Audio Engineering

Acoustic Environment 91

As the sound source passes the listener and recedes, the pitch swings from 1084 Hz to

F







1130 0

1130 88

1000

⎡

⎣

⎢

⎢

⎤

⎦

⎥

⎥

928 Hz.

This rapid sweep of 156 Hz is called the Doppler effect. A very large excursion
low-frequency driver can exhibit Doppler distortion of its signal. Moving vanes in
reverberation chambers can produce Doppler effects in the refl ected signals that can cause
unexpected diffi culties in modern spectrum analyzers.

3.9 Refl ection and Refraction ........................................................................................

Sound can be refl ected by hitting an object larger than one-quarter wavelength of the
sound. When the object is one-quarter wavelength or slightly smaller, it also causes
diffraction of the sound (bending around the object). Refraction occurs when the sound
passes from one medium to another (from air to glass to air, for example, or when
it passes through layers of air having different temperatures). The velocity of sound
increases with increasing temperature. Therefore sound emitted from a source located
on the frozen surface of a large lake on a sunny day will encounter warmer temperatures
as the wave diverges upward, causing the upper part of the wave to travel faster than the
part of the wave near the surface. This causes a lens-like action to occur, which bends the
sound back down toward the surface of the lake ( Figure 3.4 ).

Sound will travel great distances over frozen surfaces on a quiet day. Wind blowing against
a sound source causes temperature gradients near the ground surface that result in the
sound being refracted upward. Wind blowing in the same direction as the sound produces
temperature gradients along the ground surface that tend to refract the sound downward. We

Cooler

Warmer Cooler

Warmer

Figure 3.4 : Effect of temperature differences between the ground and the air on the

propagation of sound.