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Old 18th September 2008
Gear Nut

Well, the primary reason for splayed control room walls given your layout constraints is as much about practicality as it is acoustics - it's the only way to create both a reflection-free zone in the control room (essential for monitoring) as well as good sight-lines through to the studio proper (helpful for producer/engineer-musician communication.)

If you can do without a control room window, then yes, the most straightforward way to ensure your CR will perform is to make it rectangular with good proportions and simply treat the first reflection points with absorption (not forgetting speaker/listener placement.)

I should note that in the drawings I previously posted pertaining to your room, the angle of splay for each side is 12 degrees. The thinking is this;

a) because of the relatively short distance to the side boundaries a higher degree of splay may be necessary to effectively create an RFZ.
b) this degree of splay seems to lend itself well to an ergonomic and reflection-free 5.1 setup in terms of surround speaker placement (would mean moving the listener/centre of the circle back slightly.)
c) the 12 degree splay, carried over to the studio side of the wall, should be enough of an angle to provide some acoustical benefit vs. a smaller angle (since the opposite wall of the studio proper would not be splayed.)

The other thing to consider is that although, yes, you are making the CR smaller by adding the angle -half the space you're losing from the CR you're gaining in an already uncomfortably small studio room. Yes, there is about 20 square feet of "dead" space on the other side, but if it's possible to translate this into an exterior window to provide some natural light, then it would be a more than worthwhile trade-off IMO.

Regarding taking splayed walls into consideration in predicting the modal response of the final dimensions - the conventional internet wisdom seems to be to use the average width created by the splayed wall as the width in your calculations (probably fine, provided you're already well within the envelope of a known good proportion.) However I've recently altered my perception of the matter slightly - my current thinking is: the dimensions which are most relevant to the conventional method of predicting rectangular room modal response (e.g. the bobgolds calculator) are the actual remaining rectangular dimensions - i.e. the dimensions of the back of the room. This is based on my (possibly incorrect?) thinking that the splayed walls will diminish the impact of the width axial mode at the front of the room (while making the contribution of oblique and tangential modes more complex and difficult to predict) but the contribution of the width mode at the (rectangular) back of the room remains relevant (if possibly now of less consequence than the length and height axial modes at the listening position.)

Anyway, I agree with the others that if you're serious about this, I'd employ the services of a professional designer - you'll likely learn a lot more that way than by doing it (and possibly messing it up) yourself!