Hearing I:
Sound & The Ear
Overview of Topics
Chapter 5 in Chaudhuri
• Philosophical Aside: If a tree falls in the
forest and no one is there to hear it...
• Qualities of sound energy and sound
perception
• Anatomy of the ear
• Auditory brain areas
•
•
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Sound vs. Sound
Sound Waves
If a tree falls in the forest and no one’s there,
does it make a sound?
Depends on two definitions of “sound”
•
Physical definition - sound is pressure changes
in the air or other medium (answer:Yes)
•
Perceptual definition - sound is the experience
we have when we hear (answer: No)
3
•
Objects make sound by moving back and forth rapidly
(20 to 20000 times/second) through a medium (air)
•
Example: A speaker produces sound like this:
•
The diaphragm of the speaker moves out, pushing air
molecules together (compression)
•
The diaphragm moves in, pulling the air molecules
apart (rarefaction)
•
The cycle of this process creates alternating highand low-pressure regions that travel through the air
4
Sound Waves
•
Sound waves are longitudinal waves, meaning that
variations in intensity (air density) are parallel to the
wave’s direction of travel.
•
This is unlike light or water waves, which are transverse
waves, meaning that variations are perpendicular to the
direction of the wave’s travel.
•
Note that it is the sound pressure that moves through
the air, and not the air itself that moves. (Think of “the
wave” at a sports stadium)
Linear vs. Transverse Waves
Transverse Wave (water or light)
Longitudinal Wave (sound)
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Movement Makes
Sound Waves
Questions
• Define sound (hint: give two definitions)
• How is a sound wave like “the wave” at a
Slow
(low freq.)
stadium?
moves through the air in a sound
• What
wave?
Fast
(high freq.)
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Pure Tones
• Pure tone: The simplest form of sound wave.
• Pressure variations are sinusoidal.
• Can be defined by two qualities:
• Amplitude (µPa) ≈ subjective loudness
• Frequency (Hz) ≈ subjective pitch
• All other sounds are made up of
A tuning fork
vibrates sinusoidally,
thus producing a
pure tone
combinations of (usually many) pure tones.
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Two waves with
same frequency but
different amplitudes
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Two waves with
same amplitude but
different frequency
Larger amplitude ≈
subjectively louder
Higher frequency ≈
higher pitch
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Amplitude
Amplitude
• Amplitude = difference in sound pressure between
high and low peaks of wave
• Alternatively, we speak in terms of the ratio of
squared pressures: Ps2 / Pr2
• Sound pressure (P) is typically measured in
• Sound intensity is square of pressure (I = P ) and is
• Reference pressure (P ) is typically 20 µPa.
• This is the lowest pressure variation amplitude
• Usually, we speak in terms of the ratio of sound
• Called “sound pressure level” or SPL
r
micropascals (μPa)
2
more directly related to subjective loudness.
intensity to some reference (Is / Ir)
detectable by an average human at 1000 Hz
frequency
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Decibels
Decibels
•
The range of pressure ratios that humans can hear covers 7
orders of magnitude (i.e., from 1 to 10000000)
•
The range of intensity ratios thus covers 14 orders of
magnitude (from 1 to 100000000000000)
•
This makes direct use of pressure/intensity measures
unwieldy
•
We instead take the log (base 10) of intensity ratios, log10(Is / Ir), to get bels
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• Bels make for a somewhat coarse range, so we take
tenths of bels to get decibels (dB).
dB = 10 × log10(Is / Ir)
dB = 10 x log10(Ps2 / Pr2)
dB = 20 x log10(Ps / Pr)
• Most often we use dB
SPL (“decibels, sound pressure
level”), indicating that Pr is 20 µPa.
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Decibels
Pressure (μPa)
dB = 20 × log10(Ps/Pr)
•
Ps is the difference between maximum and minimum
pressure and Pr is an arbitrary reference pressure
•
Pr is usually taken to be 20 μPa, referred to as SPL or
Sound Pressure Level
•
Each increase by a decibel yields approximately the same
increase in subjective loudness.
