Experimental manifold extension mid bass hybrid IB.

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A thought experiment which is likely to produce copious sawdust.
Read into that what you will:

What if a typical, closed end IB manifold was extended well beyond the norm? This suggestion often crops up with the fledgling, wannabe IB builder. He/she[?] has a suitable IB enclosure volume nearby but not in close enough proximity to the desired subwoofer position. Amongst these wannabes I must include myself here.

My present 8 x 15" driver IB manifold sits in a corner a couple of feet to the left of the left Main speaker. I would much rather the manifold was somewhere between the speakers. Being so far off axis it badly affects the frequency response by cancellation with the far [R] Main speaker. Leaving a severe frequency trough between the IB output and that of the Main speakers. This trough coincides with the maximum energy area of the upper bass spectrum. Precisely where one normally feels a nice thump in the chest from loud rock music, drums and bass guitars.

The present 6' tall manifold IB can easily lay waste over a large area but lacks this desirable wallop. It is insanely powerful and clean in the infrasonic and lower bass. It just lacks that something on rock music. This area is usually covered by powerful speakers. My Mission 753F floor standing speakers have been measured repeatedly at 110dB(C) but just can't do chest thumping bass.

The sort of bass you hear regularly at Hifi and AV shows. The Wilson Watt Puppy and M&K pro speakers excelled at this sort of thing. As can many others of course. I am not in the market for better speakers. I like what they do. Many speakers strike me as too hard and too bright. Certainly impressive, but difficult to live with in the longer term. I hardly need to mention the high cost of such transducers. Their size alone makes them difficult to house in my attic AV space.

Which brings me nicely back to the real world. Extending the manifold before it joins the large IB enclosure volume would be such an easy option for many of us. Yet the received wisdom is that manifolds be kept as short as possible. The exact opposite of my plans.

The basic idea is shown alongside. Only three drivers are shown instead of the intended four. My skills at drawing matching circles to scale with only a mouse are rather limited.

Such a strange layout as this would not be dissimilar to a huge reflex enclosure but with the driver[s] effectively closing the rather large and ridiculously long port. One could even build a large tunnel straight out from the speaker wall. Then arrange the drivers on the closing baffle. Or placed either side in an opposed driver manifold. Perhaps to bring them into much closer proximity to the main speakers to avoid phase issues.

Since multiple drivers would/should normally be applied to any IB the foolishly long "port" of this hybrid IB suggests they [the drivers] could easily be staggered. Arranged along the length, in a linear array provides a number of advantages. The deliberate spread of the drivers might well confuse the sharp tuning of this port as a closed 8' organ pipe. [@64Hz] Any likely pipe resonances should be killed by the multiple driving points along its length. Each driver being fed the same signal simultaneously should help to break up any fundamental or early harmonic pipe resonances.

The main problem is likely to be the extremely complex pressure waves in the "pipe". Each driver will see a constantly changing level of back pressure depending on the phase and frequency of the wave fronts at any particular moment. The rear pressure wave output from the drivers should hopefully be lost in the adjoining and heavily damped enclosure through the pipe's open mouth.

Though there is likely to be a reflection of each pressure wave from the exit opening back up the pipe. The problem is the sheer complexity of these pressure waves. Would the pressure waves/pulses effectively turn each driver into an ABR? [Auxiliary Bass Radiator] Albeit ABRs which are under the active control of the drivers' power amplifier. Rather than imitating the floppy cones of passive ABRs.

In my own case I would have the option of simply halving an 8'x4' sheet to produce a long, horizontal baffle. Each half could be glued and screwed to the other half to effectively double the baffle for extra mass and stiffness. The upper edge of the vertical baffle would rest against the 45 degree sloping ceiling of my attic AV space.

A simple batten could be fixed along the floor to stop the baffle from sliding outwards and further improve its sealing against the floor. The top of the baffle would rest firmly against the ceiling thanks to the weight of the four 15" drivers. Possibly requiring no mechanical help or fixing.

