PSYC 212 | Chapter 5 – The Auditory System: Sound and the Ear
- Used so that chemosensory information can be used for communication
- Is a distant sense stimulus can be detected from far away
- Fastest among all sensory systems
A. The Physics of Sound
3 requirements for humans to hear sound:
- Something that creates the sound
- Sound must propagate through medium from its source
- Mechanism to translate sound energy into a biological signal that
ultimately produces hearing (aka our perceptual experience)
Sound is caused by transmission of vibrational waves through a medium:
- Proposed by Galileo
- Boyle’s bell jar experiment: sound produced by bell is less audible if
in a jar in which air is slowly pumped out. Becomes silent when no air
left in jar.
A1. The Creation of Sound
Sound has 2 definitions:
- Psychology definition: physical event that produced perceptual
experience (hearing). Described by a set of qualities (loudness, pitch,
etc…) since they are based on our perception.
- Physics definition: vibrational disturbance of a medium (medium can
be solid, liquid, or gas); can be associated with physical qualities. Its
existence is independent of perception
- “If a tree falls and no one hears it, does it make a sound?”
– Psychologist: yes
– Physicist: no
Vibrational properties of objects
- To produce sound, an object must possess two opposing forces (inertia
and elasticity) to able to vibrate (to impart a vibrational disturbance to
- Inertia: - “things like to keep doing what they’re doing”
- Since objects have mass If object is at rest, it will resist being moved
If object is in motion, it will resist coming to rest
- Object will have tendency to resist becoming deformed
- But once deformation starts, it will continue until opposing force
- Elasticity: - Tendency to return object to its original state; ability to
- Ensures that object’s deformation does not continue beyond certain
The tuning fork as a sound source & Impact of a sound source on the medium The opposing forces of inertia and elasticity causes the back and forth motion
of a tuning fork
Simple harmonic motion sinusoidal function
- Tuning fork’s SHM makes a vibrational disturbance in air
- Since air also possesses inertia and elasticity, will have similar effects
- Compression (increase in air pressure) immediately surrounding
tuning fork’s prongs caused but its outward movement
- Rarefaction (decrease in air pressure) caused when prong moves in
- Air molecules in immediate surrounding of prongs alternate between
states of compression and rarefaction as they vibrate
- Each air molecule acts as one vibrator, colliding with neighboring
particles, which causes them to vibrate.
- Momentary compressions and rarefactions passing from region to
region cause waves of outward vibrational changes in air pressure
(vibrational energy). This is what generates the sound waves.
- Sinusoidal SMH causes sinusoidal profile of pressure change
Results in pure tone: single sinusoidal function of air pressure
change over time
A2. The Properties of Sound
Sound: longitudinal travelling wave of pressure disturbance in a medium.
Auditory system detects the pressure changes, causing “hearing”.
Amplitude and its relationship to sound intensity
- Amplitude: pressure change from baseline to peak of sinusoidal
function (for pure tone).
The louder the sound, the greater the pressure changes, the greater
- Unit of pressure for sound: micropascal (μPa)
- Intensity: square of sound pressure I = P
- For sound, intensity in relation to reference level is used, not absolute
- We use ratios P /Ps r
- Ps: peak pressure (amplitude) of sound we want to measure
- Pr: peak pressure of reference sound (usually minimum audible sound
level: 20 μPa)
- Intensity depends on vibrational amplitude
Representing sound intensities
Since scale of sound level ratios is so large, we use decibels (one tenth of a
dB = 10log(I /Is)r= 10log(P /P )s r2
When dB values are shown as dB SPL(where SPL stands for sound pressure
level), infers that minimum audible reference value was used for I . r
- Since decibels are a logarithmic scale, two 40 dB SPLdoes not equal 80 dB SPL
Sound frequency Frequency: number of cycles (complete vibration containing one instance
and compression and one of rarefaction) in a second (measured in Hz)
- Frequency depends on the vibrational (physical) properties of a sound
source. This is why any tune fork (has one vibrational property) can
produce only one pure tone of only one frequency.
- Resonant (natural) frequency: vibrational frequency resulting from
the sound sources’ mass and stiffness (elasticity). The greater the
stiffness, the greater the vibrational frequency; the greater the mass (or
length), the lower the frequency.
