> Could you explain why a diffusor will "diffuse" your voice when you speak into it but if you are listening to music you must be several several feet away before " diffusion" occurs. <
Diffusion happens no matter how close or far you are. But when you're very close to a QRD well diffusor, the individual well resonances are audible. So that's one reason. Another reason is even a really good diffusor reflects some sound straight back at you. Those reflections create comb filtering which skew the frequency response and harm imaging.
I'm not knowledgeable enough on the math to comment at a more technical level, but these are a few of my personal observations. Peter D'Antonio of RPG is a pioneering expert with this stuff. He has a few (expensive) books out, but there are also articles you can download for free at the RPG web site:
www.rpginc.com
--Ethan