From what I've learned, and correct me if I'm wrong, but...
in general, with room treatment...
Smaller rooms require more bass trapping due to the close proximity of adjacent walls.
Since bass waves are stronger than any of the other frequencies, they tend to "crash" into each other and stomp the shit outta your higher freqs.
Small square rooms are especially bad mainly because of the "equalness" of the walls really makin a mess of the sound waves crashin and bouncing. No way you can get a clear picture of what you're doin.
In any room...bass trapping each corner to the left and right of your mixing station will always help the accuracy of your listening triangle.
sound about right so far?
Small rooms are especially bad because the frequencies of the various modes are based on the dimensions of the room and smaller dimensions move a lot of isolated modes up into important audio frequency ranges. An 8 foot ceiling gives you a first axial mode at roughly 70 Hz, which is about the D string on a standard 4-string bass.
Modes start to get denser when you get to higher orders (and higher frequencies), but in a small room you might have to get well above 300 Hz before the modes start to get dense. The mayhem will typically be from 30 to 150 Hz. In a big venue, the modes may be dense by the time you get to 20Hz.
You can't generally control it unless you build your own space, but generally you want the lower modes to be uniformly spaced (at least 5% apart so they aren't degenerate, but not more than 20 Hz apart, so they don't act as isolated modes. Then, of course, you need to strategically install bass traps, starting with corners and then moving to other key spots (edges, etc.) until you can't stand to give up more space or run out of cash!
Square rooms are especially bad because the horizontal modes from the two directions will coincide and reinforce.
Dense, thick walls are great for isolation.
They also strengthen the room modes.
Generally, you really can't do a lot about the existence of the modes. If you can get the relaxation times of all the modes short and relatively consistent, then you have done about the best you can do.
BTW, if you want a handy reference, the
Master Handbook of Acoustics by F. Alton Everest is a great choice.
Cheers,
Otto