The Ear

99
Hearing
TA B L E
3.2
Intensity of Sound Sources
Sound intensity varies across an extremely
wide range. A barely audible sound is, by
definition, 0 decibels; every increase of
20 decibels reflects a tenfold increase in
the amplitude of sound waves. So the
40-decibel sounds of an office are actually
10 times as intense as a 20-decibel whisper; and traffic noise of 100 decibels is
10,000 times as intense as that whisper.
Source
Sound
Level (decibels)
Spacecraft launch (from 45 meters)
Loudest rock band on record
Pain threshold (approximate)
Large jet motor (at 22 meters)
Loudest human shout on record
Heavy auto traffic
Conversation (at about 1 meter)
Quiet office
Soft whisper
Threshold of hearing
180
160
140
120
111
100
60
40
20
0
Source: Levine & Schefner (1981).
sound A repetitive fluctuation in the
The greater the amplitude, the louder the sensation of sound. Loudness is described in
units called decibels (abbreviated dB). By definition, 0 decibels is the minimum
detectable sound for normal hearing. Table 3.2 gives examples of the intensity, or loudness, of some common sounds.
The psychological dimension of pitch—how high or low a tone sounds—depends
on the frequency of the sound wave. Frequency is the number of complete waves or
cycles that pass a given point in one second. It is described in units called hertz,
abbreviated Hz (for Heinrich Hertz, a nineteenth-century physicist). One cycle per
second is 1 hertz. High-frequency waves are sensed as sounds of high pitch. The highest note on a piano has a frequency of about 4,000 hertz, and the lowest note has a
frequency of about 50 hertz. Humans can hear sounds ranging from about 20 to
20,000 hertz.
Most sounds are a mixture of many frequencies and amplitudes, and this mixture
creates a sound’s timbre (pronounced “tamber”), the psychological dimension of
sound quality. Complex wave patterns added to the fundamental, or lowest, frequency
of sound determine its timbre. The extra waves allow you to tell the difference between,
say, a note played on a flute and the same note played on a clarinet.
pressure of a medium such as air.
loudness A psychological dimension
of sound determined by the amplitude
of a sound wave.
pitch How high or low a tone sounds;
pitch depends on the frequency of a
sound wave.
timbre The quality of a sound that
identifies it.
pinna The crumpled part of the outer
ear that collects sound waves.
eardrum A tightly stretched membrane in the middle ear that generates
vibrations that match the sound waves
striking it. Also known as the tympanic
membrane.
cochlea A fluid-filled spiral structure
in the inner ear in which auditory transduction occurs.
basilar membrane The floor of the
fluid-filled duct that runs through the
cochlea.
The Ear
The human ear converts sound energy into neural activity through a series of accessory structures and transduction mechanisms. The crumpled part of the ear on the side
of the head, called the pinna, collects sound waves in the outer ear. (People trying to
hear a faint sound may cup a hand to their ear, because this action tilts the pinna forward and enlarges the sound-collection area. Try this for a moment, and
learn you will notice a clear difference in how sounds sound.) The pinna funnels
by
doing sound down through the ear canal. At the end of the ear canal, the sound
waves reach the middle ear (see Figure 3.14). There they strike the eardrum, a tightly
stretched structure also known as the tympanic membrane. The sound waves set up
vibrations in the eardrum. The hammer, the anvil, and the stirrup, three tiny bones
named for their shapes, amplify these vibrations and direct them onto a smaller membrane called the oval window.
2
Sound vibrations passing through the oval window enter the inner
ear, reaching the cochlea (pronounced “COK-lee-ah”), where transduction occurs. The
cochlea is rolled into a coiled spiral. (Cochlea comes from the Greek word for “snail.”)
A fluid-filled tube runs down its length. The basilar membrane forms the floor of
this tube, as you can see in Figure 3.15. When a sound wave passes through the fluid
The Inner Ear
100
FIGURE
Chapter 3 Sensation and Perception
3.14
Structures of the Ear
Malleus
(hammer)
Incus
(anvil)
The outer ear (pinna and ear canal) channels sound waves into the middle ear,
where the vibrations of the eardrum are
amplified by the delicate bones that stimulate the cochlea. In the cochlea in the
inner ear, the vibrations are converted, or
transduced, into changes in neural activity,
which are sent along the auditory nerve to
the brain.
Semicircular
canals
Oval
window
To brain
Auditory
nerve
Cochlea
Ear canal
Tympanic
membrane
Stapes
(stirrup)
Pinna
in the tube, it moves the basilar membrane (Ren, 2002). This movement, in turn, bends
hair cells on the membrane. These hair cells make connections with fibers from the
auditory nerve, a bundle of axons that goes into the brain. Bending the hair cells
stimulates the auditory nerve, which sends coded signals to the brain about the amplitude and frequency of the sound waves (Griesinger, Richards, & Ashmore, 2005). These
signals allow you to sense loudness, pitch, and timbre.
FIGURE
3.15
Deafness The middle and inner ear are among the most delicate structures in the
body, and damage to them can lead to deafness. One form of deafness is caused by problems with the bones of the middle ear. Over time they can fuse together, preventing
accurate conduction of vibrations from one bone to the next. This condition, called conduction deafness, can be treated by surgery to break the bones apart or to replace the
natural bones with plastic ones (Ayache et al., 2003). Hearing aids that amplify incoming sounds can also help.
Nerve deafness results when the auditory nerve or, more commonly, the hair cells are
damaged. Hair cell damage occurs gradually with age, but it can also be caused by very
loud sounds, including amplified rock music (Goldstein, 2002). High-intensity sound
can actually rip off the hair cells of the inner ear. Generally, any sound loud enough to
produce ringing in the ears causes some damage. In humans, small amounts of damage gradually build up and can produce significant hearing loss by middle age—as
many older rock musicians, and their fans, are finding out (Levine, 1999).
Hair cells can be regenerated in chickens’ ears (Cotanche, 1997), and a related kind
of inner-ear hair cell has been regenerated in mammals (Malgrange et al., 1999). Scientists hope that human hair-cell regeneration might someday be accomplished by treating
damaged areas with growth factors similar to those being used to repair damaged brain
cells (Shepherd et al., 2005; see the chapter on biology and behavior) or by inserting
The Cochlea
This drawing shows how the vibrations of
the stirrup set up vibrations in the fluid inside the cochlea. The coils of the cochlea
are unrolled in this illustration to show the
path of the fluid waves along the basilar
membrane. Movements of the basilar
membrane stimulate hair cells, which
transduce the vibrations into changes in
neural firing patterns.
Stapes
(stirrup)
Unfolded
cochlea
Wave
Basilar
membrane
Hair cells