•
That is, log nature of the decibel scale compensates
(roughly) for response compression in the auditory system
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Questions
Example for Self-Test
What is log10 of 10? 100? 1000?
db = 20 x log10(40000/40) db = 20 x log10(1000)
db = 20 x 3 = 60 db
•
Reference pressure is 20 µPa and a pressure
wave is hitting your ear with an amplitude of 20
µPa. How many decibels is that?
•
Reference pressure is 20 µPa and a pressure
wave is hitting your ear with an amplitude of
20000 µPa. How many decibels is that?
Is that really loud? Really soft?
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20
fj.j dBSPL
Ambient (reference) pressure is 40 µPa and a
pressure wave is hitting your ear with an
amplitude of 40000 µPa. How many decibels is
that?
j.jj dBSPL
•
•
20
200
2 000
20 000
2 000 000
20 000 000
200 000 000
2 000 000 000
The Doppler Effect
Frequency
- number of amplitude cycles
• Frequency
within a given time period
• Measured in Hertz (Hz): 1 Hz is 1 cycle
per second
of pitch is related to frequency
• Perception
(but there’s more to it)
•
When a sound source approaches at high speed, its sound
seems to increase in frequency, then decrease as it passes
due to compression and expansion of sound waves
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Complex Sounds
Complex Sounds
•
•
Higher frequency elements of a sound are called overtones
Rather, they are made up a complex set of
combined pure tones, each of which has its own
frequency and amplitude
•
Together, the tones that make up a sound are referred to as
its frequency spectrum, which is in part responsible for a
sound’s timbre.
The lowest frequency element of a set of tones is
called the fundamental frequency
•
Good musical instruments produce sounds which have
mostly harmonics and not many non-harmonic partials. (e.g.,
Stradivarius violins)
•
Natural sounds do not have a single frequency
and/or amplitude.
•
•
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Overtones whose frequency is a whole integer multiple of the
fundamental frequency are called harmonics.
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Frequency spectra for 3
instruments playing a tone
with a fundamental
frequency of 196 Hz (G3).
The coloured lines
indicate the frequencies
and intensities of the
harmonic overtones.
Very different sets of pure
tones add up to the same
note, but with different
timbre.
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Attack & Decay
•
Other qualities that make
up timbre:
•
•
Attack & Decay
Attack of tones buildup of sound at the
beginning of a tone
Attack Only
Decay of tones decrease in sound at
end of tone
No Attack or Decay
Decay Only
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Questions
• Describe the relationship between pure
tones and natural sounds.
• Define the following: Fundamental frequency,
overtone, harmonic (or harmonic overtone),
timbre, attack, decay.
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sounds have patterns that repeat
• Periodic
across time. E.g., musical notes, vowel sounds
• Aperiodic sounds have no repeating pattern.
E.g., hissing/fricative sounds, thumps, etc.
Periodic Sounds
Periodic and
Aperiodic Sounds
• AKA “noise”. If equal energy at all
frequencies, we call it “white noise”
Aperiodic Sound
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Fourier
Analysis/Synthesis
•
Fourier discovered that any function could be
mathematically broken down into a series of sine
wave elements (Fourier Analysis)
•
It is also therefore true that any function can be
built up from a series of sine wave elements
(Fourier Synthesis).
•
Remarkably, your auditory system (and visual
system) do something very much like Fourier
Analysis with the information they receive.
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Fourier Synthesis
Five Sound Examples
(a) Pressure changes for a pure
tone with frequency of 440 Hz. 1) 440 Hz tone
2) 880 Hz tone
(b) The 2nd harmonic overtone of
this tone. Frequency = 880 Hz.
3) 440 + 880 Hz tone
4) 1320 Hz tone
5) 440 + 880 + 1320 Hz
tone
(c) The 3rd harmonic overtone. Frequency = 1,320 Hz. (3x440)
Note how the combinations
sound more “complex”, i.e.,
closer to real-world sounds.
(d) The sum of the three
harmonics creates the waveform
for a complex tone.
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(a)
(b)
(c)
Frequency
Fourier spectrum for the last tone on the previous
slide. The heights of the lines indicate the amplitude
of each of the frequencies that make up the tone.
