I have a theater that's just starting to come to fruition with an existing customer. The room is terrible, basically an echo chamber.

Dimensions are 19'7" x 19'7" x 8'10", I tried to get him to let me bring the side walls in, but he insists on having at least 10 seats, row of 6 and row of 4 so its going to stay wide.

I can build it out however I want and treat the walls with anything I need. There's going to be some nice audio gear going in and I want to get as much out of the room as I can. He'll spend the money if needed on real engineering plans, but I'd like to explore not having to hire it out.

Any one ever deal with a room that's this square?

The fact that two dimensions are the same means you'll have some problems, but the SPL used will matter, quite a bit. At low SPL, it may not be much of a problem but at higher SPL, you're going to need some hlp in the 60Hz range because of the standing waves. If you can make changes to the walls, make sure they aren't totally flat- if possible, make them non-parallel. Diffusion is generally better than total absorption because the latter reduces the total energy and that means you'll need more power and/or speaker sensitivity but I would also recommend keeping an open mind about the pre-construction thoughts that it will be a nightmare because the results can be surprising. I think I posted about a place that I was calling "an acoustical nightmare", but it turned out to be pretty nice, without my doing ANY treatment. The WAF won out over my desire to provide the best sound, so the only things in the room that do anything are a large rug and a very large sofa. The walls are plaster over poured concrete walls with a small space between them, the ceiling is wood lath and plaster, the floor is terazzo. The first thing I said when I walked in was "Wow!" because of the echo- I used the ClapIR app on my phone and saw RT60 of more than 6 seconds at some frequencies, yet the results were very good. I used a sealed sub, which didn't excite the room modes as much as a vented model would (lower Q) and when I walked around the room while music or movies were playing, the bass response was even, not boomy and that goes against all theories I have read/heard.

If you can pre-sell the client on using pilasters on the walls and columns in the corners, you'll be able to make some kind of bass traps to tame the modes, but again, keep an open mind. The seating will help- what other furniture and decorations will be in the room?

I would look for some kind of room mode calculator that shows a graphical representation of the grouping of the standing waves- they should have broad distribution, rather than being bunched in a narrow range.

Equalization helps, but some problems (standing waves) can't be EQ'd out because it's not a matter of simple mid or high frequency reflections- the low frequency energy is higher and it's more a matter of pressurization and resonance.

I have used HAA to help with a very difficult room. The design, acoustical modeling and acoustical materials worked miracles. We are getting ready to design another nice room and will be using them again for design and materials. Below is a link to some pictures of the previous room.

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