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Responding to Loud and Soft Sounds
TEACH 1.6-11 How do we detect loudness? If you guessed that it’s related to the intensity of a hair cell’s
response, you’d be wrong. Rather, a soft, pure tone activates only the few hair cells attuned
Active Learning to its frequency. Given louder sounds, neighboring hair cells also respond. Thus, your brain
interprets loudness from the number of activated hair cells.
(15 minutes) Students may get the If a hair cell loses sensitivity to soft sounds, it may still respond to loud sounds. This
place and frequency matching theo- helps explain another surprise: Really loud sounds may seem loud to people with or with-
ries of hearing confused. Have them out normal hearing. Given my hearing loss, I [DM] have wondered what really loud music
must sound like to people with normal hearing. Now I realize it sounds much the same;
engage in a directed paraphrasing where we differ is in our perception of soft sounds (and our ability to isolate one sound
activity, by imagining that a first amid noise).
grader saw their psychology text- Hearing Different Pitches
book and asked them to explain the How do we know whether a sound is the high-frequency, high-pitched chirp of a bird or
difference. In pairs, have each student the low-frequency, low-pitched roar of a truck? Current thinking on how we discriminate
explain one theory using only words pitch combines two theories.
a first grader would understand. • Place theory (also called place coding) presumes that we hear different pitches because
different sound waves trigger activity at different places along the cochlea’s basilar
Time saver: You can have students do membrane. Thus, the brain determines a sound’s pitch by recognizing the specific place
this individually in writing. (on the membrane) that is generating the neural signal. When Nobel laureate-to-be
Georg von Békésy (1957) cut holes in the cochleas of guinea pigs and human cadavers
and looked inside with a microscope, he discovered that the cochlea vibrated, rather like
a shaken bedsheet, in response to sound. High frequencies produced large vibrations
ENGAGE 1.6-11 near the beginning of the cochlea’s membrane. Low frequencies vibrated more of the
membrane and were not so easily localized. So, there is a problem: Place theory can
(5 minutes) Invite a student volunteer to explain how we hear high-pitched sounds but not low-pitched sounds.
sit with eyes closed in a chair facing the • Frequency theory (also called temporal coding) suggests another explanation that
class. Clap at varying locations around accounts for our ability to hear low-pitched sounds: The brain reads pitch by monitor-
ing the frequency of neural impulses traveling up the auditory nerve. The whole basilar
the volunteer’s head. The student membrane vibrates with the incoming sound wave, triggering neural impulses to the
will confidently and accurately locate brain at the same rate as the sound wave. If the sound wave has a frequency of 100 waves
sounds coming from either side (which per second, then 100 pulses per second travel up the auditory nerve. But frequency
theory also has a problem: An individual neuron cannot fire faster than 1000 times
strike the two ears differently) but will per second. How, then, can we sense sounds with frequencies above 1000 waves per
have more difficulty locating sound in second (roughly the upper third of a piano keyboard)? Enter volley theory: Like soldiers
the 360° plane equidistant between the who alternate firing so that some can shoot while others reload, neural cells can alter-
nate firing. By firing in rapid succession, they can achieve a combined frequency above
two ears (overhead, in back, or in front). 1000 waves per second.
Use this activity to open your discussion Copyright © Bedford, Freeman & Worth Publishers.
So, place theory and frequency theory work together to enable our perception of
place theory in hearing, the
theory that links the pitch we
pitch. Place theory best explains how we sense high pitches. Frequency theory, extended
of sound location. Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.
hear with the place where
the cochlea’s membrane is by volley theory, also explains how we sense low pitches. Finally, some combination of
stimulated. (Also called place the place and frequency theories likely explains how we sense pitches in the intermediate
coding.) range.
ENGAGE 1.6-11 frequency theory in hearing,
the theory that the rate of Localizing Sounds
(5 minutes) Explain to students that nerve impulses traveling up Why don’t we have one big ear — perhaps above our one nose? “All the better to hear you
we localize sound by detecting small the auditory nerve matches with,” as the wolf said to Little Red Riding Hood. Thanks to the placement of our two ears,
the frequency of a tone, thus
differences in the loudness and timing enabling us to sense its pitch. we enjoy stereophonic (“three-dimensional”) hearing. Two ears are better than one for at
least two reasons (Figure 1.6-21). If a car to your right honks, your right ear will receive a
(Also called temporal coding.)
of the sounds received by our two ears. more intense sound, and it will receive the sound slightly sooner than your left ear.
Using a 4-inch flexible plastic tubing or
hose, have a student hold each end of 140 Unit 1 Biological Bases of Behavior
the tube up to each ear, while the circle
is kept down and in front of the body.
Another student then taps the tube
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with a pencil. A sound wave will move
in both directions to the ears. If the
tube is tapped at any point other than
the middle, the sound will reach the
two ears at different times. Thus, the
sound will seem to come from different
directions. The perceived direction of
a sound is related to differences in the
time at which the sound is received by
each ear.

Information from Coren, S., Ward, L. M., & Enns,
J. T. (1999). Sensation and perception (5th ed.).
Harcourt Brace.

140 Unit 1 Biological Bases of Behavior

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