Digital Sound & Music: Concepts, Applications, & Science, Chapter 2, last updated 6/25/2013
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Figure 2.2 Air pressure amplitude and sound waves
Assume that a tuning fork creates a single frequency wave. Such a sound wave can be
graphed as a sine wave, as illustrated in Figure 2.1. An incorrect understanding of this graph
would be to picture air molecules going up and down as they travel across space from the place
in which the sound originates to the place in which it is heard. This would be as if a particular
molecule starts out where the sound originates and ends up in the listener‟s ear. This is not what
is being pictured in a graph of a sound wave. It is the energy, not the air molecules themselves,
that is being transmitted from the source of a sound to the listener‟s ear. If the wave in Figure
2.1 is intended to depict a single frequency sound wave, then the graph has time on the x-axis
(the horizontal axis) and air pressure amplitude on the y-axis. As described above, the air
pressure rises and falls. For a single frequency sound wave, the rate at which it does this is
regular and continuous, taking the shape of a sine wave.
Thus, the graph of a sound wave is a simple sine wave only if the sound has only one
frequency component in it that is, just one pitch. Most sounds are composed of multiple
frequency components multiple pitches. A sound with multiple frequency components also
can be represented as a graph which plots amplitude over time; it‟s just a graph with a more
complicated shape. For simplicity, we sometimes use the term “sound wave” rather than “graph
of a sound wave” for such graphs, assuming that you understand the difference between the
physical phenomenon and the graph representing it.
The regular pattern of compression and rarefaction described above is an example of
harmonic motion, also called harmonic oscillation. Another example of harmonic motion is a
spring dangling vertically. If you pull on the bottom of the spring, it will bounce up and down in
a regular pattern. Its position that is, its displacement from its natural resting position can be
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