Thanks Evosilica

Dunno. Digital allpass filters for EQ or crossover tasks can be made with very short delay times (one sample dalay) but with a low center frequency and "rather long" decay time. That would be a high Q allpass filter, a very resonant allpass.

For typical reverb usage one could use the same structure, but use a longer loop delay for longer decay time with "not as much" feedback. Lower Q (but still rather high).

I don't have deep understanding of the topic, but ringing allpass filters can satisfy the allpass requirement and still sound rather bad. Which is part of why it is easy to make an allpass filter but difficult to make a truly good sounding digital reverb out of that building block.

For instance a 15 foot room dimension would be a 13.3 ms delay element. A digital allpass has equal amounts of feedback and feedforward surrounding the delay. Similar situation with analog allpass so far as that goes. The feedback alone would make a comb filter, so far as I know the same mechanism which causes room modes.

If one could arrange (perhaps via dsp and/or multiple subs) a feedforward which can "undo" the comb filtering and restore the allpass characteristic, then perhaps one could devise an acoustic situation where the frequency response is fairly flat but the time response is maddeningly long.

That is why I earlier asked SteveGTR how much electronic tweaking he used along with the four subs. Apparently some tweaking was involved. If he had said "oh, we just hooked up the four subs and it was instantly flat" then it would have been discouraging that I hadn't tried that "the very first thing".

Steve said his results flattened the FR without increasing decay time. My crude 2 sub tests increased decay time along with un-doing my null. But my room would be fairly inconvenient to place four subs, even small ones. My room also is perhaps "too open" having two always open doorways in the rear wall. It is good for dumping some excess bass into the rest of the house, but OTOH results in rather long-delayed low level reflections re-entering the room from the rest of the house thru the same doorways.

It results in a "2 slope" impulse response with fairly fast initial decay, but a longer low-level impulse response tail. The room doesn't sound so bad for a low budget amateur effort, but the low level bass decay looks rather long on waterfalls. Maybe if the small room was "fully enclosed" the bass decay would look better and respond better to multiple subs. Dunno. In other words, in my room maybe there are not enough walls for the multiple subs to interact with.