Neil,

how do you feel about diffusors above a drum kit or above a listening
position, as opposed to absorption?

I'm not an expert with diffusion, but I can see how it would be useful
there. It won't make the ceiling disappear like absorption does, but it will
make it seem farther away. The main goal is to avoid the obvious echoes and
comb filtering, and good diffusion can do that. Note the word "good" - all
of the cheap diffusors I've heard sounded worse to me than a bare wall. But
RPG's deep well diffusors are very good if a little pricey.

Peter D'Antonio who heads RPG, and is a real diffusion expert, says
diffusion is not useful if it's too close to your ears. Dr. D'Antonio says
the minimum distance is about 10 feet, but I've heard others say 8 feet is
okay. So that probably rules our diffusion as a practical solution in a
small control room having a normal 8 foot ceiling.

--Ethan