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I read up on bass absorbing panels, membrane absorbers and Helmholtz resonators. All very nice, but with a lowish SAF. How about active traps?
1. classic DBA (delay between arrays)
The one thing I came accross is a DBA (dual bass array), as it is known in German discussion boards (maybe there is another name for it in English, as I didn't get any hits on this board or diyaudio?). The idea is that you have two, preferably four subs along one side of the room, and an equal number along the opposite side. Then you feed the rear array with a signal that has been inverted and delayed by the speed of sound x the distance between the arrays.
So, ideally, you have flat wavefront that emanates from the first array, propagates along the room and is completely absorbed by the second array, i.e. a perfect transmission line. Practically, you would still get some reflections from the side walls and ceiling to break the coherence, so cancellation is not perfect, and you would propably have to attenuate the signal to the rear array somewhat for optimum performance. Strangely, the discussions I have seen are very partisan and do not discuss the implication of those reflections. Has anybody tried this or can give a link to a more serious discussion of the effects?
2. undelayed DBA
An interesting variation on this concept would be to feed the rear array with the very same signal, i.e. not delayed, not inverted, as the front array. You would get cancellation along the center line of the room (or more correctly, the center plane between the two arrays) and have a transmission line from both front array to center line and rear array to center line. If you were to sit close to the rear wall, which is not unheard of, you would hear an essentially resonance free bass (which would be ahead of the midrange signal, so you would probably still want to use delay, but this time on both front and rear bass signals).
The great thing about this DBA setup where front and rear signals are identical is that as long as the room and the arrays are symmetrical about the center plane, you also cancel all first order reflections. Surely, somebody has tried this?
3. active bass trap
Finally, one could use a conventional setup with one or two subs close to the main speakers and place active bass traps in two or more corners. These would consist of a sub and a microphone placed close to the sub, a low pass and a feedback circuit. There is an old patent by Nelson Pass on this, and from his comments on diyaudio, you can manage about 6 - 12 dB cancellation near the trap, which is a lot more than you can achieve with a passive absorber of the same size.
1. classic DBA (delay between arrays)
The one thing I came accross is a DBA (dual bass array), as it is known in German discussion boards (maybe there is another name for it in English, as I didn't get any hits on this board or diyaudio?). The idea is that you have two, preferably four subs along one side of the room, and an equal number along the opposite side. Then you feed the rear array with a signal that has been inverted and delayed by the speed of sound x the distance between the arrays.
So, ideally, you have flat wavefront that emanates from the first array, propagates along the room and is completely absorbed by the second array, i.e. a perfect transmission line. Practically, you would still get some reflections from the side walls and ceiling to break the coherence, so cancellation is not perfect, and you would propably have to attenuate the signal to the rear array somewhat for optimum performance. Strangely, the discussions I have seen are very partisan and do not discuss the implication of those reflections. Has anybody tried this or can give a link to a more serious discussion of the effects?
2. undelayed DBA
An interesting variation on this concept would be to feed the rear array with the very same signal, i.e. not delayed, not inverted, as the front array. You would get cancellation along the center line of the room (or more correctly, the center plane between the two arrays) and have a transmission line from both front array to center line and rear array to center line. If you were to sit close to the rear wall, which is not unheard of, you would hear an essentially resonance free bass (which would be ahead of the midrange signal, so you would probably still want to use delay, but this time on both front and rear bass signals).
The great thing about this DBA setup where front and rear signals are identical is that as long as the room and the arrays are symmetrical about the center plane, you also cancel all first order reflections. Surely, somebody has tried this?
3. active bass trap
Finally, one could use a conventional setup with one or two subs close to the main speakers and place active bass traps in two or more corners. These would consist of a sub and a microphone placed close to the sub, a low pass and a feedback circuit. There is an old patent by Nelson Pass on this, and from his comments on diyaudio, you can manage about 6 - 12 dB cancellation near the trap, which is a lot more than you can achieve with a passive absorber of the same size.
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