Converting two-car garage to listening room. What would you do?


My wife is angling to get my mountain of audio gear out of the living room and I have graciously consented. :-) So off to the two-car garage I go, but first it has to be made livable, and we've started talking to contractors. It's not a huge space: roughly 18 x 19.5 feet, with a vaulted ceiling that crests at about 16 feet. I'll be leaving the basic structure in place but everything else is up for grabs: I can play with dimensions (a bit), lighting, where I put the (20 amp) outlets, acoustic treatments, and so on.

My main question is currently if I should "toe in" the walls so as to not make them parallel. Would it help the acoustics if I had four walls that measure, say, 18 feet and 17 feet on the shorter sides, and 20 and 18.5 feet on the longer sides? In that case, no corner would be at precisely an 90-degree angle. I read somewhere that that will eliminate or reduce room nulls and peaks, but I don't know much about this stuff. However, I'm willing to learn! Any acousticians here? Words of advice? Thanks!
passthedutchie
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Check out "Acoustics Insider" on YouTube. It is a series done by a professional acoustician, a studio and control room designer. Many questions are answered, including yours.

I agree with MC that the issue would be similar dimensions, not parallel walls, unless you can do something really radical.

Make sure there is adequate moisture control, including of water vapor coming through the slab.

The best listening room I ever had was in a garage. Great dimensions and a higher ceiling can make for very nice sound.

passthedutchie


Your off to a flying start with a cement slab floor spike your speaker into it and adjust the spikes so they don't move/rock even the tiniest bit in any plane
 
For the best imaging and depth perception, design it so there nothing between the speakers, and as far back as possible. (put your equipment rack off to the side not between the speakers).
The rest is fine tuning placement from back wall for the correct bass loading for your speaker/room 

Cheers George
A reviewer in one of the magazines converted his garage and I have heard it and it sounds good.  You will find a lot of advise, especially if you visit studio sites such are gearslutz (which is changing it's name).  After helping build my brothers recording studio, one of the most important aspects from staring from scratch is determine the proper dimensions and make sensible alterations if possible.  The other lesson is to make sure the room is as sturdy as possible, including using double walls and or thicker sheetrock.  Also sealing all cracks and gaps with acoustic calk including the floor seam where the structure meets the floor.    
Non parallel sidewalls and a sloping ceiling helps to reduce what we at ASC call "head end ringing." With parallel sidewalls and floor and ceiling sound emitted sideways from the speakers gets trapped at the front of the room and relatively slowly oozes it's way toward the listener. A 10 degree angle or more between the sidewalls and the floor and ceiling will greatly reduce the problem. 
https://www.acousticsciences.com/media/head-end-ringing

The problem with building that way is it tends to be more difficult and waste space. Head end ringing can also be dealt with by using bass traps in the front of the room, especially the two front corners but also directly off to the sides of the speakers.