Digital Sound & Music: Concepts, Applications, & Science, Chapter 2, last updated 6/25/2013
crowd together and then to move apart. The alternate crowding together and moving apart of
these molecules in turn affects the surrounding air pressure. The air pressure around the
vibrating object rises and falls in a regular pattern, and this fluctuation of air pressure,
propagated outward, is what we hear as sound.
Sound is often referred to as a wave, but we need to be careful with the commonly-used
term “sound wave,” as it can lead to a misconception about the nature of sound as a physical
phenomenon. On the one hand, there‟s the physical wave of energy passed through a medium as
sound travels from its source to a listener. (We‟ll assume for simplicity that the sound is
traveling through air, although it can travel through other media.) Related to this is the graphical
view of sound, a plot of air pressure amplitude at a particular position in space as it changes over
time. For single-frequency sounds, this graph takes the shape of a “wave,” as shown in Figure
2.1. More precisely, a single frequency sound can be expressed as a sine function and graphed as
a sine wave (as we‟ll describe in more detail later). Let‟s see how these two things are related.
Figure 2.1 Sine wave representing a single-frequency sound
First, consider a very simple vibrating object – a tuning fork. When the tuning fork is
struck, it begins to move back and forth. As the prong of the tuning fork vibrates outward (in
Figure 2.2), it pushes the air molecules right next to it, which results in a rise
in air pressure corresponding to a local increase in air density. This is called
compression. Now, consider what happens when the prong vibrates inward.
The air molecules have more room to spread out again, so the air pressure
beside the tuning fork falls. The spreading out of the molecules is called
decompression or rarefaction. A wave of rising and falling air pressure is
transmitted to the listener‟s ear. This is the physical phenomenon of sound,
the actual sound wave.
0 0.0023 0.0045 0.0068 0.0091 0.0113 0.0136 0.0159 0.0181
time in seconds