Originally Posted by Bill Fitzmaurice
The main concern with having two subs in the front of the room is that both will excite the same potential cancellation dip from the reflection off the wall in back of the LP. If the LP is close enough to the wall in back of it, ie., less than 3.5 feet with an 80Hz crossover, it might not be a problem. Otherwise it's usually better to have one sub in the front of the room and the other in the back of the room.
... it seems that several concepts of boundary reflections shall be considered, once placing the subs and speakers:
1) room modes and effect of standing waves
Avoid nodes (nulls) at any cost as they can't be cured with EQ. Avoid antinodes (peaks) if possible, though can be partially cured with EQ. "Problematic" frequencies are dependent on specific room size. Proper placing of seat positions is a solution. Anthony Grimani gives a great talk on this at HTG 178 episode, starting with minute 4.
2) SBIR-effect (S is for Speaker)
Arises due to speaker positioning relative to the wall. Speaker positioning within 1/4 wavelength from the wall leads to cancelation of the frequency with this wavelength (non-modal frequency null). As far as I understand, cancelation (and nulls correspondently) occurs at specific points (where speaker is and each 1/2 wavelength further: 1/4, 3/4, 5/4 etc.). Can be avoided with proper speaker positioning (Bill shared a great reading on that).
Natural question here is: If cancelation occurs only at specific points, rather than the whole wave (as if two waves with 180 phase shift meet), can't it be cured with moving a listener position instead, so it is not in 1/4, 3/4, 5/4 of this specific wavelength? or while cancelation occurs at the source (speaker), then it distorts the whole sound field across the room?
PS. To be more precise, as you move from the line perpendicular to the wall, connecting you, speaker and the wall, things become not that straightforward, because the path of the wavelength changes. But low frequencies (below the Schroeder frequency for the room) wavelength is so much bigger than typical room, that these changes are pretty minor and perpendicularity of the wall doesn't matter.
3) LBIR-effect (L is for Listener) or LBE (Listener Boundary Effect)
Arises due to the listener positioning relative to the wall. The physics is the same as for SBIR: when sound wave hits the wall it cancels at 1/4 of its wavelength (and each 1/2 further), so you need to move seating position to avoid cancelation at the given frequencies.
Thus issue with 2 subs at front wall comes exactly from the 3rd concept (LBIR): at given frequency (seating distance from back wall = 1/4 wavelength, 3/4, 5/4 etc.) the seating area is in null and both subs contribute equally in cancelation at this point due to similar wave paths. Is it correct?