A good seal is readily ensured using simple weather stripping foam, draught excluder. The cost of the whole project is as low as obtaining a cheap 8' x 4' x 3/4" sheet of suitable material. Or rather its metric equivalent. [2440 x 1220 x 18mm]

The drivers can be spaced along the baffle entirely at the builder's own whim. Allowing for plenty of batten, or even concrete, reinforcement between the drivers.

Such an arrangement would offer a very small cross sectional area to the 'pipe.' In narrow musical instruments such as an oboe or saxophone, a narrow pipe produces a strongly nasal tone. This is due to the spread of the many, quite dominant harmonics. [Though the reed also helps of course] The fundamental is not nearly as strong in a narrow pipe compared with a much larger area, organ, flue pipe.

Is all this a good or a bad thing as far as an IB is concerned? Probably all bad. I have been warned by those far more expert that it will not work. An ideal IB offers zero back pressure and no reflecting surfaces anywhere near the driver cones. I am deliberately going against this advice and building one to put myself beyond any further doubt in the matter.

How will it perform in practice? Well, it should have much lower back pressure than any box small enough to fit on my stage. So it won't need massive power to reach maximum cone displacement. I have never heard of a reflex enclosure or hybrid where the drivers are fixed in the port. If it were a pure reflex then it would be tuned incredibly low! Single figures of Hz is a very likely tune with 600 cu feet working against an 8' long port.

It certainly isn't a tapped horn because no tapering is involved. What about a bandpass box? It has vague similarities to the 4th order bandpass. Except that the drivers are not sitting in an internal baffle and the enclosure/port is overlong and a continuation of the same cross sectional area. So the likely frequency of the pass band is a complete unknown. It isn't a quarter wave anything because the drivers are staggered along the length.. Nor is it a labyrinth. My hope is merely that it will offer the drivers a smooth, low pressure, resonance free environment to do their thing within a fairly narrow, upper bass, pass band.

Fluffy insulation could be laid along the floor of the pipe below the drivers and fixed to the sloping ceiling above. This would help to reduce direct reflections from passing straight back out through the cones. If enough of fluffy stuff were used it might also soften any remaining pipe resonances. There is no problem in filling the entire tubular space if it helps to smooth things out.

The proof of the pudding is easily achieved by obtaining any suitable 8' x 4' sheet of plywood, chipboard or MDF to give the idea a try. If it performs as an upper bass augmenter then I shall be happy. I really don't care about an extreme bandwidth provided it adds some serious upper bass wallop.

The most obvious mechanical downside is the reaction forces of having four 15" drivers facing forwards on a simple baffle. This arrangement of drivers has become a horizontal line array. Albeit one housed in a seriously stiff and heavy part of the roof structure. Fixing the baffle to the rafters might prove necessary if the baffle vibrates badly in use. Fortunately appearance hardly matters in my case. The baffle will lie along the floor in the darkness, behind the concealing curtains of the overhanging ceiling area behind the LCD TV.

The existing BFD can be used to flatten the response electronically. Though I have no plans to boost the low bass as has been the norm with the older, 32Hz AEIB15s when housed in the manifold. This should help to reduce the reaction forces by rolling them off much earlier.

The open end of  the baffle/pipe will exit into the existing IB enclosure. A 600 cu ft storage area. The other end will be sealed from the AV room with a triangular plate of plywood or MDF. I shall begin with the substandard, vinyl coned, 32 Hz AEIB15s. I have no need that these drivers dig very deep since the infrasonic area is already taken care of by the later 13Hz AEIB15 drivers. Which are presently sitting in the lower half of the manifold. The box will be halved in height if the experimental layout works well.

My personal IB battle has always been the lack of a space above or below the speaker/TV platform. There isn't any useful IB enclosure space above. If there was it would chill the room through the very thin, speaker cones. I rebuilt the roof myself just to put 40cm /16" of rock wool up there. So I am not about to make large holes in it now!

