There's a lot of talk about the importance of dealing with room modes in for mixing, and how rectangular rooms are better despite the parallel walls because the room modes are more predictable, and flutter echo is easily dealt with.

But:

1) How much do modes matter in a tracking room, where instruments are moves around alot from session to session? Aren't you fine as long as your not getting weird frequency response at the mic, and you have reliable monitoring, e.g., headphones?

2) Room mode prediction are almost never perfect anyway, right? And won't your flutter echo treatments also affect your modes? So you're probably going to end up treating a small room for modes no matter what shape you go with, right?

All this makes me think that, for a tracking room in particular, alot of the advice about rectangular ratios for good modes doesn't much matter, and I should optimize the tracking room size & shape for the available space, not to meet some modal goal before the room is even built and test. Anyway, if I end up hating my size/shape, I can change it later. Treatments are easily added if mode correction is needed later. Am I missing something?

This is too easy. Do you want the constant fixed frequency mode boost from the room modes in your tracks?

But its not fixed, because unlike a mixing room, equipment and other treatments are often moved around. Of course I don't want modal effects in my tracks, that's why I use treatments.

Modes are only constant when the room shape is relatively constant, right? But the room "shape", as far as sound waves are concerned, changes, often significantly, when large amps, drums sets, etc, are moved around the room, in all 3 dimensions. If otherwise, then there's something about sound waves that I really don't understand yet...

The room modes remain. Do you want peaks and holes in your recorded sound ad hoc? With even modes you can record in any key and mode. What key would you design your room for?

OK, maybe I'm not using the terms quite correctly. The room modes remain, but the position in the room where the level varies will change depending on the surfaces in the room that waves bounce off. In a mixing room, you generally can't move the sound source (monitors) and there's a sweet spot for the listener (ears). But in a tracking room, there's usually lots of freedom to move the sound source (instrument) and listener (microphone). Every recording engineer used to small spaces has lots of experience moving mics slightly and hearing significant changes in frequency response, even at the same proximity. So if you have a mode at 57.2 Hz and some instrument that emits that frequency, you move stuff around the room so that the peak level of that wave is in different spot from the microphone, because modal peaks and valleys aren't everywhere, they are in specific locations depending on the shape of the room. Or else, what have I said in this response that is factually incorrect?

The frequency amplitude change but not the decay.

of course one way to eliminate these problems is to limit the amount of purely acoustic instruments - DI is your friend in many cases. and the other as Matt has correctly pointed out, moving things around to find the best sound spot for a given instrument. drums - use the low tom and move it around to find a spot where it sounds best, then try to set up the kit there and tweak as need. and like any situation, you'll learn where the spots are for certain instruments to have an affinity.

i think the original question was "angled live room walls or not?" i think if you're trying to maximize the space and angled walls do that for you, then deal with the acoustic aspects afterwards. rectangular rooms have some advantages when it comes to building out some iso booths, closet space, and acoustic treatments but not everyone will have the perfect shaped space to do that.

still a good idea to "flatten" the room response from a modal perspective and ideal to have options for reflections to create live/dead/muted spots in the room.

If we are specifically talking about dividing an 18x18x8 garage into 3 or 4 spaces, one of which is a mix/tracking space, how much room is there practically speaking to move the drum kit around? There's an entry door that needs a clear path, and a mix position that's centered on the L/R axis, in a symetrical portion of the room. Theres also 2 or 3 other doors that need a clear path. And space for walking and mic stands cable runs. (Lol sometimes the hardest part about micing drums is fitting all the stands! Thanks for mic clips)

There is also the reasonable possibility that by creating an uneven distribution of modes you require more treatment than otherwise necessary, and considering the above space limitations, may be locked into an area that has an extremely skewed response and is inescapable.

(Attachments from Acoustics of Small Rooms, Kleiner/Tichy.)