Diffuser depth sets a lower frequency below which no scattering occurs.

Evidence

If a 284. diffusing surface is decided on, there is again a decision about the thickness, and again the answer will be based on wavelength. Figure 9.19 illustrates the basic principle underlying diffusing surfaces. Each diffuser, depending on its design and depth, will have a frequency below which it does not scatter the sound. It is therefore desirable that the scattered/diffused sound should cover the bandwidth above the transition frequency, to avoid distorting the spectrum of the reflected sound field. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1032

Categorization

• QUESTION: Worth chasing: the exact relationship between diffuser depth and transition frequency — is it a quarter-wavelength rule like absorbers?