Introduction to Computer Music: Volume One

4. What are the characteristics of sound waves?

Sound waves are often characterized by four basic qualities, though many more are related:

frequency, amplitude, wave shape and phase*

Some sound waves are periodic, in that the change from equilibrium (average atmospheric pressure) to maximum compression to maximum rarefaction back to equilibrium is repetitive. The 'round trip' back to the starting point just described is called a cycle.

It is possible to measure frequency in seconds per cycle or periods, but it is far more common for sound measurements to use cycles per second or simply cps .

Periodic motion depends on two prime factors; 1) elasticity, in that the medium being distorted returns to its original state (equilibrium), and 2) a source of energy to initiate and sustain motion. In the case of sound waves, the atmospheric pressure will return to the ambient pressure without an energy source to disturb it, and any vibrating surface will constitute an energy or excitation source.

Simple harmonic motion is represented in sound as a sine wave, which traces the mathematical shape of its namesake. A sinusoidal wave (which also includes a cosine wave) is the only wave shape that produces a single frequency, as we will see in the waveform chapter. With any minute deviations in the sine shape, additional frequencies will be generated.

*It could be argued that phase is not a characteristic of a single wave, but only as a comparison between two or more waves.

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