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More examples of Fourier Synthesis
Amplitude
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Questions
Spectrum
(frequency domain)
Pressure
Amplitude
Waveform
(time domain)
Time
• What is the basic idea behind Fourier
Analysis? Fourier Synthesis?
Frequency
• What is a Fourier Spectrum?
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Inverse Square Law
Sound Transmission
• Sound waves must move through a medium
(e.g., air or water)
• Speed of sound depends on density and
elasticity of medium.
dense = slower transmission
• More
More elastic = faster transmission
• Speed of sound in air ≈ 331 m/s
Speed of sound in water ≈ 1400 m/s
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•
As sound moves out from a point source, its energy is
spread over a larger and larger area
•
•
Area is proportional to square of the relative distance
2×distance = 1/4 intensity
3×distance = 1/9 intensity
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Sound & Objects
•
Sound & Objects
Objects create different sounds based on their size, mass,
and elasticity
• E.g., the longer and thicker a piano wire, the lower
frequency sound it produces
•
Objects also vibrate when sound wave hit them. Each object
has a resonant frequency at which it vibrates most strongly
•
This will be a lower frequency the larger the object is, and
higher for more elastic objects.
•
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Sound & Objects
Sound & Objects
Objects also absorb, reflect, transmit, or diffract sound based
on their physical properties.
• The larger and denser an object is, the more it will absorb
sound
• The more elastic an object is, the more it will tend to
reflect sound (e.g., large rock walls cause echoes)
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•
The shape of an object is
also important in how it
will absorb or reflect
sound.
•
Acoustic foam has a
shape designed to trap
sound waves
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Sound & Objects
•
•
•
Diffraction occurs when
sound waves encounter
objects.
•
Sound waves tend to re-form on the other side
of small objects (red) but
not larger ones (yellow)
Acoustic Impedance
•
Sound is reflected when it moves from one medium to
another that has higher acoustic impedance
•
For example, when moving from air to water, 99.9% of
sound energy is reflected
•
This will become important when we consider the inner ear,
which is fluid-filled.
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Sound Illusion:
Shepperd’s Glissando
Sound/Vision Illusion:
The McGurk Effect
Pitch seems to drop forever
But how can this be?
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Questions
does it mean for a sound to diffract
• What
around an object?
• What different processes can occur to a
sound when it encounters an object?
• If the sound intensity is 180 units at distance 10 m, what will it be at 30 m?
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Outer Ear
•
•
Outer ear = pinna and auditory canal
•
•
Pinna helps with sound localization (more later)
Auditory canal ≈ 3 cm long tube. Protects the
tympanic membrane at the end of the canal
Resonance Effect: The resonant frequency of the outer
ear amplifies frequencies between 2,000 and 5,000 Hz
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Middle Ear
•
≈2 cm3 cavity separating
inner from outer ear
•
Contains the three
ossicles: Malleus, Incus, &
Stapes
•
Also Eustachian tube,
which equalizes pressure
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Function of Ossicles
• Outer and middle ear are filled with air
• Inner ear filled with fluid that is much
Function of Ossicles
•
denser than air
• Pressure changes in air transmit poorly into
Condensation Effect:
•
Eardrum is larger than
stapes footplate
•
So force is concentrated
down on a smaller area
to create higher pressure
the denser medium (-30 dB!)
• Ossicles act in two ways to amplify the
vibration for better transmission to the fluid
•
By far the larger of the two
effects, at about 25 dB
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Function of Ossicles
•
Lever Effect:
• Ossicles are set up like a lever arm (malleus is longer than
•
Fluid-filled snail-like structure set into vibration by
the stapes
• Allows weaker force of air vibrations to move liquid in
•
Divided into three canals by two membranes: Scala
vestibuli, (Reissner’s membrane), cochlear duct/
scala media (Cochlear partition) and scala tympani.
