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is called rarefaction . That resulting solitary wave of pressure will head off through the
air to the right in Figure 26-1 at the speed corresponding to the speed of sound in air.
If the piston, or speaker cone, pulses back and forth, as illustrated in Figure 26-2 , a series
of these high-/low-pressure regions will be created, resulting in a continuous series of
waves—a sound wave—propagating to the right.
Figure 26-2. Sound wave
The wavelength of this sound wave (i.e., the distance measured from pressure peak to
pressure peak) is a function of the frequency of pulsation, or vibration, of the cone. The
resulting sound wave's frequency is related to the inverse of its wavelength—that is, f =
1/λ. The pressure amplitude versus time waveform for this scenario is illustrated in
Figure 26-2 . We've illustrated the pressure wave as a harmonic sine wave, which need
not be the case in reality since the sound coming from a speaker could be composed of
an aggregate of many different wave components. We'll say more on this later.
One thing we do want to point out is that a sound wave is a longitudinal wave and not
a transverse wave like an ocean wave, for example. In a transverse wave, the displacement
of the medium due to the wave is perpendicular to the direction of travel of the wave.
In a longitudinal wave, the displacement is along the direction of travel of the wave. The
higher density and pressure regions of a sound wave are due to compression of the
medium along the direction of travel of the wave. Thus, sound waves are longitudinal
So, sound waves are variations in density and pressure moving through a medium. But
how do we hear them? In essence the pressure wave, created by some mechanical vi‐
bration like that of a speaker cone, gets converted back to a vibration in our inner ear.
And that vibration gets interpreted by our brains as sound—the sound we hear.
Figure 26-3 illustrates this concept.
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