Chapter 6 Sensation and Perception 2053
Timbre is the distinguishing quality of a sound.
It is the dimension of auditory experience
related to the complexity of the sound wave, the
relative breadth of the range of frequencies that
make up the wave. A pure tone consists of only
one frequency, but pure tones in nature are ex-
tremely rare. Usually what we hear is a complex
wave consisting of several subwaves with different
frequencies. Timbre is what makes a note played
on a flute, which produces relatively pure tones,
sound different from the same note played on an
oboe, which produces complex sounds.
When many sound wave frequencies are pres-
ent but are not in harmony, we hear noise. When
all the frequencies of the sound spectrum occur,
they produce a hissing sound called white noise.
Just as white light includes all wavelengths of the
visible light spectrum, so white noise includes all
frequencies of the audible sound spectrum.an ear on the World Lo 6.17
As Figure 6.8 shows, the ear has an outer, a mid-
dle, and an inner section. The soft, funnel-shaped
outer ear is well designed to collect sound waves,
but hearing would still be pretty good without
it. The essential parts of the ear are hidden from
view, inside the head.
A sound wave passes into the outer ear and
through an inch-long canal to strike an oval-
shaped membrane called the eardrum. The ear-
drum is so sensitive that it can respond to the
movement of a single molecule! A sound wave
causes it to vibrate with the same frequency and
amplitude as the wave itself. This vibration is
passed along to three tiny bones in the middle
ear, the smallest bones in the human body. These
bones, known informally as the hammer, the anvil,
and the stirrup, move one after the other, which
has the effect of intensifying the force of the vi-
bration. The innermost bone, the stirrup, pushes
on a membrane that opens into the inner ear.
The actual organ of hearing, the organ of Corti,
is a chamber inside the cochlea, a snail-shaped
structure within the inner ear. The organ of Corti
plays the same role in hearing that the retina
plays in vision. It contains the crucial receptor
cells, which in this case are called hair cells and are
topped by tiny bristles, or cilia. Brief exposure to
extremely loud noises, like those from a gunshot
or a jet airplane (140 dB), or sustained exposure
to more moderate noises, like those from shop
tools or truck traffic (90 dB), can damage these
fragile cells. The cilia flop over like broken blades
of grass, and if the damage affects a critical num-
ber, hearing loss occurs. In modern societies, with
their rock concerts, crowded bars, leaf blowers,timbre The distinguish-
ing quality of a sound;
the dimension of auditory
experience related to the
complexity of the pres-
sure wave.organ of Corti [core-
tee] A structure in the
cochlea containing hair
cells that serve as the re-
ceptors for hearing.cochlea [KoCK-lee-uh]
A snail-shaped, fluid-
filled organ in the inner
ear, containing the organ
of Corti, where the re-
ceptors for hearing are
located.people with hearing impairment feel strongly
about teaching deaf children American Sign
Language (ASL) or other gestural systems, which
allow them to communicate with other signers.What We Hear Lo 6.16
The stimulus for sound is a wave of pressure cre-
ated when an object vibrates (or when compressed
air is released, as in a pipe organ). The vibration (or
release of air) causes molecules in a transmitting
substance to move together and apart. This move-
ment produces variations in pressure that radiate in
all directions. The transmitting substance is usually
air, but sound waves can also travel through water
and solids, as you know if you have ever put your
ear to the wall to hear voices in the next room.
As with vision, physical characteristics of the
stimulus—in this case, a sound wave—are related
in a predictable way to psychological aspects of our
experience:1
Loudness is the psychological dimension of
auditory experience related to the intensity of
a wave’s pressure. Intensity corresponds to the
amplitude (maximum height) of the wave. The
more energy contained in the wave, the higher it
is at its peak. Perceived loudness is also affected by
how high or low a sound is. If low and high sounds
produce waves with equal amplitudes, the low
sound may seem quieter.
Sound intensity is measured in units called
decibels (dB). A decibel is one-tenth of a bel, a unit
named for Alexander Graham Bell, the inventor
of the telephone. The average absolute threshold
of hearing in human beings is zero decibels. Un-
like inches on a ruler, decibels are not equally dis-
tant; each 10 decibels denotes a 10-fold increase in
sound intensity. On the Internet, decibel estimates
for various sounds vary from site to site; this is be-
cause the intensity of a sound depends on things
like how far away it is and the particular person
or object producing the sound. What you need to
know is that a 60-decibel conversation is not twice
as loud as a 30-decibel whisper; it is 1,000 times
louder.2
Pitch is the dimension of auditory experi-
ence related to the frequency of the sound
wave and, to some extent, its intensity. Frequency
refers to how rapidly the air (or other medium)
vibrates—the number of times per second the
wave cycles through a peak and a low point. One
cycle per second is known as 1 hertz (Hz). The
healthy ear of a young person normally detects
frequencies in the range of 16 Hz (the lowest note
on a pipe organ) to 20,000 Hz (the scraping of a
grasshopper’s legs).loudness The dimension
of auditory experience
related to the intensity of
a pressure wave.pitch The dimension
of auditory experience
related to the frequency
of a pressure wave; the
height or depth of a tone.