•
Cochlear partition extends from the base (stapes
end) to the apex (far end)
•
Subsection of the cochlear partition is the Basilar
Membrane
incus)
cochlea
•
The Inner Ear:
The Cochlea
A small effect, however (about 2 dB)
Resistance of
cochlear fluid
Force of Air
Movement
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The Cochlea
Cochlea: Partly Unwrapped
The cochlea is shown here in its real, coiled, position
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The Cochlea:
Fully Unwrapped
•
•
•
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Cochlear partition is narrow at apex and wide at base.
Basilar membrane is a part of the cochlear partition that
is opposite: Wide at apex and narrow at base.
Spiral lamina make up the rest of the cochlear partition.
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Questions
• Describe the basic structure of the cochlea
is the function of the ossicles? How
• What
do they accomplish it?
do we have a pinna? What are the
• Why
functions of the auditory canal?
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The Organ of Corti
•
Consists of inner and outer hair cells and their
supporting structures
•
Rests on the Basilar membrane, which vibrates in
response to sound stimuli, activating hair cells
•
•
•
Inner hair cells are the receptors for hearing
Cross-section of the
cochlea, showing how
the Organ of Corti
rests on the basilar
membrane.
Tectorial membrane extends over the hair cells
Transduction at the hair cells takes place due to
the bending of tectorial and basilar membranes
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Close-up of the Organ of Corti, showing how hair cells’
stereocilia extend between basilar and tectorial membranes
Animation of the Organ of Corti, showing how hair cells’
stereocilia are bent due to different fulcrum locations of
basilar and tectorial membranes.
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Questions
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Fundamental Concept:
Theoretical Synthesis
•
• Which cells transduce sound into neural
• Thesis: Someone proposes a theory
• Antithesis: A counter-proposal is made, often apparently
signals?
two membranes move relative to one• What
another to stimulate the IHCs?
contradictory or even mutually exclusive of the thesis
• Synthesis: The two are brought together for a more
complete model
•
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Hegel* suggested that understanding progresses through
three stages:
Science often progresses this way. Watch out for false
dichotomies. There is often potential for synthesis.
* The attribution of this idea to Hegal is debatable...Also, there’s way more to it than this.
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Neural Signals for
Sound Frequency
•
There are two ways nerve fibres signal frequency
•
•
Neural Signals for Frequency
Base
Apex
Which fibres are responding (the Place Theory)
•
Hair cells at different points along the OoC
fire to different sound frequencies
How fibres are firing (the Frequency Theory)
•
Rate or pattern of firing of nerve impulses
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Place Theory
Békésys’ Place Theory
• Hz of sound is coded by the place on the
organ of Corti that has the greatest vibration,
and thus greatest stimulation of neurones
• Békésy provided evidence for this in two ways
observation of basilar membranes
• Direct
from cadavers
• Physical properties of the basilar membrane
• Base of the membrane (by stapes) is
• 3 to 4 times narrower than at the apex
• 100 times stiffer than at the apex
• Therefore, the resonant frequency of the base is much
higher than the apex
• Building a model of the cochlea using the
• Indeed, resonant frequency of basilar membrane changes
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physical properties of the basilar membrane
systematically from 20 to 20000 Hz from apex to base.
Békésy suggested that sounds produce a travelling wave along
the basilar membrane.
The peak of this wave occurs at the point where the
membrane’s resonant frequency matches that of the sound’s
frequency.
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Békésys’ Place Theory
• The peak of the envelope* of the traveling
wave indicates the point of maximum
displacement of the basilar membrane
cells at this point are stimulated the
• Hair
most strongly, leading to the nerve fibres
firing the most strongly at this location
A single travelling wave at 3 points in time. Dashed line shows envelope over entire wave.
P is the point of maximum displacement.
* You may want to review “envelope functions”
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Evidence for Place Theory:
Frequency Tuning of Hair Cells
Envelope functions of
basilar membrane vibration
at various frequencies
(Békésy,1960).
