Originally Posted by sdurani
If you blow across an empty Coke bottle, you can get that small chamber to resonate (make that boooooooh sound). If you enlarge that chamber to the size of your room, it will still resonate (of course at different frequencies than a Coke bottle).
The 15-foot width of your room will have resonances (room modes, standing waves, whatever you want to call them) at 38Hz, 75Hz and 113Hz. Sounds at those frequencies will exhibit big peaks & dips along the width of your room (meaning the bass will sound very different from seat to seat).
Placing a pair of subs on opposite sides of the room (like you have them now) will cancel the 38Hz and 113Hz modes. Moving them in to the quarter-points of room width will additionally cancel the 75Hz mode. Those seat to seat variations will be minimized. The auto EQ in your receiver will thank you for the improved consistency.
Your calculations are nice, simple predictions, but rarely do they translate into real world rooms.
A: You are assuming a perfectly rectangular room with perfectly reflective walls, floor and ceiling. I've never encountered one of those.
B: Tangential and Oblique modes also come into play. They could reinforce, or cancel, axial modes.
C: The listening position is the most important factor for determining whether a mode is a cancellation or a peak. Move the LP a foot or two left or right, front or back, or raise it a foot, and you'll get completely different FR's.
D: In-room measurements are the only way to verify what the room mode calculators predict.
If the room is 43 ft. long, I don't think I would place a sub on the rear wall and a sub on the front wall. If the LP is 10 or 12 ft from the front of the room, that would put the front sub 10 ft away and the rear sub 33 ft. away. Using level-matching would make the rear sub 6 to 10 dB higher than front sub. System headroom will suffer badly.
If the the room is 43 ft. long, I suggest placing the two subs at the mid-sidewall positions. This will ensure that they can be gain matched, and should provide the best headroom capability. It won't provide the best room reinforcement, but it should provide the smoothest in-room response.
mantaraydesign, what subs do you have? Where do you plan to place the seating?