Speed of sound
- Depends only on the medium, not the source.
- Characteristics that affect sound speed: inertia and elasticity
- Inertial property: expressed by medium’s density. The greater the
density, the lower the sound.
- Temperature affects this relationship (especially for gas and liquids) -
as temperature increases, medium expands and decreases in density
so speed of sound increases
- Elasticity: the greater the elasticity of a medium, the greater the
speed of sound.
- Speed of sound in steel is much greater than in air. Although steel is
much more dense than air, it is also much more elastic.
A3. Complex Sounds
Most sounds do not display SHM, as their vibrational sequence is more
Complex periodic sounds
Period sounds are produced when the pattern of pressure change repeats
itself at regular intervals over time.
Simple periodic wave: produces a wave with a sinusoidal profile (ex: pure
Complex periodic waves: repeating wave that does not have sinusoidal
- Contain discrete patterns that are exactly duplicated in a cyclic manner
- Created by combining multiple sinusoidal waves that differ in frequency
- For any point in time, simply add the respective amplitudes of each
wave to produce the compound wave of the complex periodic sound.
Notes on a musical instrument have periodic profile characterized by the
summation of series of sine waves (harmonic series). Strings on a guitar
vibrate as a whole at a fundamental frequency, but parts of the string also
vibrate separately, but simultaneously, to create overtone harmonics.
Overtone frequencies are whole number multiples of the fundamental
frequency (2 , 3 ,…) harmonic. (Discussed further in Chapter 7)
Complex aperiodic sounds
Noise: sounds produced by aperiodic vibration
- Vibrations are random, impossible to find any time intervals where
pressure change is identical. - Can be created by combining many sound frequencies with random,
White noise: noise pattern composed of all the frequencies within a
particular range (ex: range of human hearing) (same amplitude)
- Ex: traffic, noise from a large crowd. Can also be soothing: waterfall.
Any function, no matter how complex, can be decomposed intro a series
of sine-wave functions, without prior knowledge of what those constituent
Fourier spectrum: sine wave functions derived from Fourier analysis in terms
of amplitude and frequency. X- axis = frequency, y-axis (height) = amplitude.
Ohm’s law of hearing
Complex sound waves can be subjected to Fourier analysis and this operation
occurs in hearing.
A4. Sound Transmission
Sound intensity becomes dissipated with distance.
Sound waves must interact with objects along their path, which can interfere
with, causing it to enhance or dampen.
The inverse square law Intensity = 1/r (only true if there are no obstacles
Sound from a point source radiate in all direction, creating a moving spherical
Sound intensity dissipates with distance because the energy contained in
sound must be distributed over a progressively larger area of the sphere as
sound propagates outward.
The area of a sphere grows according to the square of this distance, so sound
intensity decreases correspondingly.
For example: doubling the distance would cause sound intensity to be
decreased to ¼ of its original value.
Interaction of sound with objects
Sound can interact with obstacles in its path in 3 ways: reflection, absorption,
As with light, a boundary can serve as a reflective surface if the second
medium has a greater resistance to sound transmission.
Examples: Hard surfaces and air-water interface.
Acoustic impedance: resistance to sound. The greater the impedance
difference at a surface, the greater the amount of sound reflection.
Echoes/reverberations: reflected sounds.
Reverberant room: room with highly reflective walls.
Interface (border) between different surface is absorbed and could be
transmitted though the second. Absorption coefficient: the proportion of sound energy absorbed to that
contained in the incident wave. Denotes the magnitude of absorption.
Examples: water has low absorption coefficient, while rubber and Styrofoam
can absorb up to 1/3 of the incident sound.
Anechoic room: room with walls made with high sound absorbance materials
Diffraction: sound waves bend around objects when the object is too small
for reflection or absorption to occur.
Sound waves of lower frequency span a larger space, so they diffract more
easily. Sound waves of high frequency have more compressed undulations
(ripples) of pressure change, and do not diffract easily.
Properties of absorption and reflection need to be considered for
Wallace Sabine: important factor in acoustic sustainability is sound
reflection, quantified by reverberation time.