• Record activity from single hair cell and measure
Based on measurements of
cadavers’ cochleas.
how intense a sound (dB) is required to activate
the cell at each frequency (Hz)
• i.e., measure the absolute threshold for the cell
Envelopes much more
sharply peaked in living
cochleas due to active
feedback (about which,
more later).
across the frequency spectrum
• Resulting function is the frequency tuning curve
• Hz to which the cell is most sensitive (lowest
threshold) is the characteristic frequency (CF)
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Evidence for Place Theory:
Tonotopic Organization of CFs
Frequency Tuning Curves of 4 Hair Cells
•
Characteristic
frequencies of hair cells
along the Cochlea shows
tonotopic map*
•
i.e., Cochlea shows an
orderly map of frequency
response along its length
* You may want to review topological maps
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Frequency Theory
• At first, a competing theory with place theory
• Proposed that all hair cells fire to every sound, and
their rate of vibration indicates frequency of sound
•
Problem: Whole cochlea physically can’t vibrate across
the range 20 - 20000 Hz, so this cannot be the whole
story. Also, max neurone firing rate = 500 Hz
• However, action potentials from IHCs are found to be
Phase Locking
•
Phase is a characteristic of sine waves
describing where the wave starts
•
The red and blue waves (top right) are
≈90° out of phase with one another
•
The upper set of blue waves (bottom
right) are in phase while those below
are out of phase
phase locked to the sound signals
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Volley Theory
Phase Locking
•
IHCs spike when a pure tone is at
highest compression
•
But max firing rate is 500 spikes/sec,
so how to code for higher
frequencies?
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•
No single neurone can signal each pressure maximum for
tones above about 500 Hz
•
But, by aliasing and firing out of phase with one-another,
several neurones can encode high frequencies.
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Synthesis
Questions
• Both place and volley theories are partly correct
• For sounds below 1000 Hz, only volley theory
• The hair cells are sandwiched between what
applies
• For those between 1000 and 5000 Hz, both place
and volley theory apply
two membranes?
end of the basilar membrane codes
• Which
for low frequency sounds?
• For 5000 Hz and above, only place theory applies
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Transduction:
Inner Hair Cells
• Hearing requires extremely rapid response.
• To response to a 20000 Hz tone, for
instance, cell must be able to encode
changes (not fire!) every 50 microseconds!
• GPCRs, used in other sensory systems, are
too slow for this
• Instead, a direct physical mechanism is used
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Inner Hair
Cells
•
Transduction occurs at the inner
hair cells, specifically at the
stereocilia
•
When stereocilia are bent, tiny
filaments between their tips, called
“tip links” are pulled
•
The tip links directly and
mechanically open ionic channels,
causing the cell to alternately hyper
polarize and depolarize
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Tip Links
Active Response:
Outer Hair Cells
• OHCs do not transduce sound, but they play
an important role in boosting IHC response
• When sound stimulates the OHCs, they
rhythmically contract and expand, like
muscle cells
• This amplifies the motion of the basilar and
tectorial membranes, which in turn more
actively stimulate the IHCs
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Active Response:
Outer Hair Cells
Effect of Active
Response
Living
Chochlea
What
Bekésy
saw
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Questions
• Describe the basic structure of the cochlea
is the function of the ossicles? How
• What
do they accomplish it?
do we have a pinna? What are the
• Why
functions of the auditory canal?
Amplitude Transduction
can sense amplitudes between 20
• Humans
µPa and 20 000 000 µPa (7 orders of
magnitude)
• Neurones can vary firing rates from about 1
to 500 spikes per second (≈2.5 orders of
magnitude)
a single class of neurones cannot code
• So
for the whole range
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Amplitude Transduction
The Basilar Membrane
& Complex Tones
•
Instead, different hair cells
code for different limited
ranges of amplitude
•
“A” codes for a low range,
starting at ≈10 dB &
saturating at ≈50 dB. It has a
high resting response rate
•
“B” responds only to upper
range, starting at ≈50 dB and
saturating at ≈100 dB
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•
Fourier analysis - mathematical process that separates
complex waveforms into a series of sine waves
•
Research on the response of the basilar membrane
shows the highest response in auditory nerve fibers with
characteristic frequencies that correspond to the sinewave components of complex tones
•
In this sense the basilar membrane does a Fourier
Analysis of the incoming sound signal, breaking it down
into component pure tones.
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Fourier Analysis
Fourier Analysis
(a) A complex sound wave.
