Wall projections must exceed one-seventh wavelength to scatter sound usefully

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- the acoustics and…, 1033

Categorization

• DEFINE: Presents the 1/7-wavelength rule of thumb as a concrete design criterion for diffuser depth.
• CANDY: Textured paint being acoustically worthless despite visual appeal is a fun, tweetable room-acoustics fact.