Page 181 - Master Handbook of Acoustics

FIGURE 9-6 Generally, concave surfaces are undesirable, whereas convex surfaces are very
desirable. (A) Concave surfaces tend to focus sound. Concave surfaces should be avoided if the goal
is to achieve well-diffused sound. (B) Convex surfaces tend to diffuse sound.

Convex Surfaces: The Polycylindrical Diffuser

The polycylindrical diffuser (poly) is an effective diffusing element, and one relatively easy to
construct; it presents a convex section of a cylinder. Three things can happen to sound falling on such
a cylindrical surface made of plywood or hardboard: The sound can be reflected and thereby
dispersed as in Fig. 9-6B; the sound can be absorbed; or the sound can be reradiated. Such

cylindrical elements serve as absorbers in the low-frequency range where absorption and diffusion
are often needed in small rooms. The reradiated portion, because of the diaphragm action, is radiated
almost equally throughout an angle of roughly 120°, as shown in Fig. 9-7A. A similar flat element
reradiates sound in a much narrower angle, about 20°, as shown in Fig. 9-7B. Therefore, reflection,
absorption, and reradiation characteristics favor the use of the cylindrical surface. Some very
practical polys and their absorption characteristics are presented in Chap. 12. The dimensions of such
diffusers are not critical, although to be effective their size must be comparable to the wavelength of

the sound being considered. The wavelength of sound at 1,000 Hz is a bit over 1 ft and at 100 Hz it is
about 11 ft. A poly element 3 or 4 ft across would be effective at 1,000 Hz, much less so at 100 Hz. In
general, a poly base or chord length of 2 to 6 ft with depths of 6 to 18 in meet most needs. Axes of
symmetry of the polys on different room surfaces should be mutually perpendicular.