Absorption. This is to control early reflection off the ceiling, and to force a standing wave to become a traveling wave in this location, as you cannot put an absorber on the floor(at least that is my understanding. (some may disagree as per traveling wave)
One school of thought says diffusers in a control room should only be used to diffuse specular reflection off a rear wall, and only if the distance from the wall to the engineering position is 3 times that of the distance from the mix position to the monitors. Any closer and you won't get the benifit of diffusion, as they are 1/4 wavelength silencers also. Therefore, at close range, they produce combfiltering and other sonic artifacts.
Actually, there is another school of thought that says a diffuse sound field cannot exist in small rooms. Yet others say it can, within a mid-high band of frequencies. There are other experts that have designed large quadratic residue type diffusers for diffusion of low frequencies, however, just as absorption by resistance absorbers take up deep floor space via 1/4 wavelength principles, so do low frequency diffusers. Well depths can approach 3' and and width of a foot. Yet there are others who swear by them and even haul a few QRD's to live recording venues for placement on the stage. Almost all pro studios use them in the live rooms, although the rooms are usually very large, where large expanses of wall areas need to have specular reflection diffused.
Personally, I am a little disappointed with diffuser device hype. As one well known acoustician told me, you can acheive good diffusion(within reason and room dimensions) by using patchwork pattern application of absorption panels. Why go to the trouble of building bulky and space consuming devices
fitZ
PS. My disclaimer is in full effect here...ie..I ain't no expert. I'm all ears for disagreement and clarification from those that are.
fitZ
