Overview

Sound can be defined as a wave motion in air or other elastic media (stimulus) or as that excitation of the hearing mechanism that results in the perception of sound (sensation). Which definition applies depends on whether the approach is physical or psychophysical. The type of problem dictates the approach to sound. If the interest is in the disturbance in air created by a loudspeaker, it is a problem in physics. If the interest is how it sounds to a person near the loudspeaker, psychophysical methods must be used. Because this book addresses acoustics in relation to people, both aspects of sound will be treated.

These two views of sound are presented in terms familiar to those interested in audio and music. Frequency is a characteristic of periodic waves measured in hertz (cycles per second), readily observable on a cathode-ray oscilloscope or countable by a frequency counter. The ear perceives a different pitch for a soft 100 Hz tone than a loud one. The pitch of a low-frequency tone goes down, while the pitch of a high-frequency tone goes up as intensity increases. A famous acoustician, Harvey Fletcher, found that playing pure tones of 168 and 318 Hz at a modest level produces a very discordant sound. At a high intensity, however, the ear hears the pure tones in the 150-300 Hz octave relationship as a pleasant sound. We cannot equate frequency and pitch, but they are analogous.

The same situation exists between intensity and