Originally Posted by OpusOfTrolls
It might sound academic, but did you check the ceiling? If it is 8ft, ~210hz is the third axial mode.
It could be coming from the area behind the speakers as well. I do notice the panels on the ceiling in front of them. Use your mic at the ceiling in front of and behind the speakers to look for a resonant peak. Also check the front wall in between the speakers.
A HH resonator might actually work well for this, but if it cannot be placed in the correct area then you should look at other options.
I wasnt going to go into this now, but since offers of help seem to be coming for this, I will plunge into it, at least a little.
Here is what i have done. I got out a sine wave CD and played the 200hz tone (the closest one to 210hz that I could find) and walked around the room with a RS digital meter. I did indeed discover a heightened reading at a place between each speaker at a location just in front of where you see the equipment (CD player / amp).
The spot I am describing happens to be where the ceiling is 93" tall (the ceiling is vaulted, so therefore is a different height at different places in the room), and 93" from each side wall (the room is 186" wide). This spot also corresponds to just after the ceiling cloud stops and where those large side wall aborbers end (except at floor level). The front wall is 72" away from this point.
What I havent worked out yet is whats special about 93" other than its a point where the height of the room is exactly half its width. While there is certainly something going on there, I havent figured out what approach to take with it. I did take (2) large pieces (2'x4') of 2" OC703 and spaced them 16" away from each side wall (1/4 the wavelength of 210hz = 64") and took before and after FR reading with seemingly no difference. I HAVE noticed that blocking the front wall (2-3' away) with the same OC703 panels DID make some difference, but only 4db or so.
Like said before, I havent given this my full attention yet. There maybe something obvious that ive just missed. I havent even thought it through far enough to say if its a modal or reflection problem. To this point, ive been working on the ETC almost exclusively. Thats why I only included that graph in my initial post. The ETC part of the room equation is the only part I feel ive pretty well mastered for the room.