Page 25 - Master Handbook of Acoustics

Waves created by the wind travel across a field of grain, yet the individual stalks remain firmly
rooted as the wave travels on. In a similar manner, particles of air propagating a sound wave do not
move far from their undisplaced positions, as shown in Fig. 1-3. The disturbance travels on, but the
propagating particles move only in localized regions (perhaps a maximum displacement of a few ten-

thousandths of an inch). Note also that the velocity of a particle is maximum at its equilibrium
position, and zero at the points of maximum displacement (a pendulum has the same property). The
maximum velocity is called the velocity amplitude, and the maximum displacement is called the
displacement amplitude. The maximum particle velocity is very small, less than 0.5 in/sec for even a
loud sound. As we will see, to lower the level of a sound, we must reduce the particle velocity.

FIGURE 1-3 An air particle is made to vibrate about its equilibrium position by the energy of a
passing sound wave because of the interaction of the elastic forces of the air and the inertia of the air
particle.

There are three distinct forms of particle motion. For sound traveling in a gaseous medium such as
air, the particles move in the direction the sound is traveling. These are called longitudinal waves, as
shown in Fig. 1-4A. A second type of wave motion is illustrated by a violin string, as shown in Fig.

1-4B. The tiny elements of the string move transversely, or at right angles to the direction of travel of
the waves along the string. Thirdly, if a stone is dropped on a calm water surface, concentric waves
travel out from the point of impact, and the water particles trace circular orbits (for deep water, at
least), as shown in Fig. 1-4C.