Throttling the beast

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.


Gilding the lily

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 45 degree, sloping ceiling. The enclosure 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.

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The big lift

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.


Polishing the gilding.

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.



If it aint broke..

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.

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.
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:

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!



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!

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

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.

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.

I thought this was interesting:

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.

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?



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!


The + 3dB shelf?

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!




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

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

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... 


IB Distortion

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. ;-)