Page 263 - The Master Handbook Of Acoustics

238 CHAPTER TEN

concave surface. Spherical concave surfaces
are common because they are readily
formed. They are often used to make a
microphone highly directional by placing it
at the focal point. Such microphones are fre-
quently used to pick up field sounds at
sporting events or in recording songbirds or
other animal sounds in nature. In the early
A days of broadcasting sporting events in
Hong Kong, a resourceful technician saved
the day by using an ordinary Chinese wok,
or cooking pan, as a reflector. Aiming the
microphone into the reflector at the focal
point provided an emergency directional
pickup. Concave surfaces in churches or
auditoriums can be the source of serious
problems as they produce concentrations of
FIGURE 10-3
sound in direct opposition to the goal of
Plane sound waves impinging on a convex irregularity uniform distribution of sound.
tend to be dispersed through a wide angle if the size The effectiveness of reflectors for
of the irregularity is large compared to the wave-
length of the sound. microphones depends on the size of the
reflector with respect to the wavelength of
sound. A 3-ft-diameter spherical reflector
will give good directivity at 1 kHz (wave-
length about 1 ft), but it is practically
nondirectional at 200 Hz (wavelength
about 5.5 ft).

Reflections from Parabolic
B
Surfaces

A parabola has the characteristic of
focusing sound precisely to a point (Fig.
10-5). It is generated by the simple equa-
2
FIGURE 10-4 tion y = x . A very “deep” parabolic sur-
face, such as that of Fig. 10-5, exhibits far
Plane sound waves impinging on a concave irregular-
ity tend to be focussed if the size of the irregularity better directional properties than a shallow
is large compared to the wavelength of the sound. one. Again, the directional properties