosure. This would also better distribute the bass and quite possibly overcome the present trough around 150hz. My attempt to use 8 drivers has killed the IB goose which laid the golden SQ eggs. Such flagrant abuse of the IB enclosure sizing rules deserved no better reward.

Given a solid concrete box in a basement to contain the pressures involved I might have got away with it. Probably not. It would become a large sealed box rather than an IB. IB drivers aren't designed to fight an air cushion. It is not worth reinforcing the baffle wall as it stands because the antique, glazed, double doors are by far the weakest point. Totally rebuilding this area as a heavy stud wall with a single, modern, fire door might improve matters but I have no such plans. More light would have to be found to replace the large area of glass.

My wife informed me that she could clearly hear Bass Outlaws "Stereo Bass" at the bottom of the garden as I played it at an indicated 100dB(C) on the Galaxy SPL meter.. This never occurred with only four 15" drivers. So the 8 driver IB must be putting far more energy into the roof structure. This is not a criticism of the IB principle. It just demands far more respect than I have been showing. I pushed the boundaries of undersized enclosures almost to destruction. I lost (almost) as much as I gained by doubling the number of drivers. Though the SQ still improved. Note to self: I must try harder not to break the (IB) rules. ;-)