Applying Fourier analysis to
this wave indicates that it is
made up of the three
components in (b), (c), (d).
These can be represented as a
Fourier Spectrum, below
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Questions
• What is the active response? What is its
function?
• What does it mean to say that the basilar
membrane performs Fourier analysis?
Of Sound Mind
Subcortical and Cortical Auditory Processing Areas
are two ways that frequency is coded
• What
for by cochlear neurones? How is phaselocking involved?
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Pathway from the
Cochlea to the Cortex
•
Auditory nerve fibres synapse in a series of
subcortical structures (C? SONIC MG is A1!)
•
•
•
•
•
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)
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Cochlear Nucleus
Superior Olivary
Nucleus
•
Has tonotopic organization: Ventral = low Hz, Dorsal = high Hz
•
Bushy cells code for different frequencies and inhibit
one another to provide sharper frequency tuning.
•
Stellate cells fire for duration of stimulus (slow
adapting) with rate indicating intensity of sound.
•
Octopus cells fire at sound onset/offset (fast adapting),
and may provide sound timing information.
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•
•
•
Has tonotopic organization
First site of binaural activity.
First analysis of sound direction (horizontal only),
which works via analysis of:
•
•
Timing differences between the 2 ears.
Intensity differences between the 2 ears.
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Inferior Colliculus
• Receives inputs from many higher areas,
such as A1
be a “switchboard” for regulating
• May
auditory attention
an integrating area for multi-modal
• Also
perceptual responses such as startle reflexes
and reflexive looking.
Medial Geniculate
Nucleus
• Nucleus of the thalamus
• Tonotopically organized, but in a complex
fashion
• Processes all aspects of sound. May be the
first site of complex pitch (as opposed to
simple frequency) perception.
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Descending Pathways
Questions
•
Connections do not only go from ear up to cortex
(afferent), but also back down (efferent)
•
For instance, olivocochlear neurones--going from superior
olivary down to cochlea--can turn down the gain on IHCs to
allow them to process higher sound volumes
•
Descending connections also cause contractions in small
muscles attached to ossicles, causing them to be less mobile
and thus “turning down the volume” on high amplitude
sounds
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• What is the sequence of subcortical nuclei
that carries auditory information from ear
to cortex? (remember the mnemonic?)
• What do the descending (efferent) fibres
leading into the cochlea do?
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Auditory Areas in the
Cortex
•
Signals from MGN arrive in A1, primary auditory
cortex
•
A1 seems to process relatively simple sound
information regarding frequency and location
•
•
A2 processes more complex aspects of sound
Organization of A1
• A1 is organized in isofrequency sheets, which run
left-right.
•
Neurones within a sheet have the same CF
• Transverse to the sheets are aural dominance
columns and suppression/summation columns
Other areas (Wernicke’s, Broca’s) process speech
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Schematic of A1
Columnar Organization
16kHz
Suppress
Sum
Suppress
Sum
Suppress
Sum
Suppress
Sum
Suppress
Sum
Suppress
Sum
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8kHz
Left
Binaural
Right
Left
Binaural
Right
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4kHz
2kHz
1 kHz
Effect of Experience on
Tonotopic Maps
Effect of Training on
Tonotopic Maps
• Owl monkeys were trained to discriminate
between two frequencies near 2,500 Hz
monkeys showed tonotopic maps
• Trained
with enlarged areas responding to 2,500 Hz
compared to untrained monkeys
of humans with brain damage to this
• Cases
area show perception difficulties with pitch
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What and Where
Streams for Hearing
What, Where, & Integration
•
It has been proposed that there are separate “what” and
“where” processing circuits in auditory cortex
•
What stream starts in the anterior portion of temporal
lobe and extends to the prefrontal cortex
•
•
•
It is responsible for identifying sounds
Where stream starts in the posterior temporal lobe and
extends to the parietal and prefrontal cortices
•
It is responsible for locating sounds
A similar division of labour is well-established in vision
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Questions
• What is an isofrequency sheet?
• Describe the columnar organization of A1.
• What do we mean by “what and where
streams”?
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