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I have been watching a lot of films and listening to lots of music since I fitted the new drivers.
The new drivers have greatly improved the performance at the bottom end. The IB is also incredibly quick on LFE impacts. I watched Elektra for the first time and the shocks through the floor were almost painful to my feet. Brutal hammer blows with zero overhang! Since my IB is built into the wall this is purely an acoustic effect of pressure waves acting on the floor. The manifold itself does not move physically to driver reaction forces so the floor is not receiving any vibration by mechanical conduction from that source. Nor is the floor well connected to the IB enclosure.
The much smaller BFD filters I need now are not sapping the power as much either. I have gained about 8dB over my old drivers which had used massive bass boost at 20Hz. (+16dB) The new drivers are also seeing more power because they are in parallel pairs for 4 Ohms impedance per channel instead of series pairs per channel for 8 Ohms. This has offered a very worthwhile increase in output. On a couple of films with really "hot" bass levels I have even backed off the bass by a further 6dB to increase comfort levels and headroom. Relative bass levels are very much a matter of taste. Which depends a great deal on programme material and one's mood.
I love the way the IB locks onto the bass drum and bass guitar on rock music. There is absolutely no sense of the subwoofer being off-centre. Listening to Satriani CDs at a steady 100dB(C) is certainly entertaining these days. The clarity and impact on drums is excellent. The IB cones aren't visibly moving at these levels. Perhaps a fraction of a millimetre. It can be felt with a fingertip on the cone surround but it is not usually visible even watching from close to.
Only film LFE provides the levels and frequencies which get the cones moving and then only rarely to a serious degree.
An update:
Well, I seem to have found my new drivers' absolute limits. The four AEIB15s seem to be capable of nearly 120dB(C) (Galaxy 140 SPL meter uncorrected) measured at 8 feet from the manifold. Watching the Fantastic 4 DVD I heard a couple of deep knocking sounds when the heat seeking missile exploded on the river barge. When I checked the levels again (after reducing the subwoofer gain by 6dB for safety) I hit 112.8dB(B) on Max hold/Fast on the SPL meter during this scene. Adding the 6dB back on, there is no doubt that the IB subwoofer was giving out well over 120dB(C) if the REW SPL meter correction factors are also added in. Until then the whole room had really been shaking. There were times when the floor went completely soft beneath my feet and the air itself could be felt shaking vigorously.
I approached the manifold at one point in the film with a torch to check the drivers' excursion and found I could hardly catch my breath during LFE rumbles. The acoustic vibration at the manifold opening was incredibly fierce yet the drivers were hardly moving. This reminds me of a kind of forced air heater used to warm or dry out large halls and buildings after a flood. These industrial, forced air fan heaters produce vast quantities of VLF and infrasonics. The effect of standing near the manifold is rather similar. In fact an acoustic furnace is quite a good description of the manifold during action films.
Occasionally I notice some strange transient cone movements on films which produce well over 1/2" (12mm) of excursion but nothing audible is heard. There seems to be nothing on-screen to suggest any reason for these rather odd transients. The speaker cones shoot out simultaneously and then return slightly more slowly.
Needless to say I am absolutely delighted with the performance of my IB despite the slight possibility of having bottomed the drivers. Though it may well have been a structural rattle. The bass levels were truly shocking until that point and I see no reason (whatsoever) to criticize this level of performance.
Since I seem to have found the safe limits of my IB I ought to press ahead with the second manifold to gain some extra VLF headroom. The LFE effects I like. Risking these amazing AE IB15 drivers is not my purpose. John E Janowitz the designer and builder of the AE speakers claims that these drivers can't be physically bottomed so the knocking sounds I heard may be sympathetic structural movements. The double doors out to the IB enclosure are the most likely culprits here. It is amazing to see glazed wooden doors bend back and forth by an inch as they used to do with my array. Leaning my full weight on them and pressing hard with my shoulder or hands had no effect at all on this flexure. It may be that the doors still flex to sudden acoustic pressure waves on film LFE transients.
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