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This is due to a node or anti-resonance in this region at the listening position in Wes's room. It does disappear in front of or behind the listening position.
This a little strange, as this kind of measured behavior is generally very position-dependent. Is your room rectangular, or does it have recesses and alcoves, or openings into other rooms?
John Atkinson
Editor, Stereophile
Yes, strange and very frustrating. BTW, thank you for your response! The room is 21 ft long, 16.3 ft wide with a nine foot ceiling. The speakers fire down the long dimension of the room. There are two sets of doors leading from the room, one to a library/office (back of room) and one to the foyer (side of room). Both sets of doors are kept shut while listening. Closing the doors improves bass response by 1-2 dB. I suspect the problem is a large bay window alcove (appx. 9 ft wide and 3.5 ft deep) at the front of the room behind the speakers. I am able to measure high SPL's within the bay window area for the frequencies in question, e.g., 40Hz, but not within the main room itself. Not sure what this means or what I can do about it.
I don't know why this is true everywhere in the room, but I put the dimensions in a room mode calculator and you do have some problematic modes in that region:
34.7 hz
43.8%
32'7", 16'3", 8'2"
(0,1,0 Axial)
Start iso, End iso
39.0 hz
11%
28'12", 14'6", 7'3"
(2,0,0 Axial)
39.8 hz
2%
28'5", 14'2", 7'1"
(1,1,0 Tangential)
Caveat, the actual modes tend to vary from the predicted ones.
I could be wrong, but I'm guessing it isn't the bay window. They do focus sound but at these frequencies the window is << a wavelength.
Calculator is at:
http://www.bobgolds.com/Mode/RoomModes.htm