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Originally Posted by Brad McGowan  Okay another question for you two gurus: would you put absorbtion or diffusion of the rear wall behind the mix position if the room is only about 16-17 feet long?
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Absorption. No question (IMHO). I typically recommend that people not use diffusors where they will be any closer than 10 feet from your head (i.e., listening position) can muck up your imaging anywhere from a bit to a whole lot, depending on the type and quality of the diffusors, and you will often be hearing a bit too much of the anomalies (i.e., comb filtering) that happen across the face of diffusors. These issues tend ESPECIALLY to be the case with cheap, poorly designed diffusors. Same issues with imaging using diffusors as opposed to absorption at first reflection points.
That said, Ethan and I have talked about this a bit, and I believe he has found our diffusors to not be problematic even closer than 10 feet (if I'm not mistaken, he's tried the "near" version of our diffusors at closer range, but I don't know if he tried the "far" version that close), but we haven't had a chance to do enough adequate testing in this regard, so I tend to stick to standard recommendations even with our diffusors when I'm asked for my recommendations. Ethan and I have been meaning to get together to do some more subjective testing (you really need more than one person to do the necessary testing), but we haven't yet had a chance, since Ethan's up in CT and I'm in NYC (and I don't have a car at the mo).
If you are going to use diffusors in a small control room, you really need something that will deal with low frequencies as well, and not just highs and mids -- which means you either need some *really* deep welled diffusors (and good quality ones like this are *very* expensive), or something that does diffusion for the highs and mids, and absorption for the low frequencies. And anything that will do a good job in this role is going to be more expensive. For this reason, too, I think absorption is a better option for small rooms (and particularly for small control rooms!), as it is much more cost-effective.
One of the problems with cheap plastic diffusors is that they don't necessarily scatter sound evenly in the way you really want them to, and they may not really have the most even balance across the frequency spectrum for which they are intended to address (which is why, while things like book shelves, etc. do diffuse sound, they are not really proper diffusors). Also . . . some of the cheap hollow plastic diffusors may have some resonance of their own that is not particularly ideal. (In those cases, it may be worth your while to stuff those diffusors with fluffy fiberglass, which would cut down resonance and also potentially give them at least some capability for LF absorption.)
Mind you, I'm actually not trying to disparage your use of the diffusors you have in your studio (and I actually quite like the panels on those light fixtures that are a bit like small polys!). Some of this stuff is much less of an issue in a tracking room, where you just want to cut down on the flutter echo and retain a little bit of presence/ambiance in the room, where precise stereo imaging is not the critical aim (or even necessary for recording certain things, for that matter), but in a control room where you will be making critical mixing decisions, you want to be very careful with this.
To give you an idea, we did an acoustics makeover for a certain grammy nominated hip hop producer's studio last summer. He had QRD diffusors across the back wall of his control room, somewere around 9 to 12 feet behind the listening position (don't remember the exact dimensions of the room at the mo).
One of the first things I noticed when I first visited the studio was that his imaging was a mess (and he was managing some good sounding mixes despite the imaging problems -- I think he had just gotten so used to working that way). I initially intended to leave the diffusors in, but even after adding a significant amount of treatment to his control room, including treating the first reflection points, the imaging still was not to my satisfaction. So I pulled diffusors down and replaced them with some MiniTraps and MondoTraps. The imaging was *exponentially* better, and of course he was thrilled with it.
When I talked with him later, he said that he found he had to go back and redo a bunch of mixes on some projects he was working on, because he started hearing so much more in his mixes. He said he could hear so much deeper into the mix, and he had never been able to hear so deeply into a mix before in any studio he had been in.
About a month ago, I had the chance to hear some more recent tracks he had mixed. While I always liked the sound/production of his tracks, the more recent stuff had a deep, three-dimensional image that had not been present in his mixes before. He had stuff that was sitting way to the back of the sound stage (but still very clearly audible), and there was stuff that was popping out front of the speakers and tickling your earlobes. So much more depth and space in his mixes.
Anyway . . . if you had a large control room where it was possible to have things like 700 ms reverb time, etc., then my recommendations might be a bit different, but for small control rooms, I really feel that absorption is the best, most cost-effective option. You're really in too close quarters for there to be enough time delay between direct sound from your monitors and reflected sound that your ears will readily identify the difference between the two.
Hehe . . . sorry for the long, anecdotal post, but hopefully that will be helpful to you (and/or others who may happen across the thread). It's a question that come up fairly often, so I figured I'd riff on it a bit.
