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If the infrasonics really are the most important part of the IB's performance then adding a high pass filter at (say) 25Hz should cripple the IB's advantage over box subs. In fact when I engage the 25Hz filters on my CX2310 crossover there is only a very slight loss of impact on low frequency, electronic programme material.
I tried Bass Outlaws: Illegal Bass: "Extreme Woofer Test" at a steady 95 and then at 100dB(C) read straight off the Galaxy 140 SPL meter at the listening position. The non-filtered output was only a little more full with a few more rattles from the room's surfaces. So much for my essential IB infrasonics theory! There was zero visible cone movement with the 100dB "Woofer Test" yet the whole room was shaking and most of the house along with it.
My guess is that the IB's SQ advantage lies with very low distortion and remarkable transient performance. The large cone area is able to respond instantly to programme input due to the very small linear cone movements usually required. The individual cones each have adequate motor power to closely follow whatever is called for with minimum air disturbance.
A single heavy cone in a sealed box is constantly fighting against the resistance of the air in the box and/or the inertia of that in the port. The single cone must move many times as far as the multiple IB cones to achieve the same output. The single cone is under constant heavy acceleration which must make life difficult if it wants to add smaller nuances on top of the major acoustic output frequency. So detail is inevitably lost unless serious cone breakup comes into play. At low frequencies it is difficult to imagine the amount of breakup required to achieve simultaneous low frequency sine waves at different frequencies. The air movement required for each frequency is considerable at high levels. It is no wonder the multi-driver IB is so capable since it can move so much air with no visible come movement whatsoever.
Even a low frequency sinewave at higher levels is demanding in itself. The cone must move to the maximum excursion required for the acoustic output demanded. It must do so in both directions without undershoot or overshoot. How closely the cone can follow the signal is vital to the waveform reproduced. Would you trust multiple, large driver cones (each with their own motor) moving a tiny fraction of an inch? Or a single cone moving well over an inch in each direction and trying to follow and reproduce a number of perfect waveforms all at the same time at various sound pressure levels? (SPLs)
It is interesting that some authorities deny that subwoofers have variable speed depending on the design. They suggest that the rise time for the signal is far too slow for a subwoofer to know the difference. They obviously haven't felt the sharp edge of an IB exciting the floor through acoustic pressure alone on an LFE pulse.
Doppler distortion would seem very likely with a single long-throw cone in comparison with multiple cones moving over tiny distances in unison.
Mechanical noise and turbulence must be higher in a long-throw "air beater". Imagine a small fan whizzing round to move as much air as a large fan moving much more slowly. You can't even hear the large fan as it effortlessly moves hundreds of cubic meters of air per minute. The small fan quickly drives you out of the room with its racket. Or you try to turn down the speed to get some peace. Only to lose the required air movement to keep you cool. So what do you do when you get fed up with fan noise? You put up a huge, slow, ceiling fan for a bit of quietness. The large ceiling fan doesn't even need carefully shaped blades to achieve its low noise cooling effect. It can make do with flat plates arranged at a suitable angle of attack. No wonder a multi-driver IB wins over a single cone.
Ignoring displacement, the question is whether matching cone area by purchasing a stack of large box subs adds up to an IB on sound quality. My immediate thoughts would be that any colouration would be additive with the boxes. The cost of going this route would of course be many times that of the IB with similar cone area. The power requirements of multiple box subs would need a special cable to be run. All of these potential gremlins may be subjectively inaudible of course. So it is just as well this is just a thought exercise and none of my own money changes hands.
If reality follows my musings then the Thigpen fan subwoofer should have a much larger but much slower moving fan. To kill the unwanted noise. The fan blades of this clever subwoofer must deliberately change their angle of attack with frequency. But each angle of attack must require a completely different aerofoil section to avoid stall and/or turbulence. Carefully shaped cowl rings on both sides of the baffle in which the fan rotates would no doubt help reduce noise. Even if I had a fan sub to play with my home is just too structurally noisy to tolerate such serious output at deep infrasonics frequencies. Just playing "The Ultimate Woofer Test" at serious levels already finds every rattle and point of flexure. This strange track has vastly more serious LF content than any other track I have found so far. It sounds completely different and almost innocuous through headphones. No doubt it sounds very different through box subs and car subs too. Played at 110dB through my IB it has the impact of a wrecking ball! Rather like the server room scene in the film "Pulse" but considerably more musical.
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