Page 23 - Master Handbook of Acoustics

weight moves in what is called simple harmonic motion. When the weight is at rest, the system is said
to be in equilibrium. If the weight is pulled down to the -5 mark and released, the spring pulls the
weight back toward 0. However, the weight will not stop at 0; its inertia will carry it beyond 0
almost to +5. The displacement of the weight defines the amplitude of the motion.

FIGURE 1-1 A weight on a spring vibrates at its natural frequency because of the elasticity of the
spring and the inertia of the weight.

The weight will continue to vibrate, or oscillate. Each up/down repetition is called a cycle, and
the motion is said to be periodic. In the arrangement of a mass and spring, vibration or oscillation is

possible because of the elasticity of the spring and the inertia of the weight. Elasticity and inertia are
two things all media must possess to be capable of conveying sound. In this practical example, the
amplitude of motion will slowly decrease due to frictional losses in the spring and the air.
Harmonic motion is a basic type of oscillatory motion, and it yields an equally basic wave shape

in sound and electronics. To illustrate this, if a pen is fastened to the weight’s pointer, as shown in
Fig. 1-2, and a strip of paper is moved past it at a uniform speed, the resulting trace is a sine wave.
The sine wave is a pure waveform closely related to simple harmonic motion. In this figure, the sine
wave traced by the pen has completed one full period and is more than half way through a second
period. The periodic motion of the weight will continue to trace the sine wave indefinitely. (For a
moment, we are ignoring the frictional losses that would decrease amplitude.) This simple oscillatory
system will always create sinusoidal motion; without outside forces, no other motion is possible.

This graph of a sine wave, showing amplitude versus time, sets precedence for plotting all kinds of
wave shapes.