Page 24 - Master Handbook of Acoustics

FIGURE 1-2 A pen fastened to the vibrating weight traces a sine wave on a paper strip moving at
uniform speed. This shows the basic relationship between simple harmonic motion and the sine

wave.

As another example of oscillatory motion, consider a piston in an automobile engine that is
connected to the crankshaft by a connecting rod. The rotation of the crankshaft and the up-and-down
motion of the pistons illustrate the relationship between rotary motion and linear simple harmonic
motion. As with the weight on a spring, the piston position plotted against time produces a sine wave.

Sound in Media

The weight and spring system in the previous example models the motion of air molecules. 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.

An elastic medium is essential to the existence of sound waves. Sound is readily conducted in
gases, liquids, and solids such as air, water, steel, concrete, and so on, which are all elastic media.
Imagine a friend stationed a distance away, who strikes a railroad rail with a rock. You will hear two
sounds, one sound coming through the rail and one through the air. The sound through the rail arrives
first because the speed of sound in the dense steel is faster than in tenuous air. Similarly, underwater
sounds can be detected after traveling 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, the air inside the jar, 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 atmosphere within a
spaceship or a spacesuit.

Particle Motion