Page 138 - Master Handbook of Acoustics

Reflections from Convex Surfaces

Considering sound as rays is a simplified view. Each ray should really be considered as a beam of
diverging sound with a spherical wavefront to which the inverse square law applies. Spherical
wavefronts from a point source become plane waves at greater distance from the source. For this
reason, impinging sound on various surfaces can usually be thought of as plane wavefronts. Reflection
of plane wavefronts of sound from a solid convex surface scatters the sound energy in many
directions as shown in Fig. 6-4. The size of the irregularity must be large compared to the wavelength

of the sound. This reflecting sound returns and diffuses the impinging sound. Polycylindrical sound-
absorbing modules both absorb sound and contribute diffusion in the room. The latter is created by
reflection from the cylindrically shaped convex surface of the modules.

FIGURE 6-4 Plane sound waves impinging on a convex irregularity tend to be dispersed through a
wide angle if the size of the irregularity is large compared to the wavelength of the sound.

Reflections from Concave Surfaces

Plane wavefronts striking a concave surface tend to focus to a point as illustrated in Fig. 6-5. The
precision with which sound is focused is determined by the shape and relative size of the concave