It is often useful to graph fluctuations in air pressure (or the analog or digital representations) that occur at one point in space over time.
Example 4-1: Sound can be graphed as a waveform to show the fluctations in voltage created by sound.
Points above the zero axis correspond to points at which the air molecules are more dense. Points below the zero axis correspond to points at which the air molecules are less dense, or rarefied. The zero axis itself is the point of equilibrium. This graphic depiction of sound is called a waveform. The shape of the waveform contributes greatly to the timbre perceived by the listener.
Diagram 4-2: Waveform: high pressure, low pressure.
When a waveform repeats continuously it is a periodic waveform. Usually periodic waveforms are represented as a single occurrence of the pattern. One complete pattern is a cycle, and the time occupied by a single cycle is a period.
Example 4-3: A periodic waveform is one that repeats continuously; each complete unit of the pattern is a cycle.
In some cases a waveform does not have a recurring amplitude pattern. A waveform which does not consist of a repeating single cycle is called a non-periodic waveform.
Example 4-4: Most sounds are comprised of waveforms which are non-repeating. A non-periodic waveform contains non-repeating amplitudal patterns.