Digital Sound & Music: Concepts, Applications, & Science, Chapter 2, last updated 6/25/2013
For clarity, we‟ve thus far simplified the picture of how sound propagates. Figure 2.2
makes it look as though there‟s a single line of sound going straight out from the tuning fork and
arriving at the listener‟s ear. In fact, sound radiates out from a source at all angles in a sphere.
Figure 2.3 shows a top-view image of a real sound radiation pattern, generated by software that
uses sound dispersion data, measured from an actual loudspeaker, to predict how sound will
propagate in a given three-dimensional space. In this case, we‟re looking at the horizontal
dispersion of the loudspeaker. Colors are used to indicate the amplitude of sound, going highest
to lowest from red to yellow to green to blue. The figure shows that the amplitude of the sound
is highest in front of the loudspeaker and lowest behind it. The simplification in Figure 2.2
suffices to give you a basic concept of sound as it emanates from a source and arrives at your ear.
Later, when we begin to talk about acoustics, we'll consider a more complete picture of sound
Figure 2.3 Loudspeaker viewed from top with sound waves radiating at multiple angles
Sound waves are passed through the ear canal to the eardrum, causing vibrations which
pass to little hairs in the inner ear. These hairs are connected to the auditory nerve, which sends
the signal onto the brain. The rate of a sound vibration – its frequency – is perceived as its pitch
by the brain. The graph of a sound wave represents the changes in air pressure over time
resulting from a vibrating source. To understand this better, let‟s look more closely at the
concept of frequency and other properties of sine waves.
2.1.2 Properties of Sine Waves
We assume that you have some familiarity with sine waves from trigonometry, but even if you
don‟t, you should be able to understand some basic concepts of this explanation.
A sine wave is a graph of a sine function . In the graph, the x-axis is the
horizontal axis, and the y-axis is the vertical axis. A graph or phenomenon that takes the shape
of a sine wave – oscillating up and down in a regular, continuous manner – is called a sinusoid.