through any material medium with a speed that depends on the properties of the
medium. As the waves travel through air, the elements of air vibrate to produce
changes in density and pressure along the direction of motion of the wave. If the
source of the sound waves vibrates sinusoidally, the pressure variations are also sinu-
soidal. The mathematical description of sinusoidal sound waves is very similar to that
of sinusoidal string waves, which were discussed in the previous chapter.
Sound waves are divided into three categories that cover different frequency
ranges. (1) Audible waves lie within the range of sensitivity of the human ear. They can
be generated in a variety of ways, such as by musical instruments, human voices, or
loudspeakers. (2) Infrasonic waves have frequencies below the audible range. Elephants
can use infrasonic waves to communicate with each other, even when separated by
many kilometers. (3) Ultrasonic waves have frequencies above the audible range. You
may have used a “silent” whistle to retrieve your dog. The ultrasonic sound it emits is
easily heard by dogs, although humans cannot detect it at all. Ultrasonic waves are also
used in medical imaging.
We begin this chapter by discussing the speed of sound waves and then wave inten-
sity, which is a function of wave amplitude. We then provide an alternative description
of the intensity of sound waves that compresses the wide range of intensities to which
the ear is sensitive into a smaller range for convenience. We investigate the effects of
the motion of sources and/or listeners on the frequency of a sound. Finally, we explore
digital reproduction of sound, focusing in particular on sound systems used in modern
motion pictures.