I'm thinking why alpha and impression series plates are called "diffusors"? Isn't it mandatory for diffusor to have different depths and/or deformed surfaces (like polyfusors)? My understanding is that alpha and impression plates simply reduces reflection area (because of holes). But I don't understand why they are called diffusors. Or am I missing something?
Also, real diffusors (QRD and skyline) must have distance to work properly. What about alpha/impression plates? Is it important to have distance from listening position or do they also work directly behind listener’s head? What do you think?