Chapter 11:
Sound, The Auditory System,
and Pitch Perception
Overview of Questions
 If a tree falls in the forest and no one is there
to hear it, is there a sound?
 What is it that makes sounds high pitched or
low pitched?
 How do sound vibrations inside the ear lead to
the perception of different pitches?
 How are sounds represented in the auditory
cortex?
Pressure Waves and Perceptual Experience
 Two definitions of “sound”
 Physical
definition - sound is pressure
changes in the air or other medium.
 Perceptual
definition - sound is the
experience we have when we hear.
Sound Waves
 Loud speakers produce sound by:
 The
diaphragm of the speaker moves out,
pushing air molecules together called
condensation.
 The
diaphragm also moves in, pulling the air
molecules apart called rarefication.
 The
cycle of this process creates alternating
high- and low-pressure regions that travel
through the air.
Figure 11.1 (a) The effect of a vibrating speaker diaphragm on the
surrounding air. Dark areas represent regions of high air pressure,
and light areas represent areas of low air pressure.
Figure 11.1
(b) When a pebble is dropped into still water, the resulting
ripples appear to move outward. However, the water is
actually moving up and down, as indicated by movement
of the boat. A similar situation exists for the sound waves
produced by the speaker in (a).
Sound Waves - continued
 Pure tone - created by a sine wave
 Amplitude
- difference in pressure between
high and low peaks of wave
Perception of amplitude is loudness
Decibel (dB) is used as the measure of
loudness
The decibel scale relates the amplitude of
the stimulus with the psychological
experience of loudness.
Figure 11.2 (a) Plot of sine-wave pressure changes for a pure tone;
(b) Pressure changes are indicated, as in Figure 11.1, by darkening
(pressure increased relative to atmospheric pressure) and lightening
(pressure decreased relative to atmospheric pressure.)
Figure 11.3 Three different amplitudes of a pure tone. Larger
amplitudes are associated with the perception of greater loudness.
Relative amplitudes and decibels for environmental sounds
Sound Waves - continued
 Frequency - number of cycles within a given
time period
 Measured
in Hertz (Hz) - 1 Hz is one cycle
per second
 Perception
 Tone
of pitch is related to frequency.
height is the increase in pitch that
happens when frequency is increased.
Three different frequencies of a pure tone.
Higher frequencies are associated with the perception of
higher pitches.
All pitches are same loudness (amplitude).
Complex Periodic Sounds
 Fundamental frequency is the repetition rate
and is called the first harmonic.
 Periodic complex tones consist of a number of
pure tones called harmonics.
 Additional
harmonics are multiples of the
fundamental frequency.
Complex Periodic Sounds - continued
 Additive synthesis - process of adding
harmonics to create complex sounds
 Frequency spectrum - display of harmonics of
a complex sound
 Attack of tones - buildup of sound at the
beginning of a tone
 Decay of tones - decrease in sound at end of
tone
a. Complex sound
b. Fundamental (200 Hz)
c. 2nd harmonic (400 Hz)
d. 3rd harmonic (600 Hz)
e. 4th harmonic (800 Hz)
Left: Waveforms of (a) a complex periodic sound with a fundamental
frequency of 200 Hz; (b) fundamental (first harmonic) = 200 Hz; (c)
second harmonic = 400 Hz; (d) third harmonic = 600 Hz; (e) fourth
harmonic = 800 Hz. Right: Frequency spectra for each of the tones on
the left. (Adapted from Plack, 2005)
Complex Periodic Sounds - continued
 Timbre - all other perceptual
aspects of a sound besides
loudness, pitch, and duration
It is closely related to the
harmonics, attack and decay of
a tone.
Guitar
Bassoon
Alto saxophone
Figure 11.10 Frequency spectra for a guitar, a bassoon, and an alto
saxophone playing a tone with a fundamental frequency of 196 Hz. The
position of the lines on the horizontal axis indicates the frequencies of
the harmonics and their height indicates their intensities.
Musical Scales and Frequency
 Letters in the musical scale repeat.
 Notes with the same letter name (separated
by octaves) have fundamental frequencies
that are multiples of each other.

These notes have the same tone chroma.

We perceive such notes as similar to one another.
Figure 11.8 A piano keyboard, indicating the frequency
associated with each key. Moving up the keyboard to the
right increases the frequency and tone height. Notes with
the same letter, like the A’s (arrows) have the same tone
chroma.
Range of Hearing
 Human hearing range - 20 to 20,000 Hz
 Audibility curve - shows the threshold of
hearing in relation to frequency

Changes on this curve show that humans are most
sensitive to 2,000 to 4,000 Hz.
 Auditory response area - falls between the
audibility curve and the threshold for feeling

It shows the range of response for human audition.
pain
Figure 11.9 The audibility curve and the auditory response
area. Hearing occurs in the light green area between the
audibility curve (the threshold for hearing) and the upper curve
(the threshold for feeling). Tones with combinations of dB and
frequency that place them in the pink area below the audibility
curve cannot be heard. Tones above the threshold of feeling
result in pain.
The Ear
 Outer ear - pinna and auditory canal

Pinna helps with sound location.

