Diffuser depth must scale with the lowest frequency it can scatter.

Evidence

As we have seen, these sequences achieve their performance by spreading the phase of the reflected wavefront over at least one cycle of the incident wavefront. In order to do this, their maximum depth must be λ 2 at the lowest design frequency. This means that to achieve diffusion, a reasonable depth is required. For example, to have effective diffusion down to 500 Hz, a depth of 34 cm (13.5 inches) is required. To get down to 250 Hz, one would need to double this depth. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2096

Categorization

• DEFINE: Explains that the maximum well depth must reach half a wavelength at the lowest design frequency for reflection phase to spread across a full cycle.
• XREF: Ties back to earlier chapters’ wavelength-frequency relationship, where halving frequency doubles wavelength. — connects to: Effective low-frequency diffusion requires physically deep wells, about 34 cm for 500 Hz — David Howard, Jamie Angus, coustics and Psychoacoustics, 2143