Having built the two new, 2 x15" manifolds some domestic resistance was encountered to cutting holes in the walls. So they were simply planted open side down on the carpet at the back of the stage for the moment. The true IB remained as the top half of the tall manifold with the four newer drivers. A shelf halfway down isolates the drivers from the rest of the box. Heavy cardboard has been temporarily stapled over the open driver cut-outs.
I did some REW sweeps to see how the new boxes were performing alone and together with the IB. As usual these older drivers (Fs32Hz) rolled off very early without serious bass boost. So I added their usual +16dB @ 20Hz 120BW steroid injection with the DSP1124 BFD. This lifted the bottom end nicely leaving a very broad hump peaking around 90Hz. I doubt they will suffer from over-excursion in such small boxes if they aren't harmed in an IB. Besides, they have the four 15" drivers in the IB to support them.
This is the IB + the two boxes + the main speakers up to 500Hz with 1/3 octave smoothing. 120Hz indicated setting on the CX2310, 24dB/0ctave, active crossover. This probably means they are crossing over a bit higher going on past measurements. The idea of the two boxes was to allow a higher crossover point. I shall experiment further with this. The new manifolds/boxes contribute nicely to fill in the former deep troughs between the IB and the speakers.
I have had a quick listen to Loreena McKennitt and some Franck organ music but no films yet. I normally have the subwoofers rather more "warm" than this. Early days yet as I can easily add more bass to taste. The Behringer EP2500 is throttled right back on the control knobs. So has plenty in reserve.
As an afterthought I have switched input channels on the EP2500 to bring the boxes onto the Right channel and the IB on the Left. This should ensure what remains of any stereo information in the bass is maintained. The IB is on one channel and the two boxes are on the other. Bass sounds "drier" and lighter than before but still with good depth. Detail and weight is certainly not lacking.
The REW Waterfall graph generated from the response curve above.
I shall bide my time until resistance crumbles and I can open up the plywood wall to let the manifolds breathe as true IBs. The reason for not opening the manifolds to the under-roof space is thermal loss in winter. I'm considering a plywood slide to close off the openings when the subs are not in use. The drivers are fitted magnets outwards to the boxes so that the metalwork will remain in the room to avoid condensation. I could also hinge the boxes to the wall. Then slip a piece of thin plywood in to seal the wall when the subs are not in active use. I'll think of something to satisfy the "Management Committee Chairperson".
I moved both of the new manifolds right to the back of the stage today and then watched The Matrix Revolutions rather loudly. The manifolds are still operating as grossly undersized sealed boxes with opposed drivers. The combined subs managed to "melt the floor" under our chairs a few times. The polythene DPM out in the enclosure was rippling in time to the visible driver excursion of the IB. I probably saw +/- 1/4" cone excursion (+/-6mm) during the big fight scene in the rain. No doubt it would have been even more exciting with a "hotter" setting on the subs. I suppose I should have used the SPL meter but I was too busy enjoying the film. It's ages since I last saw it. What a truly remarkable example of special effects. One which actually works. Even the actors are well up to the task for an SF action film.
An update: I have fiddled with the phase, crossover point and levels of the two sealed manifolds and the IB relative to the speakers. The idea was to fill the trough around 180Hz. The new curve looks slightly worse than the last one. What is much worse is the sound quality. Lifting the levels on the sealed boxes has muddied everything.
Switching between the subs using the push buttons on the CX2310 it is easy to hear the difference in SQ on organ music. On films and rock music there is more drive and attack. We watched S.W.A.T last night which has lots of rock music tracks. Drum and bass both sounded better. On classical organ there is no detail or subtlety. Just loud "subwoofer" bass.
I'm still getting resistance to the idea of cutting more holes in the house. So I may just return the older drivers to the big manifold. I am thinking of switching the newer drivers to the bottom of the box. The older drivers can go up in the top section. This may not achieve anything useful but it's something I haven't tried yet.
I have a couple of 10" SEAS 25F-EWRX drivers which I built into a stereo pair of 6th order series bandpass boxes. They have been resting unused for years but could be fitted into a couple of sealed boxes just to fill the 180Hz trough. The problem always comes down to the necessary 3-way crossover and amp to drive them. They might add the missing "wallop" which the present 180Hz trough is denying me. I have tried every imaginable crossover point from around 40hz up to several hundred Hz but nothing helps to fill the trough. It is obviously room related but with a twist. The interaction between the main speakers and offset IB is still a problem.
Logic suggests that I need a balancing IB on the right of the stage. There just isn't anywhere to fit one sensibly. There is a box dormer to the right of the stage where I sit at my computer. Beyond that is solid cupboard space on both sides of the central chimney. The ceiling of the dormer is flat with the open roof space above the insulation accessible but thermal losses come into play. It's a shame because it would be otherwise ideal. Probably with room for a compact 4 x 15" manifold up there. I have been over this so many times in my mind but nothing suggests itself. At least nothing that I can easily get away with and still have a clear conscience. :-)
Update: 25th Aug. I have returned the four older drivers to the big manifold.
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