Effective sound diffusers ideally need no physical depth at all.

Evidence

However, even λ 4 at low frequencies is often too large to be useful. What one really requires is a diffuser which is effective without using any depth! — David Howard, Jamie Angus, coustics and Psychoacoustics, 2145

Categorization

• DEFINE: Clarifies the fundamental constraint on diffuser design: depth demands scale with wavelength, making low-frequency diffusion impractical.