Auditory canal - tube-like 3 cm long structure
It
protects the tympanic membrane at the
end of the canal.
The
resonant frequency of the canal
amplifies frequencies between 1,000 and
5,000 Hz.
Figure 11.11 The ear, showing its three subdivisions -outer, middle, and inner. (From Lindsay & Norman, 1977)
The Middle Ear
 Two cubic centimeter cavity separating inner
from outer ear
 It contains the three ossicles

Malleus - moves due to the vibration of the tympanic
membrane

Incus - transmits vibrations of malleus

Stapes - transmit vibrations of incus to the inner ear via
the oval window of the cochlea
Figure 11.12 The middle ear. The three bones of the
middle ear transmit the vibrations of the tympanic
membrane to the inner ear.
Function of Ossicles
 Outer and inner ear are filled with air.
 Inner ear is filled with fluid that is much denser
than air.
 Pressure changes in air transmit poorly into
the denser medium.
 Ossicles act to amplify the vibration for better
transmission to the fluid.
 Middle ear muscles dampen the ossicles’
vibrations to protect the inner ear from
potentially damaging stimuli.
Figure 11.13 Environments inside the outer, middle, and
inner ears. The fact that liquid fills the inner ear poses a
problem for the transmission of sound vibrations from the
air of the middle ear.
(a) A diagrammatic representation of the tympanic membrane and the
stapes, showing the difference in size between the two.
(b) How lever action can amplify the effect of a small force, presented
on the right, to lift the large weight on the left. The lever action of the
ossicles amplifies the sound vibrations reaching the tympanic inner
ear. (From Schubert, 1980).
The Inner Ear
 Main structure is the cochlea

Fluid-filled snail-like structure (35 mm long) set into
vibration by the stapes

Divided into the scala vestibuli and scala tympani by the
cochlear partition

Cochlear partition extends from the base (stapes end) to
the apex (far end)

Organ of Corti contained by the cochlear partition
Figure 11.15 (a) A partially uncoiled cochlea. (b) A fully uncoiled
cochlea. The cochlear partition, indicated here by a line, actually
contains the basilar membrane and the organ of Corti.
The Organ of Corti
 Key structures

Basilar membrane vibrates in response to sound and
supports the organ of Corti

Inner and outer hair cells are the receptors for hearing

Tectorial membrane extends over the hair cells
Figure 11.16 (a) Cross-section of the cochlea.
(b) Close-up of the organ of Corti, showing how it rests on
the basilar membrane. Arrows indicate the motions of the
basilar membrane and tectorial membrane that are
caused by vibration of the cochlear partition.
The Organ of Corti - continued
 Transduction takes place by:
 Cilia
bend in response to movement of
organ of Corti and the tectorial
membrane.
This
causes bursts of electrical signals.
channels
excites
resting
Figure 11.18 (a) Movement of hair cilia in one direction opens ion
channels in the hair cell, which results in the release of
neurotransmitter onto an auditory nerve fiber;
(b) Movement in the opposite direction closes the ion channels so
there is no ion flow and no transmitter release.
Neural Signals for Frequency
 There are two ways nerve fibers signal
frequency:
 Which
fibers are responding
Specific
groups of hair cells on basilar
membrane activate a specific set of nerve
fibers;
 How
fibers are firing
Rate
or pattern of firing of nerve impulses
Békésys’ Place Theory of Hearing
Hungarian who won Nobel Prize in 1961
 Frequency of sound is indicated by the place
on the organ of Corti that has the highest
firing rate.
 Békésy determined this in two ways:

Direct observation of the basilar membrane in
cadavers.

Building a model of the cochlea using the physical
properties of the basilar membrane.
Biography Sketch
 Before and during World War II, Békésy worked
for the Hungarian Post Office (1923 to 1946),
where he did research on telecommunications
signal quality. This research led him to become
interested in the workings of the ear. In 1946, he
left Hungary to follow this line of research in
Sweden.
 In 1947, he moved to the United States, working
at Harvard University until 1966. After his lab
was destroyed by fire in 1965, he was offered to
lead a research laboratory of sense organs in
Honolulu, Hawaii. He became a professor at the
University of Hawaii in 1966 and died in
Honolulu.
Hair cells all along the cochlea send signals to nerve fibers that
combine to form the auditory nerve. According to place theory, low
frequencies cause maximum activity at the apex end of the cochlea,
and high frequencies cause maximum activity at the base. Activation
of the hair cells and auditory nerve fibers indicated in red would signal
that the stimulus is in the middle of the frequency range for hearing.
Békésys’ Place Theory of Hearing - continued
 Physical properties of the basilar membrane
 Base
of the membrane (by stapes) is:
Three
to four times narrower than at the
apex.
100
times stiffer than at the apex.
 Both the model and direct observation
showed that the vibrating motion of the
membrane is a traveling wave .
Figure 11.21 A perspective view showing
the traveling wave motion of the basilar
membrane. This picture shows what the
membrane looks like when the vibration is
“frozen,” with the wave about two thirds of
the way down the membrane.
Figure 11.22 A perspective view of an uncoiled cochlea,
showing how the basilar membrane gets wider at the apex
end of the cochlea.
Békésys’ Place Theory of Hearing - continued
 Envelope of the traveling wave
 Indicates
the point of maximum
displacement of the basilar membrane
 Hair
cells at this point are stimulated the
most strongly leading to the nerve fibers
firing the most strongly at this location.
 Position
of the peak is a function of
frequency.
Figure 11.24 The
envelope of the basilar
membrane’s vibration at
frequencies ranging
from 25 to 1,600 Hz, as
measured by Békésy
(1960).
Evidence for the Place Theory
 Tonotopic map
 Cochlea
shows an orderly map of
frequencies along its length
Apex
responds best to low frequencies
Base
responds best to high frequencies
Basilar Membrane Response to Complex Tones
 Basilar membrane can be described as an
acoustic prism.

