**Sound Generators**

Any motion that is repeated is called periodic motion. Examples of periodic motion
are the moon's orbit of the earth, a beating heart, the whirring tail of a frightened
rattlesnake, the wings of a hummingbird, and the movement of the valve on a revolving
bicycle tire. A vibrating body in contact with the atmosphere will produce sound
waves. One simple example is a vibrating piston.

**Basic Features of Periodic Motion**

There are thus three parameters which characterise the basic features of any periodic
motion.

1. The period, T is the time required for one complete cycle.

2. The frequency, f is the number of cycles occurring in a given time period.

3. The Amplitude, A. The "extent" of the motion.

Thus the frequency is the inverse of the period (and vice versa), i.e.:

Frequency used to be expressed in cycles per second (cps). The unit **Hertz**
(abbreviated Hz) is now used. 1 cycle/second = 1 cycle sec.-1 = 1 Hz.