Page 30 - The Master Handbook Of Acoustics

5
FUNDAMENTALS OF SOUND

Propagation of Sound
If an air particle is displaced from its original position, elastic forces of
the air tend to restore it to its original position. Because of the inertia
of the particle, it overshoots the resting position, bringing into play
elastic forces in the opposite direction, and so on.
Sound is readily conducted in gases, liquids, and solids such as air,
water, steel, concrete, etc., which are all elastic media. As a child, per-
haps you heard two sounds of a rock striking a railroad rail in the dis-
tance, one sound coming through the air and one through the rail. The
sound through the rail arrives first because the speed of sound in the
dense steel is greater than that of tenuous air. Sound has been detected
after it has traveled thousands of miles through the ocean.
Without a medium, sound cannot be propagated. In the laboratory,
an electric buzzer is suspended in a heavy glass bell jar. As the button is
pushed, the sound of the buzzer is readily heard through the glass. As
the air is pumped out of the bell jar, the sound becomes fainter and
fainter until it is no longer audible. The sound-conducting medium, air,
has been removed between the source and the ear. Because air is such a
common agent for the conduction of sound, it is easy to forget that other
gases as well as solids and liquids are also conductors of sound. Outer
space is an almost perfect vacuum; no sound can be conducted except
in the tiny island of air (oxygen) within a spaceship or a spacesuit.

The Dance of the Particles

Waves created by the wind travel across a field of grain, yet the indi-
vidual 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. 1-4. The disturbance trav-
els on, but the propagating particles do their little dance close to home.
There are three distinct forms of particle motion. 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 in Fig. 1-5(A). Another type of wave motion is illus-
trated by a violin string, Fig. 1-5(B). The tiny elements of the string
move transversely, or at right angles to the direction of travel of the
waves along the string. 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, Fig. 1-5C.