Effective sound diffusers must be physically comparable in size to the waves they scatter

Evidence

The original source of scattered sound in concert venues was irregular architectural shapes, opera boxes, and statuary. Modern halls use engineered irregular and curved surfaces. Diffusers must manipulate sound waves of significant size and must therefore be of comparable size. One of the early popular shapes was the hemi-cylinder or, in a slightly flattened form, the polycylindrical absorber/diffuser. Large ones were covered with thin membranes so that they also served as absorbers in the upper bass region. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1386

Categorization

• ADDENDUM: The size-of-device-to-wavelength constraint connects to the Grafting & Growth article only loosely — actually more relevant to the reader’s broader acoustics infrastructure interest.
• DEFINE: Explains why ornate opera boxes and statuary were acoustically useful — they were unplanned diffusers.
• XREF: connects to: Effective low-frequency diffusion requires physically deep wells, about 34 cm for 500 Hz — David Howard, Jamie Angus, coustics and Psychoacoustics, 2143