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Responding to Loud and Soft Sounds
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
to its frequency. Given louder sounds, neighboring hair cells also respond. Thus, your brain
interprets loudness from the number of activated hair cells.
If a hair cell loses sensitivity to soft sounds, it may still respond to loud sounds. This
helps explain another surprise: Really loud sounds may seem loud to people with or with-
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;
where we differ is in our perception of soft sounds (and our ability to isolate one sound
Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.
amid noise).
Hearing Different Pitches
How do we know whether a sound is the high-frequency, high-pitched chirp of a bird or
Copyright © Bedford, Freeman & Worth Publishers.
the low-frequency, low-pitched roar of a truck? Current thinking on how we discriminate
pitch combines two theories.
• 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
membrane. Thus, the brain determines a sound’s pitch by recognizing the specific place
(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
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
explain how we hear high-pitched sounds but not low-pitched sounds.
• Frequency theory (also called temporal coding) suggests another explanation that
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
membrane vibrates with the incoming sound wave, triggering neural impulses to the
brain at the same rate as the sound wave. If the sound wave has a frequency of 100 waves
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
per second. How, then, can we sense sounds with frequencies above 1000 waves per
second (roughly the upper third of a piano keyboard)? Enter volley theory: Like soldiers
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
1000 waves per second.
place theory in hearing, the So, place theory and frequency theory work together to enable our perception of
theory that links the pitch we pitch. Place theory best explains how we sense high pitches. Frequency theory, extended
hear with the place where by volley theory, also explains how we sense low pitches. Finally, some combination of
the cochlea’s membrane is
stimulated. (Also called place the place and frequency theories likely explains how we sense pitches in the intermediate
coding.) range.
frequency theory in hearing,
the theory that the rate of Localizing Sounds
nerve impulses traveling up Why don’t we have one big ear — perhaps above our one nose? “All the better to hear you
the auditory nerve matches
the frequency of a tone, thus with,” as the wolf said to Little Red Riding Hood. Thanks to the placement of our two ears,
enabling us to sense its pitch. we enjoy stereophonic (“three-dimensional”) hearing. Two ears are better than one for at
(Also called temporal coding.) least two reasons (Figure 1.6-21). If a car to your right honks, your right ear will receive a
more intense sound, and it will receive the sound slightly sooner than your left ear.

140 Unit 1 Biological Bases of Behavior

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