And, if the experts are proved right, all I have lost is the cost of the materials. Plus the fun time wasted in cutting the holes and fitting the drivers. This daft idea has been nagging me for years now. Unless I try it I will never know how badly it performs. These older, vinyl coned, AEIB drivers are [probably] unsuitable for smaller boxes and were rather disappointing in open baffles. If this idea works better than expected I could always swap the drivers for the newer ones for more infrasonic bass. Though the infrasonic performance may be limited by the very design I intend to build.

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How are the mighty fallen!

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My Sony S-790 BDP is fast becoming just another piece of crap. Its drive shakes, rattles and vibrates. It's getting progressively worse! The racket can be clearly heard above quieter moments in films and music. I measured the background noise in my room at about 38dB(C) The Sony produced 57dB(C) while playing a perfectly normal music CD! The player will have to go back. Probably to linger over-long in a busy repair workshop before being returned with exactly the same drive and a claim of no unusual problems. My, presumably much cheaper, DVD drive in my PC is silent when spinning exactly the same disks.

The Sony player was made in Malaysia. Unless Sony has suddenly developed a foreign factory, as a sign of of its charitable kindness, these players are made there because wages are lower than at home. In other words: They pay slave wages by Japanese standards and the staff are treated contemptibly. Presumably that would explain Shimano having their (expensive) bicycle pedals made there as well. It may even be the same factory. With armed guards to stop the overworked staff from committing suicide by jumping off the factory roof.

Despite the savings in wages and avoiding the expense of proper working conditions, in much of Asia, there is nothing new for the gullible customer in paying over the odds for crap products. Not just in the Japanese home electronics industry either. I have a British Naim NAP180 power amp which has to have the mains plug inserted at the rear every time I want to use it. Switched mains sockets are considered bad karma in subjective audio circles. But not (apparently) the essential switch on all mains powered equipment. It's probably required by law. So the few  remaining (endlessly gullible) subjectivists can only repeat the old mantras and turn a deaf ear.

The Naim isolated push switch design is aimed at some sort of unspoken but completely impractical ideal of isolation. The switch actually floats on its connecting cables, within the case and is actuated by an offset, plastic plunger. Nice theory until the switch will no longer keep still as it dangles limply on its own leads. Then the front panel, push button, on-off switch moves uselessly in and out without having the desired effect.

I thought I had finally solved the problem with a rubber packing piece between the switch and the inner drawer of the metal case. However, my fix was short lived and it was back to shoving the plug up the arse of the amp every single time I want to use it. Naim usually insists on leaving their equipment constantly powered up to obtain the best audio performance. As a committed believer in Global Warming I cannot bring myself to use that get-out clause for totally crap switch design!

Another odd thing about the Naim amps is their highly resonant alloy cases. The outer sleeve is made of a single, tubular, rectangular, alloy extrusion. It rings like a bell! So much so that simply switching the amp on produces a loud metallic ring with exactly the same tone as the tubular case bell! I kid you not! Even plugging it in, rather than using the completely non-functional on-off switch, produces the same, loud, metallic kerrang!

Naim's claim to fame doesn't end there though. It uses large diameter, construction kit, control knobs. These knobs have a plastic face plate held only by friction within a rubber band fitted around the plastic rim. Of course, you've guessed it, eventually the rubber stretches and the amp now has no security for its knob's face-plates. Last time I looked there was quite a market in overpriced, Naim style control knobs on eBay. Since the control knob face plates hide the fixing screw, which holds the bløødy knob on, then gluing the face plate on with something like super-glue is a probably a very bad idea.

There was a time [in the last century] when Linn turntables had a mutually parasitic association with Naim amplification in Subjective Circles. Until, inevitably, Linn decided to branch out from its mechanical roots and build the electronics as well. Most of Linn's reputation was built on the infamous LP12 Sondek turntable. It was far more expensive than competitors, like Thorens, but laid claim to improved sound quality over "lesser" record playing devices. The subjectively orientated audio magazines bought into this line, hook and bias counterweight.

