218
Chapter 7
Sound Transmission in the Ear
The fi rst step in hearing is the entrance of sound waves into
the
external auditory canal
(
Figure 7–34
). The shapes of
the outer ear (the pinna, or auricle) and the external audi-
tory canal help to amplify and direct the sound. The sound
waves reverberate from the sides and end of the external audi-
tory canal, fi lling it with the continuous vibrations of pressure
waves.
The
tympanic membrane
(eardrum) is stretched across
the end of the external auditory canal, and as air molecules
push against the membrane, they cause it to vibrate at the same
frequency as the sound wave. Under higher pressure during a
zone of compression, the tympanic membrane bows inward.
The distance the membrane moves, although always very small,
is a function of the force with which the air molecules hit it
and is related to the sound pressure and therefore its loudness.
During the subsequent zone of rarefaction, the membrane
returns to its original position. The exquisitely sensitive tym-
panic membrane responds to all the varying pressures of the
sound waves, vibrating slowly in response to low-frequency
sounds and rapidly in response to high-frequency ones.
The tympanic membrane separates the external auditory
canal from the
middle ear,
an air-fi lled cavity in the temporal
bone of the skull. The pressures in the external auditory canal
and middle ear cavity are normally equal to atmospheric pres-
sure. The middle ear cavity is exposed to atmospheric pressure
through the
eustachian tube,
which connects the middle ear
to the pharynx. The slitlike ending of this tube in the pharynx
is normally closed, but muscle movements open the tube dur-
ing yawning, swallowing, or sneezing. A difference in pressure
can be produced with sudden changes in altitude (as in an
ascending or descending elevator or airplane). When the pres-
sure outside the ear and in the ear canal changes, the pressure
in the middle ear initially remains constant because the eusta-
chian tube is closed. This pressure difference can stretch the
tympanic membrane and cause pain. This problem is relieved
by voluntarily yawning or swallowing, which opens the eusta-
chian tube and allows the pressure in the middle ear to equili-
brate with the new atmospheric pressure.
The second step in hearing is the transmission of sound
energy from the tympanic membrane through the middle ear
cavity to the
inner ear.
The inner ear, called the
cochlea,
is a
spiral-shaped passage in the temporal bone fi lled with a fl
uid
called
endolymph.
The temporal bone also houses other pas-
sages, including the semicircular canals, which contain the
sensory organs for balance and movement. These passages are
connected to the cochlea but will be discussed later.
Because liquid is more diffi
cult to move than air, the
sound pressure transmitted to the inner ear must be ampli-
fi ed. This is achieved by a movable chain of three small bones,
the
malleus, incus,
and
stapes
(
Figure 7–35
). These bones
act as a piston and couple the vibrations of the tympanic
membrane to the
oval window,
a membrane-covered open-
ing separating the middle and inner ears. The total force of a
sound wave applied to the tympanic membrane is transferred
to the oval window, but because the oval window is much
(a)
(b)
(c)
(d)
(e)
Pressure
Time
Air molecules
Zones of
compression
Zone of
compression
Zones of
rarefaction
Number of cycles per second = frequency = pitch
Amplitude = loudness
Figure 7–33
Formation of sound waves from a vibrating tuning fork.
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