Qualities of Sound


Sound is any audible vibration in an elastic medium. In a fluid, such as air, sound is propagated as a longitudinal wave. Regions of alternating high and low pressure travel away from the sound source, at a speed dependent upon the elastic properties of the medium. The perception of sound is called hearing.


The frequency of sound is the measure of the number of vibrations in a period of time, and is given as cycles per second, or Hertz (Hz), after the German physicist Heinrich Hertz. For hearing, frequency determines pitch.


Amplitude refers to the measure of pressure, or air displacement, produced in a sound wave. High pressure is positive amplitude; low pressure is negative amplitude. Amplitude is the single most important factor contributing to the perceived loudness of sound.


With these qualities of sound, we may construct a representation of a waveform, or wave profile.

Amplitude is plotted on the vertical axis; time, on the horizontal. Regular, simple harmonic motion produces a sinusoidal wave, or sine wave, such as the one depicted here. The central horizontal line in the illustration represents zero amplitude, which corresponds to normal atmospheric pressure in air. For a longitudinal wave, the region above the center line indicates positive amplitude, or high pressure, which is compression of the medium. The region below the center line indicates negative amplitude, or low pressure, which is rarefaction of the medium. The entire waveform represents one cycle. The frequency of the corresponding sound depends upon the period of time in which it occurs. If, for instance, the horizontal length in the illustration were to represent 50 milliseconds (50 ms), or 50/1000 seconds, a complete cycle would occur in one twentieth of a second; thus, the frequency of the sound would be 20 Hz.