This is a great question... To measure it, you really need something like a TEF that does Energy/Time/Frequency tests... If you can see the return from the rear (diffuse) wall, you can start to quantify what levels we are looking for in relation to the direct signal.. I don't have the software with me right now, but I'll ask Lars to make some suggestions as to the level of the Return after the Intitial Time Gap... Relating that to a percentage of surface area is a bit of the design experience that I do more intuitively than mathematically. I've done it so many times that I look at the size of the room, the distances (and times) involved and I come up with a reasonable area for the main diffusor bank...Let me see if I can quantify this better and get back to this thread..
Lars... chime in please!!
ps: I do see way too much diffusion sometimes, and you certainly can hear it (IMHO)... yes, 8 SF of diffusors is probably a little light for a "small to medium" sized room.. Remember the affect of properly used diffusors is "envelopment" and it's supposed to be subtle.. It's not supposed to be an in your face return... That's always the way I've approached it.
Man.. you guys are getting good with the questions!!!
btw.. the 24x24 module is SOOOO popular because it fits into a tegular ceiling grid system... many of the products are meant to drop into such ceiling grids...