Fullness: when rooms have high reverberation
Clarity: when rooms have less reverberation
B. Auditory Processing of Sound – Physical Characteristics
B1. Anatomical Components of the Human Ear
Sound waves are transmitted though series of structures in the ear. These
structures must retain the frequency characteristics of the original sound
stimulus and transmit it without causing (or minimally) disruption in
The outer ear 3 parts: pinna, external auditory cannal, and tympanic
Pinna: - made of cartilage, has bumps and grooves that enhance sound
- The sound funnel
- Some animals have pinna muscles that can move in the direction of
- Humans do not have this muscle, so human pinnas are passive
receptors of sound
External auditory canal: - pinna funnels sound into here
- Is S-shaped, ~25mm in length, 5-7mm in diameter
- Outer half lined by cartilage, inner part covered by skin lining the
- Canal is lined with wax secreting glands that protect eardrum from
small foreign objects
Eardrum: - interior boundary of the external ear.
- Thin elastic membrane, ~10 mm in diameter
- Attached to skin of EAC’s innermost end, and seals it completely
- Incoming sound waves set off vibrational pattern on it, which then pass
to middle ear
- Protects middle ear against foreign objects. The middle ear – air filled space within temporal bone; immediately adjacent
- Rectangular chamber; eardrum forms wall, other walls formed by thin
plates of bone. Except: front wall opens into narrow Eustachian tube,
which connects middle ear to back of throat. This allows equalization
of any pressure differences across eardrum, so eardrum can work under
different atmospheric pressures. (Ears popping on plane)
- Increased/decreases outside pressure makes the eardrum
inward/outward, respectively. These two cases create tension in the
eardrum and reduce its vibrational response to incoming sounds waves
= reduced hearing.
Sound waves transmitted from eardrum to middle ear happens in 1 of 3 ways:
1) Transmitted through the bony matrix of the head
2) Conducted through air hat is present in the middle ear space
These two play a role in sound transmission, but their effect is negligible due
to dampening of sound stimulus. Third mechanism evolved to physically
conduct sound vibrations through series of bony levers (auditory ossicles) to
the inner ear. These bones from the outside in: mallus, incus, and stapes.
Ossicles are suspended in middle air space by ligaments, connected to form
Suspension allows them to vibrate freely and provide efficient transmission of
the original acoustic pattern.
Malleus: made of long bony protrusions (called “handle”) which are attached
- Head is tightly connected to body of incus.
Incus: - its shaft (the “long process”) forms a joint with head of the stapes.
Stapes: lies horizontally at right angle to incus, has oval-shaped footplate
that makes a tight connection with oval window of inner ear.
Vibration of eardrum conducts through this ossicular chain, and then transfers
to inner eat at oval window.
Temperal bone cavity = bony labyrinth vesticular system and cochlea
Cochlea: coiled fluid-filled bony structure, contains sensory transduction
apparatus for hearing
- Where physical energy in sound waves cause electrochemical signal
transmitted to the brain
- Begins at its basal, makes almost 2 ½ turns and ends at apex.
- ~35 mm in length, ~9 mm at diameter at base (but less at apex)
- Divided into 3 fluid-filled channels (in order): scala vestibuli, cochlear
duct, scala tympani
- scalae vestibuli (largest channel) and tympani are connected by
helicotrema (part of apex) so contain same fluid
Cochlear duct: smallest channel (7% of cochlea)
- Reisner (thin & delicate)/basilar separates it from scala
- Self-contained chamber filled with different fluid (although similar
consistency, but different ionic concentrations, does not come in
contact with the other scalae. Sound transmission through the ear
Funneled by pina transmitted through external auditory canal eardrum
(vibrational pattern set off, conducted by ossicles, goes through oval windown
to) cochlea and its fluids stapes does back and forth motion on oval
window, initiates compressional sound wave in scala vestibulli fluid Sound
now characterized by movement of liquid molecules, not air (so faster).
- Moving stapes inward on the oval window creates momentary pressure
increase, which then distributes in the cochlear fluids. Increased
pressure in the scala vestibuli goes ascorss Reissner’s membrane,
through the cochlear duct, and across basilar membrane into scala
- The round window (elastic membrane at basal end of scala tympani)
relieves the increased fluid pressure in the tympani by its outward
- This whole process is in opposite direction when stapes moves outward.
