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I am struggling with my environmental conscience in having a large area of uninsulated, paper-thin driver cones and an unheated and poorly insulated IB enclosure. More than that I'd like to perforate the ceiling to use the remaining roof space for an enclosure. This would allow me to reduce the number of drivers in the present manifold. Which would solve the problem of having an undersized enclosure. The enclosure is ideal for a 4 x 15" (or two x 18") but not 8 x 15". My previous house curve has evaporated with the addition of four more 15" drivers. Using the roof space would require that I make large holes in my 12"-16" of rockwool. I carefully placed every single sheet of the multiple layers of insulation and snuggled them together when I rebuilt the roof. This was following a severe storm some ten years ago. It would be thermal suicide to make holes in my lovely insulating blanket. Which would probably result in massive condensation and heat loss in winter.
So what to do? The older (4 x 15") 3/4" manifold had already sweated inside in winter as the warm, moist air from the house condensed on the cold plywood. I have already had to reverse the magnets-outward drivers to bring the massive lumps of freezing cold metal back into the warmth. As they lagged behind the temperature curve of the enclosure they literally dripped onto the enclosure floor! Now my manifold is made up of two layers of 3/4" plywood so it is better insulated than before. I can't easily cover the drivers in anything without stopping the vital air movement. On the other hand the inside of the manifold acts as a giant compression driver. Its power to move lots of air is concentrated in the manifold opening. Could I use this pumping capacity without spoiling the IB? A DPM (damp proof membrane) would also be nice to stop the damp indoor air from ever reaching the drivers and the insides of the box...
I'd made a large ABR in my younger days but lacked a suitable driver to get it moving at the time. This had consisted of a piece of polystyrene foam ceiling tile with a soft rubber suspension cut from a baby's cot, rubber underblanket. The ABR had moved nicely on its suspension provided it was vertical. Could I make a large ABR to fit in the mouth of my manifold? What would be its effects on air movement? It would have an unknown mass and compliance leaving considerable doubt over its natural resonant frequency. Excursion might be dramatic! The foam would certainly insulate and the area of the suspension would be very much lower than 8 x 15" naked cones. The problem would be to make it damp proof as well. If the insulation reduced the temperature inside the manifold it would be even more prone to damp as the ABR would let moisture through unhindered. Covering the foam to make it damp proof would considerably increase the mass lowering the driver's resonant frequency.
Given a big enough wall it might be fun to build a huge ABR to sit on a frame in front of the IB to be driven by air pressure. Who knows what the effect would be? My triangular baffle wall offers no space for such a device as the tall manifold already reaches the 45 degree sloping ceiling/wall.
Back to a stretched sheet DPM over the manifold opening idea then. At first I saw a choice between a simple sheet of sponge, polythene or thin rubber permanently covering the manifold mouth. The sheet materials would be fixed to an airtight frame with just enough tension to keep them taught to avoid sagging or actual rattles. The sponge would allow moisture through. Being able to breath would reduce the risk of sound quality effects at the cost of likely increased condensation problems.
A thin sheet of rubber would be airtight and moisture proof to a much higher degree compared with sponge. Its mass and tensile strength would be considerably higher than sponge too. But would it flex/stretch enough to make it disappear as far as the drivers were concerned? Too high an internal manifold pressure would raise the drivers free air resonance (Fs) considerably. Probably resulting in a serious loss of lower bass and introducing compression effects. There were a lot of unknowns whatever I tried but at least I could monitor the effects with REW. Would a single sheet of thin rubber insulate enough (by trapping still air inside the box) when not in use to make the exercise worthwhile? What if I made the rubber sheet considerably larger than the manifold opening to reduce internal pressure effects? Polythene sheet would be easy and cheap to do but it would be likely to be very noisy and inflexible unless very thin. Its insulation qualities would be minimal.
I had already discounted a well insulated cover for the manifold opening. The removal of a cover in cold weather would cause a rush of warm, most air onto the metalwork resulting in massive condensation. I'd need a fitted drip tray in the bottom of the manifold with a drain! Heating the manifold artificially is out of the question too. Nothing sits on standby in our house.
A much more practical alternative might be insulated covers for the driver panels of the manifold. The metalwork of the drivers would stay warmer from being indoors. So it shouldn't affect anything provided the insulation is removed when the IB is active. The rest of the manifold could also be lightly insulated to further reduce the risk of condensation in winter.
It might be possible to have a frame covered in open cell foam fitted permanently over the drivers. A mounting frame around the outer edges of the box would spread the compression effects over a larger area than the manifold mouth but result in a rather small enclosed volume. Deepening the frame might help but open cell foam can breathe rather well anyway. There should be no condensation problems as the drivers and box would seal moisture indoors. It remains to be seen whether the foam can breathe well enough not to affect the driver parameters. Only practical experimentation will prove whether any of this is really worth the effort. I can even monitor the temperature inside the insulating covers to see if this would make a roof manifold practical. Though I doubt 1/2" of foam can compete with 12-16" of rockwool. Perhaps several layers of thin sponge sheet would help to trap warm air between them without reducing the IB's airflow too dramatically?
Whoah! I'm looking at this problem completely backwards! All I really need to do is to keep the drivers on the warm side of the insulation. So simple I wonder how I missed it! An "outie" manifold (with baskets and magnets exposed) on the ceiling. It can have a well insulated, plywood box trapdoor on top to close the mouth off from the roof space when not in use. Normally closed, hinged, weighted, foam sealing strip, operating string through an eyelet in the ceiling ring over a screw hook in the dormer wall? The driver's metalwork always remains in the warmth of the room. Only the extended manifold mouth peeks above the roof insulation. Two drivers at 45 degree in two dormers? One pair up front right? The other pair rear right? Or 4 in a downward pointing pyramid up high at front right? So many choices. :-)
So near yet so far. Winter is already upon us! I can't open up a 4' x 2.5' hole in the ceiling to replace the dormer ceiling with a plywood panel + finished manifold. I'd have to work from above on a roof ladder. Remove the dormer roof, remove the rockwool insulation and clean up well. Only then can the T&G boarded dormer ceiling panel be removed downwards in the AV room. The Head Gardener would have a complete fit! This requires careful planning...
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