Room acoustics
Diffusion
Scattering sound reflections evenly in many directions with a shaped surface, so a room stays lively without discrete echoes or flutter.
A diffuser is a surface, often a pattern of wells of varying depth or a curved shape, that breaks a single reflection into many weaker ones spread over time and angle. Unlike an absorber it does not remove energy; it redistributes it so no one reflection dominates.
Absorption alone can leave a room sounding small and dead, especially on the rear wall and between surround speakers. Diffusion there keeps envelopment and a sense of space while still killing slap echo and comb filtering.
Quadratic residue diffusers work over a band set by well depth; a 4 inch deep unit scatters from roughly 800 Hz up. The scattered field needs distance to form, so keep listeners at least 3 m from a diffuser, which usually means the rear wall at home.