.The lack of "killer" bass from my IB required a serious rethink. The array was sitting in a weak wall which would never improve no matter how much I tried to reinforce it by fixing on more 4" x 2" timbers with 6" long screws.
So after much procrastination I finally decided to rebuild my IB as an opposed driver manifold. (box) Just to see if mechanical vibration really was my IB's Achilles heel. The box would be made of 3/4" plywood and I would use a single layer. I would judge how it performed first before beefing up the box with a second layer. I was still full of doubt abut the advantages of a manifold at this point.
The desired manifold size could be cut from a single sheet of 18mm plywood with little or no waste. I carefully selected a nice, flat sheet from the local timber merchant then brought it home in the car trailer. It didn't take long to cut everything to size and rout the mounting holes for the drivers.
Having built my heavy manifold in my garden workshop I suddenly realised that I couldn't get the large box up the narrow stairs to my attic AV room! I should have glued, clamped and screwed the box together right where it was going to be sealed into the baffle wall.
The heavy 48" x 24" x 16" box had to be run across the garden on a sack truck. Then slid up a gently sloping ladder and in through an open window. Straight into the IB enclosure. After a lot of heavy panting (and rest for a cup of coffee) I could push the manifold between the vertical timbers of the stud wall. A piece of advice: Wall studs are rarely straight without some twist or bend. Make your manifold width slightly undersized or be prepared to hack some studding away with a large chisel! This noisy exercise will not endear you to your partner. Who will assume that the confidence you had in your careful calculations was completely unfounded. They will think that the IB will be a total waste of time and money! It will not be, but don't give them any rope to hang you by at this critical stage. Build a slightly undersized manifold and then use wood or plywood packing strips. Or strips of foam plastic pressed well in. These deliberate ruses will impress your better half no end. You will appear to have everything completely under control no matter how poor your woodworking skills.
The maintenance of the balance of power is vital in an HT/AV/Hifi household. Diplomacy before a war of words will avoid expensive concessions which might be much better spent on equipment upgrades. Being allowed to make large holes in the house to install subwoofers goes completely against the nesting instincts so tread very carefully to avoid breaking too many eggs.
After the struggle to get the manifold between the twisted old timbers I had drilled holes for the speaker cables and fitted the drivers. Depending on your driver orientation it can help to fix a small batten to support the driver rim safely while you get the first fixing screw in. Once achieved you can take your time as you go around fitting the rest of the screws in place. Drivers are strangely unbalanced objects and can often benefit from a bottom fixing screw first rather than a seemingly more logical top one. It all depends on whether you fix your drivers from the front or rear of the basket rim and whether you have somebody handy to help you at this stage. I found myself having to climb right into my manifold while my wife gingerly handed me the drivers. A situation of the utmost gravity!
Once safely in place the drivers could be wired up to the amplifier again and double checked for phase. The drivers must all move towards the centre of the manifold or they will cancel each other out. Having managed this error a couple of times when moving the drivers around or rewiring after REW tests I can vouch for the lack of bass if one gets the polarity wrong.
The effect of changing to a manifold from the array was quite simply staggering! I nearly fell over with shock the first time I fired it up on a favourite bit of bassy rock music! I had mistakenly used the same amplifier settings as the baffle array. I suddenly had the bass that an IB subwoofer is famed for. The floor shook like a diesel truck on test tones. (or testosterone if you prefer)
Now I had to reduce the bass considerably to match my floor standing speakers. With the array I was always fighting for more SPLs from the IB to match the speaker output. Now I had far more bass than I could tolerate without quickly losing my nerve. It really was a night and day difference.
More importantly the wall and old French windows no longer vibrated an inch back and forth! The structure around the manifold remained almost completely free of vibration regardless of output level or frequency. The single layer of 3/4" plywood seemed quite stiff enough until I tried a 100dB(C) sinewave @ 10Hz. Even then there wasn't enough flexure to worry me unduly. The bass out in the IB enclosure also seemed to have dropped considerably and was very much less audible in the rooms below.
Just in case there is still any doubt, the manifold in an IB is not like a sealed speaker box. The manifold is certainly not the IB enclosure. The manifold merely holds the speakers rigidly in place. One side of the box is completely open to the AV room. All the drivers cones are exposed on the other side to the large IB enclosure. (Which is about 17 cu. meters or 600 cu feet in my case) The open side of the box behaves like a single, large, compression driver. The air vibrating in the mouth of the manifold produces all the bass in the AV room. Doing so with the combined power of all 4 x 15" loudspeakers simultaneously. A safe minimum number of drivers for an IB which can be exceeded at will.
The large volume of the IB enclosure means that the drivers' natural resonant frequency is not raised by an increased air pressure cushion pressing back on the cones. This allows an IB to perform to much lower frequencies at much higher SPLs than any other conventional subwoofer design. It also uses much less power into the bargain. The IB has the unique and often enormous advantage of not taking up any space (at all) in the AV room. Those who take their music and films seriously are often torn between building a huge subwoofer and their partner's natural reluctance to having such things standing in "their" living rooms. Under such circumstances the IB may be your giant subwoofer get-out clause.
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