Practical diffusers need depth set by wavelength of the lowest frequency treated

Evidence

A crude guideline is that the depth of a geometrical (e.g., semi-cylinders, multifaceted shapes) diffuser should be not less than about a foot (300 mm), and that of an engineered (e.g., Schroeder, QRD) diffuser should be not less than about 8 inches (200 mm) deep. Gilford (1959) concluded that to be useful, projections from walls must be 1/7-wavelengh or greater at the lowest frequency of interest. At 300 Hz, that is about 7 inches (180 mm). For handclaps (say 1 kHz and up), 2 inches (50 mm) will suffice—but we listen to more than handclaps. Textured paint or plaster are visually interesting, but acoustically of no value. — , Toole, Floyd E - Sound reproduction-…, 488

Categorization

• DEFINE: Gives the 1/7-wavelength rule of thumb (Gilford 1959) that sets minimum diffuser projection depth.
• CANDY: “Textured paint or plaster are visually interesting, but acoustically of no value” is a delightful myth-busting one-liner for home audio shoppers.
• XREF: connects to: Diffuser depth must exceed one-seventh wavelength to be acoustically useful; Even quarter-wavelength diffuser depths are too large for low frequencies; Diffusion depth scales proportionally with wavelength, so lower frequencies demand deeper surfaces (+3 more)

Broader Connections

• Diffusion depth scales proportionally with wavelength, so lower frequencies demand deeper surfaces #expands (0.90)
• Effective sound diffusion demands surface depths proportional to the target wavelength #corroborates (0.88)
• Even quarter-wavelength diffuser depths are too large for low frequencies #corroborates (0.85)
• Acoustic diffusion needs surface bumps sized between an eighth and a half wavelength #qualifies (0.80)