Physical Descriptors of Sound

When we characterize sounds, various descriptors are used. Sounds can be loud or quiet, soft or deafening. These are subjective descriptors of loudness. Sounds can be low- or high-pitched, bassy or tinny. These descriptors relate to the pitch of sounds. For complex sounds, words like screechy, shrill, booming, raspy can be used. Such descriptors are useful and generally understood, but defining them is more difficult. This is a challenge for subjective descriptors for which human perception is involved.

For research into the nature and the applications of sound, more precise objective descriptors are needed. Two of the key descriptors, frequency and sound pressure are described here.

The loudness of air-borne sounds can be described in terms of sound pressure, i.e., the magnitude of oscillations of atmospheric pressure about its mean value. Sound pressure is reported with units of Pascals (abbreviation Pa). The range of sound pressure can be enormous: at the threshold of hearing, sound being just barely audible, the sound pressure is about 20 micro-Pa; a jet engine at 1 m distance produces about 600 Pa. It is often convenient to use a logarithmic scale, converting to sound pressure level in decibels (abbreviation dB). In the examples, we would have 0 dB for the threshold of hearing and 150 dB near a jet engine.

To describe the tonal quality of sounds, a parameter frequency is used. Frequency counts the number of oscillations per second. These oscillations can be the motion of the arms of a tuning fork or the diaphragm of a loudspeaker; they can be the oscillations in pressure that constitute air-borne sound. Frequency is reported with units of cycles per second or Hertz (abbreviation Hz). A sound comprised of just one frequency is a pure tone. More commonly, sounds will be comprised of many frequencies.