576
UNIT 3
Regulation and Integration of the Body
15
wavelength
of the sound and is constant for a particular tone.
Te shorter the wavelength, the higher the frequency of the
sound
(Figure 15.29a)
.
Te frequency range of human hearing is from 20 to 20,000
waves per second, or
hertz
(
Hz
). Our ears are most sensitive
to frequencies between 1500 and 4000 Hz and, in that range,
we can distinguish frequencies differing by only 2–3 Hz. We
perceive different sound frequencies as differences in
pitch
: the
higher the frequency, the higher the pitch.
A tuning fork produces a
tone
—a pure (but bland) sound
with a single frequency—but most sounds are mixtures of sev-
eral frequencies. Tis characteristic of sound, called
quality
,
enables us to distinguish between the same musical note—say,
high C—sung by a soprano or played on a piano or clarinet.
Sound quality also provides the richness and complexity of
sounds (and music) that we hear.
Amplitude
Te
amplitude
, or height, of the sine wave crests
reveals a sound’s intensity, which is related to its energy, or the
pressure differences between its compressed and rarefied areas
(Figure 15.29b).
Loudness
refers to our subjective interpretation of sound
intensity. Because we can hear such an enormous range of in-
tensities, from the proverbial pin drop to a jet engine 10 million
times more intense, sound intensity (and loudness) is measured
in logarithmic units called
decibels (dB)
(des
9
ĭ-belz).
On a clinical audiometer, the decibel scale is arbitrarily set
to begin at 0 dB, which is the threshold of hearing (barely au-
dible sound) for normal ears. Each 10-dB increase represents
a tenfold increase in sound intensity. A sound of 10 dB has 10
times more energy than one of 0 dB, and a 20-dB sound has
100 times (10
3
10) more energy. However, the same 10-dB
increase represents only a twofold increase in loudness. In other
words, most people would report that a 20-dB sound seems
about twice as loud as a 10-dB sound. Te normal range of hear-
ing (from barely audible to the loudest sound we can process
without excruciating pain) extends over a range of 120 dB. (Te
threshold of pain is 120 dB.)
Severe hearing loss occurs with frequent or prolonged ex-
posure to sounds with intensities greater than 90 dB, and in the
U.S., employees exposed to occupational noise over that range
must wear ear (hearing) protection. Tat number becomes
more meaningful when you realize that a normal conversation
is in the 50-dB range, a noisy restaurant has 70-dB levels, and
amplified rock music is 120 dB or more, far above the 90-dB
danger zone.
to the molecules they bump, energy is always transferred in the
direction the sound wave is traveling. For this reason, the energy
of the wave declines with time and distance, and the sound dies
a natural death.
We can illustrate a sound wave as an S-shaped curve, or
sine
wave
, in which the compressed areas are crests and the rarefied
areas are troughs (Figure 15.28a). Sound can be described in
terms of two physical properties that you can see in this sine
wave graph: frequency and amplitude.
Frequency
Frequency
is defined as the number of waves that
pass a given point in a given time. Te sine wave of a pure tone
has crests and troughs that repeat at definite distances. Te
distance between two consecutive crests (or troughs) is the
Time (s)
(a) Frequency is perceived as pitch.
Pressure
High frequency (short wavelength) = high pitch
Low frequency (long wavelength) = low pitch
(b) Amplitude (size or intensity) is perceived as loudness.
High amplitude = loud
Low amplitude = soft
0.01
0.02
Time (s)
0.03
0.01
0.02
0.03
Pressure
Figure 15.29
Frequency and amplitude of sound waves.
Table 15.2
Summary of the Internal Ear
BONY LABYRINTH
MEMBRANOUS LABYRINTH
FUNCTION
RECEPTOR REGION
Semicircular canals
Semicircular ducts
Equilibrium: rotational (angular) acceleration
Crista ampullaris
Vestibule
Utricle and saccule
Equilibrium: head position relative to gravity, linear acceleration
Macula
Cochlea
Cochlear duct (scala media)
Hearing
Spiral organ
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