Page 139 - Master Handbook of Acoustics

surface. Spherical concave surfaces are common. For example, they may be used to make a
microphone highly directional by placing it at the focal point. Such microphones are frequently used
to pick up field sounds at sporting events or record animal sounds in nature. The effectiveness of such
concave reflectors depends on the size of the reflector with respect to the wavelength of sound. For

example, a 3-ft-diameter spherical reflector will give good pickup directivity at 1 kHz (wavelength
about 1 ft), but it is practically nondirectional at 200 Hz (wavelength about 5.5 ft). Concave
architectural surfaces, for example, domed ceilings and archways in churches or auditoriums, can be
the source of serious problems as they produce concentrations of sound in direct opposition to the
goal of uniform distribution of sound in rooms.

FIGURE 6-5 Plane sound waves impinging on a concave irregularity tend to be focused if the size of
the irregularity is large compared to the wavelength of the sound.

Reflections from Parabolic Surfaces

2
A parabola generated by the equation y = x has the characteristic of focusing sound precisely to a
point. A very “deep” parabolic surface, such as that of Fig. 6-6, exhibits far better directional
properties than a shallow one. Again, the directional properties depend on the size of the opening in
terms of wavelengths.