I myself had bought heavily into the new "Subjective Audio" movement and started with a Thorens TD150AB. Which I heavily modified with sound absorbing materials. My later purchase of a Linn Sondek with a Basik arm did not provide the immediate satisfaction I had long hoped for. Repeated set-ups by the shop staff did not excite. Even the factory staff could do no better when they had an upgrade session at the local HiFi shop. No doubt the set-up free-bee and new springs was in the hope of sales of foolishly expensive, upgraded arms and cartridges to match their magik touch. BTW: Linn liked to add a trademark 'k' to every product name to separate itself from normality. So your immediate reaction to my obvious misspelling is only half correct.

Linn Son-deks were supposed to be "set up" in a unique way which could be heavily charged for. And was a nice little earner for many Linn dealers. Lot of mystique and snake oil were essential for this task. There being no official set-up instructions provided from beyond Hadrian's Wall. Not even to those LP12 owners who lived well beyond a grasping dealer's reach.

Those dealers with true witch doctor powers over the Sondek were much admired and repeatedly visited for the "laying on of the hands." The Linn Sondek would then [allegedly] achieve levels of musical rapture which no other turntable could possibly manage. We must not be cynical here and suggest that a ride home in the car afterwards might just undo some of the expensive "magik."

After a few years of love-hate relationship with my Linn Sondek I had it "upgraded" with new drive electronics. Plus an inner platter and bearing set, much more expensive Linn arm and a very expensive moving coil cartridge. I also purchased the supposedly well respected 72/180 Naim pre-power amp combo at the same time. All from the same, highly regarded Linn/Naim dealer.

Imagine my surprise to find that when I got home that the outer platter now wobbled up and down at the rim by several millimetres! [1/8"+ in old money] That would be the Linn ultra-high precision, platter shaft engineering, would it? The cheapo Philips drive motor still ticked incredibly loudly. Just as it had done from brand new despite several replacement ball bearings and new springs. Nor would the arm board ever sit straight in the [deliberately] sagging top plate cut-out.

Perhaps I shouldn't dwell on the roughly carved slots of the Linn's Afrormosia plinth. It must have been a Friday afternoon at the factory when a vat broke at the local whiskey distillery and the contents could only be saved by lapping it up. It took me hours of rubbing at the slots with a folded piece of sandpaper to get rid of the ugly raggedness. Given Linn's claim to owning expensive CNC machinery I cannot imagine why they needed to have a registered blind person hand carve these slots with a safely blunted chisel. [In case they cut themselves, silly!] 

I can't say the rebuilt turntable sounded very much better for all the added expense. In case you are wondering: I ended up using cellotape to pack up one side of the inner platter so the outer rim rotated evenly by eye. The outer platter still rings like a bell and corrodes continuously to powdery white ugliness unless constantly repolished. That will be Linn's special, low resonance alloy. Fabled to be left outside for months to weather in the special, Scottish climate. I have often wondered if it made any audible difference if the platter storage rack faces north or south? No doubt there are those in the subjective audio movement who can quote chapter and verse on such matters. What-ever. 

Meanwhile, back at the Malaysian sweat shop: The Sony's [supposedly] touch sensitive top plate has taken umbrage with my earlier criticism. It now refuses to switch on, or off, with a stroke of the hand. Well, not with my hand, anyway. I have tried both and it seems to prefer the left even though I am right handed. When the player does work it grows increasingly sensitive about its [pristine] quality requirements in DVD disks. We borrow free DVD films from the library but are constantly plagued by stutters and total freeze-ups. The drive continues to rattle like a North Korean tank running on the gas purged, by force, from several million empty stomachs.

PS. The Sony BDPs790B has gone back to the shop [independent workshop] to be repaired. It had added freezing to its usual box of tricks. Requiring the power be switched off at the wall to reset it. I tried changing the disk but that didn't help. Even the film elapsed time clock on the little screen packed up. Leaving us with just a series of hyphens. The picture freezing and hiccuping was the last straw after a year of listening to the Sony disk drive rattling even above action films! The touch sensitive case "buttons" had also given up the ghost recently. Becoming erratic in their reaction to my increasingly desperate hand movements. Often ignoring me altogether when I wanted to switch it off.