There are peaks in the membrane’s vibration that
correspond to each harmonic in a complex tone.

Each peak is associated with the frequency of a
harmonic.
Figure 11.30 (a) Waveform of a complex tone consisting of
three harmonics; (b) Basilar membrane. The shaded
areas indicate locations of peak vibration associated with
each harmonic in the complex tone.
Hearing Loss
 Two types
 Conductive
hearing loss
Blockage of sound from the receptor cells
 Sensorineural hearing loss
Damage to hair cells
Damage to the auditory nerve or brain
Most common type is prebycusis
Hearing Loss - continued
 Presbycusis



Greatest loss is at high frequencies
Affects males more severely than females
Appears to be caused by exposure to damaging noises
or drugs
 Noise-induced hearing loss



Loud noise can severely damage the Organ of Corti
OSHA standards for noise levels at work are set to
protect workers
Leisure noise can also cause hearing loss
Presbycusis
Figure 11.34 Hearing
loss in presbycusis
(elder hearing) as a
function of age. All the
curves are plotted
relative to the 20-yearold curve, which is
taken as the standard
(Adapted from Dubno,
in press).
Figure 11.35 Sound level of game 3 of the 2006 Stanley Cup finals
between the Edmunton Oilers (the home team) and the Carolina
Hurricanes. Sound levels were recorded by a small microphone in a
spectator’s ear. The red line indicates a “safe” level for a 3-hour game.
Pathway from the Cochlea to the Cortex
 Auditory nerve fibers synapse in a series of
subcortical structures





Cochlear nucleus
Superior olivary nucleus (in the brain stem)
Inferior colliculus (in the midbrain)
Medial geniculate nucleus (in the thalamus)
Auditory receiving area (A1 in the temporal lobe)
SONIC MG
CN
SON
IC
MGN
A1
Figure 11.36 Diagram of the auditory pathways. This diagram is
greatly simplified, as numerous connections between the structures
are not shown. Note that auditory structures are bilateral -- they exist
on both the left and right sides of the body -- and that messages can
cross over between the two sides. (Adapted from Wever, 1949.)
Auditory Areas in the Cortex
 Hierarchical processing occurs in the cortex
 Neural
signals travel through the core, then belt,
followed by the parabelt area.
 Simple
 Belt
sounds cause activation in the core area.
and parabelt areas are activated in response
to more complex stimuli made up of many
frequencies.
Figure 11.37 The three main auditory areas in the cortex are the core
area, which contains the primary auditory receiving area (A1), the belt
area, and the parabelt area. Signals, indicated by the arrows, travel
from core, to belt, to parabelt. The dark lines show where the temporal
lobe is pulled back to show areas that would not be visible from the
surface. (From Kaas, Hackett, & Tramo, 1999).
What and Where Streams for Hearing
 What, or ventral stream, starts in the anterior
portion of the core and belt and extends to the
prefrontal cortex.

It is responsible for identifying sounds.
 Where, or dorsal stream, starts in the
posterior core and belt and extends to the
parietal and prefrontal cortices.

It is responsible for locating sounds.
 Evidence from neural recordings, brain
damage, and brain scanning support these
findings.
Areas in the monkey cortex that respond to auditory stimuli.
The green areas respond to auditory stimuli, the purple areas
to both auditory and visual stimuli. The arrows from the
temporal lobe to the frontal lobe represent the what and where
streams in the auditory system.
Effect of Experience on the Auditory Cortex
 Musicians show enlarged auditory cortices
that respond to piano tones and stronger
neural responses than non-musicians.
Cochlear Implants
 Electrodes are inserted into the cochlea to
electrically stimulate auditory nerve fibers.
 The device is made up of:
 a microphone worn behind the ear,
 a sound processor,
 a transmitter mounted on the mastoid bone,
 and a receiver surgically mounted on the
mastoid bone.
Figure 11.46 Cochlear implant device.
Cochlear Implants - continued
 Implants stimulate the cochlea at different
places on the tonotopic map according to
specific frequencies in the stimulus.
 These devices help deaf people to hear some
sounds and to understand language.
 They work best for people who receive them
early in life or for those who have lost their
hearing, although they have caused some
controversy in the deaf community.