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Figure 26-3. Illustration of how we hear
Our outer ears help capture and direct pressure waves into our ear canals. These pressure
waves travel down the ear canal, impinging on the eardrum, which causes the eardrum
to vibrate. This is where the pressure variations get converted back to mechanical vi‐
bration. Beyond the eardrum, biology and chemistry work their magic to convert those
vibrations into electrical impulses that our brains interpret as sound.
Our ears are sensitive enough to detect pressure waves in the 20 to 20,000 hertz (Hz)
frequency range. (A hertz is one cycle per second.) We interpret frequency as pitch.
High-pitch sounds (think tweeters) correspond to high frequencies, and low-pitch
sounds (think bass) correspond to low frequencies.
Aside from pitch, an obvious characteristic of sound that we perceive is its loudness.
Loudness is related to the amplitude of the pressure wave, among other factors such as
duration. We often think of loudness in terms of volume, or power, or intensity. All
these characteristics are related, and we can write various formulas relating these char‐
acteristics to other features of the sound wave. Sound waves have kinetic energy, which
is related to the mass of the medium disturbed by the pressure wave and the speed at
which that mass is disturbed. Power is the time rate of change of energy transference.
And intensity is related to how much power flows through a given area. The bottom
line is that the more power a sound has, or the more intense it is, the louder it seems to
you, the listener. At some point, a sound can be so intense as to cause discomfort or
Customarily, intensity is measured in units of decibels. A decibel represents the intensity
of a sound relative to some standard reference, which is usually taken as the sound
intensity corresponding to the threshold of hearing. Zero decibels, or 0 dB, corresponds
to the threshold of hearing. The intensity is so low you can't hear it. When sounds reach
about 120-130 dB, they start to cause pain. Table 26-1 lists some typical intensity values
for common sounds.
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