I bought a £50[equiv] Philips BDP1200 as backup and to provide us with our our nightly film watching experience. At least while the Sony is being repaired. Hopefully it will only need to be repaired once. Though I have very little faith remaining in any of Sony's products. The cheap little Philips BDP offers an excellent picture and cuts nicely through the background crap to the dialogue on Deadwood.  [DTS Master Audio popped up by default.] The first time I have seen it come up on the Onkyo screen. This seemed to lift the background noise at times but the dialogue remained nicely clear throughout. Fans of Deadwood will know how "convoluted" some of the character's voices are.

We have been blaming the Onkyo '818 for the lack of dialogue clarity ever since its purchase. I have been constantly fiddling with the channel levels. Though this provided no relief. We both thought we were going deaf! It seems the overpriced, seriously substandard Sony BDPs790B was to blame all along! The Philips also plays disks silently. Unlike the racket which has always emanated from the pathetic Sony drawer! The Philips also has nice "slow motion" drawer action as it opens and closes which the Sony at three times the price cannot emulate with a following wind.

Perhaps Sony thought they'd throw in a free bass shaker in the drawer. To save the purchaser from having to buy a subwoofer as well? Heaven knows what the Japanese dinosaur thinks it's doing on sourcing and QC these days. Perhaps a few of their QC directors, if there are any, should offer to fall on their swords? I just hope Sony doesn't make swords as well. They'd probably snap in half before they could do any real harm!

Talking of subwoofers: Some fuckwit is driving along a nearby road with his system banging away before 6am! He/it has woken me/us several mornings in a row now. Perhaps they should bring back the death penalty? Perhaps not. How, on earth, would they check whether he was brain dead?

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As if through a glass darkly.

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Imagine, if you will, that you wish to measure the volume of a space for your infinite baffle enclosure to which there is no reasonable, physical access. It might be an underfloor crawl space for example. Access is poor without making a rather large hole in the floor. Even if you don't lose a cat, baby or child down there any sensible partner is going to reach for their coat. Or yours!

Meanwhile, you suffer from claustrophobia and don't own a long enough tape measure anyway. Without somebody holding the other end tightly you cannot be certain that the far end of the tape hasn't followed you. As you dragged yourself along on your belly, with a torch in your teeth,  letting out your tape until you finally hit something solid. Do you return by the same arduous route to check if the tape has moved?

I was inspired to examine a similar measurement problem by the discovery of an old, disused and rather neglected farmhouse. Which we had found for sale while on an outing in the car rather a long way from home. Arranged on only one floor and well over twenty yards/metres long the old house seemed to have as many windows as an early railway carriage. The many rooms ran in a parallel series along a central, dividing wall. So there was no clear view through the property from one side to the other. The old house simply oozed character but badly needed some skilled and expensive TLC.

For some reason I became fascinated by the problem of how any interested party would be able to measure the various rooms without gaining access to the indoors. Would my fantasy farmhouse purchase offer any rooms as obvious candidates for a Home Theatre or music room? Well, one can dream!

Once inside it would be quite easy to rough out a sketch and measure the dimensions of all or some of the many rooms. Locked outside, but with easy access to every exterior window, made it a far more interesting challenge. How could I have checked the room sizes?

The answer to the measurement  puzzle was, of course, a builder's laser range finder. These devices often have a number of preprogrammed tricks to aid us. Though a smidgen of simple geometry might also be needed. Pythagoras had obvious mulled over this same problem too and provided a simple and elegant solution. No doubt he used a long cane marked at intervals instead of a laser. Though you never know.