- Pressure fluctuations in cochlear fluids need to cause movement of
membranes that separate the 3 channels.
- Neural transduction apparatus rests on the basilar membrane, so
the membrane’s up and down movement is critical.
B2. Amplitude Preservation
4 physical features make up for potentially large vibrational amplitude loss
that occurs with sounds reaching the cochlear interface on their own (directly
The lever effect
Transmission of acoustic energy thought bones of middle ear produces ~2 dB
Difference in size between the malleus and incus create a small increment in
The condensation effect – occurs from the design of the auditory components
- Sounds channels from eardrum to footplate of stapes (20x smaller than
eardrum). This results in vibrational pressures condensing from large
to small area, which produces an amplification corresponding to the
surface area difference.
- Amplification is ~25dB
The resonance effect
For sound frequencies from 2500-5000 Hz, can gain 10-15 dB due to this
Resonant properties of some outer ear components (pinna, external auditory
canal, …) amplify sounds at their according resonant frequency [range].
The directional effect
- Ossicular chain of middle ear channels all incoming sound energy upon
only the oval window.
- Sound energy at this window causes reciprocal movement of round
window. - Result: vibrational amplitudes in cochlea would be reduced by as much
as 60 dB.
- This occurs in diseases of middle ear where ossicles are impaired.
B3. Frequency Representation
The resonance theory – Hermann von Helmholtz: basilar membrane is
composed of a series of fibers tuned to different frequencies.
- Sound of a certain frequency would set an appropriate fiber in motion
that would stimulate a certain nerve fiber projecting to the brain
- Abandoned since cannot account for full range of frequencies audible to
The frequency theory – Ernest Rutherford
- Basilar membrane is capable of vibrating within the full range of
frequencies audible to humans
- Auditory never fibers are stimulated by basilar membrane; their firing
rate conveys sounds frequency information to the brain.
- Neural firing rate mirrors sound frequency, neural firing amplitude
- Neural firing is all-or-none, so they cannot encode sound intensity.
- Firing rate is constrained by time required to complete action potential,
so cannot encode frequency either.
- In this theory, basilar membrane would have to have uniform width, so
that it can oscillate at same frequency throughout. However, its width is
The place theory – George von Bekesy
- Sounds of different frequencies produce a vibrational pattern, where its
maximum amplitude occurs at different place along the basilar
- The fact that basilar membrane was uneven and the tension on
this membrane is higher at the basal end, showed Bekesy that the
membrane cannot show the same vibrational response to sounds of
different frequencies. Resonance frequency along its length is not the
- Bekesy proposed that vibrational disturbance in cochlear fluids set up a
travelling wave within the basilar membrane whereby a pure-tone of
a certain frequency produces maximum displacement only at the
point where it matches with the resonant frequency of the basilar
- The smaller width and greater tension at basal end produce higher
resonant frequency, but as you go towards the apex, width increases
and resonant frequency decreases.
- As a result, high/low frequency stimulus makes maximum vibration at
basal end/apical end of the basilar membrane, respectively
Tonotopic map: the way frequency is represented The basilar membrane changes systematically to include nearly the full
frequency range audible to humans.
The closer you are to the base, the higher the frequency.
Frequency analysis of complex sounds
The auditory system filtration process produces a frequency decomposition
similar to that of Fourier analysis.
When the ear is exposed to complex sound, vibrational disturbance is created
in cochlear fluid and follows the complex waveform of the sound stimulus
Since there are vibrational restrictions on the basilar membrane (due to its
physical characteristics), the sine-wave components of the complex sound
produce the vibrational disturbance (but they only occur where the
frequencies of the sound’s spectrum match the basilar membrane’s resonant
- The basilar membrane acts as a frequency analyzer; the distribution
of vibrational disturbances across it is then analyzed by neural
components of the auditory system.
C. Auditory Processing of Sound – Biological Mechanisms
Sound must be transduced into a neural signal, which the brain can interpret
to produce hearing
C1. Auditory Transduction
Organ of Corti: group of cells and structures that span across the cochlea in
- Since it’s so close to the basilar membrane, it can impart its vibrational
disturbances, which produce the initial bioelectric response through a
certain process, the first event of neural processing of sound.
Organ of Corti – structural features
Is in the cochlear duct, so is