Walls must be rough on many scales to scatter sound diffusely

Evidence

Diffuse surfaces, on the other hand, scatter sound in other directions than the specular. In the case of an ideal diffuser, the scattered energy polar pattern would be in the form of a hemisphere. A simple approach to calculating the effect of this can be to model the scattered energy as a source whose initial intensity is given by the incident energy. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2318 (#expands, via linked claim)

Categorization

• XREF: connects to: Perfectly hard-walled halls would produce unbearably long echoes needing leaks to tame; Diffuse surfaces make room acoustics degrade gradually away from the sweet spot; Diffuse surfaces scatter sound hemispherically rather than mirror-like — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2318 (+3 more)

Broader Connections

• Acoustic diffusion needs surface bumps sized between an eighth and a half wavelength #expands (0.92)
• Diffuse surfaces scatter sound hemispherically rather than mirror-like #expands (0.90) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2318
• Effective sound diffusion demands surface depths proportional to the target wavelength #expands (0.88)
• Diffuse surfaces make room acoustics degrade gradually away from the sweet spot #expands (0.85)
• Regularly spaced surface bumps diffuse sound only at specific frequencies #qualifies (0.85)