Let us assume that you have visual access to the interior of a room or other space. Such that you can see the entire width of the opposite wall if you only want to know the floor dimensions and area. If you want volumes or wall surface area then you will need a view of the opposite wall from floor to ceiling. Though some lasers can calculate dimensions indirectly from direct measurements.

Window glass need not be a hindrance to our measurements. Not even double/thermal glazing. Nor need you worry, too much, about the size of your viewing aperture or window. In fact the smaller the better if it helps to increase the accuracy of your laser measuring position. It also helps the accuracy of your results if you have a laser device which can have the reference measuring surface moved to its front edge. This is usually just a matter of pressing a particular button on most of these devices.

Start by taking a simple distance measurement to the opposite wall. Just make sure the spot has hit a normal bit of wall and not a window, jutting fireplace or a recess. Make sure you keep the laser square (perpendicular) to the opposite wall. We'll call this first measurement D for Depth and write it down.

Now we will mentally divide the width of the opposite wall into two lengths A and B. Their dividing point is where you measured D directly opposite your viewpoint. Your viewpoint does not even need to be in the middle of the room. Measure the distance to the far corners at the same height as your viewpoint and make a note of each measurement. These two measurements are your hypotenuses for two, right angled triangles. Hypotenuse just means the longest side of a triangle. You can call these measurements L1 and L2. Or any other name which help you avoid confusion.

If you square each measured hypotenuse in turn and then subtract the square of  D from each you will have two more figures to write down. Squaring just means multiplying a number by itself.

First half wall:
L1 x L1 - D x D = AxA.
Or L1^2 - D^2 =  A^2
(Both mean the same thing)

Second half wall:
L2 x L2 - D x D = B x B.
Or L2^2 - D^2 =  B^2.

Use a calculator, or multiply each number longhand, by itself, to obtain its square. Now you probably do need a calculator to take the square root of each resulting number A^2 and B^2 .

The square root of each will, in turn, give you the widths of the two halves of the opposite wall. A and B. Now just add these two numbers together and you have the full width of the opposite wall.

If you doubt your calculations then try using 3, 4, 5 triangle as your working example. By a very happy coincidence a triangle with sides of 3, 4 and 5 units forms a perfect right angle between the two shorter sides. The 3, 4 5 triangle is much used in building work in the absence of a precision builder's square or laser alignment tools. BTW:You can use any units you like as long as they all the same: 3, 4, 5 feet. 3, 4 , 5 yards or 3, 4 , 5 miles, leagues, meters, chains, nautical miles, light years, parsecs, AU or furlongs. Just don't mix your measurement units.

Back to our real world measurements:  If you multiply the full width by your first measurement D you will have the area of the floor. So you can order a new carpet, tiles, vinyl or lino without ever standing in that room.

You can use the same system for measuring the height of the inaccessible room assuming you can see the far wall from floor to ceiling. Some of the better laser range finders can do this calculation for you indirectly. Simply by taking distance readings from both the top and the bottom of the wall in a pre-selected mode.

Or just divide the height of the wall into two and use Pythagoras just as we did in the width calculation. Except that your two right-angled triangles will be stacked one above the other. With their adjacent sides level with your viewpoint.

You can now calculate the area of each end wall of the room. D x H.
Or the area of each long wall. A + B x H.
Or the volume of the room using the Width x Height x Depth.

An underfloor IB space (enclosure) can be just as easily measured using a laser range finder without physically entering the space. All you need is a clear enough view to point the laser and measure the full width and full depth of the space. Then measure the height and multiply the three numbers together to obtain the volume of the crawl space. Never assume that a crawl space is the entire visible floor area of a room or even your entire home. It is always best to measure it to see if it provides a suitable Total Vas multiplier of at least 5.

The same method can be applied if you forget the volume of the space behind your infinite baffle wall but can no longer access it yourself. Though a suitable hatch or door is useful for maintenance. Or adding sound absorbing, insulation material. Just don't use the access as an excuse for using the space for storage. Not unless you have plenty of room to start with.

No doors, floors, windows or tape measures were hurt in the creation of this post.

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