Meena Ramani
04/07/04
Special thanks to Dr. Mark Skowronski
Department of Electrical & Computer Engineering
Topics
• Anatomy of the Ear and Hearing
• Auditory perception
• Hearing aids and Cochlear implants.
Department of Electrical & Computer Engineering
The Incredible sense of Hearing
“Behind these unprepossessing flaps ... lie structures of such
delicacy that they shame the most skillful craftsman"
Stevens, S.S. [Professor of Psychophysics, Harvard University]
Department of Electrical & Computer Engineering
Why study hearing?
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Best example of Speech Recognition
Mimic Human Speech Processing
Hearing Aids/ Cochlear implants
Speech Coding
Department of Electrical & Computer Engineering
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The stapes or stirrup is the smallest bone in our body. It is roughly the size of a
grain of rice ~2.5mm
•
The movement of the eardrum in response to the minimum audible ## sound is
less than the diameter of a hydrogen atom
•
The inner ear has reached its full adult size and shape when the fetus is 20-22
weeks old.
•
Even during sleep the ear continues to function with incredible efficiency
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The ears are responsible for keeping the body in balance
•
Hearing loss is the number one disability in the world.
•
Percentage of people who loose their hearing at age 19 and over: 76.3%
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Dynamic Range of Hearing
•
The practical dynamic range could be said to be from the threshold of
hearing to the threshold of pain
•
Sound level measurements in decibels are generally referenced to a
standard threshold of hearing at 1000 Hz for the human ear which can be
stated in terms of sound intensity:
Dynamic range is enhanced by an effective amplification structure which
extends its low end and by a protective mechanism which extends the high
end.
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Outer Ear
Pinna /Auricle
Auditory Canal
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Focuses sound waves (variations in pressure) into the ear canal
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Sound spreads out according to Inverse Square Law
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A larger pinna captures more of the wave and hence more sound energy.
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Elephants: Hear Low frequency sound from up to 5 miles away
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Human Pinna structure: Pointed forward & has a number of curves.
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More sensitive to sounds in front
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Dogs/ Cats- Movable Pinna => focus on sounds from a particular
direction
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Pinna /Auricle
Outer Ear
Auditory Canal
Horizontal
localization
Sound Localization
Vertical
localization
Is sound on your right or left side?
Interaural Time Difference (ITD)
Interaural
Differences
Interaural Intensity Difference (IID)
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Interaural differences
The direct path from the acoustic source to
the two ears will generally be different
-The signal needs to travel further to more
distant ear
-More distant ear partially occluded by the
head
Two types of interaural difference will
emerge
- Interaural time difference (ITD)
- Interaural intensity difference (IID)
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Schematic illustration of interaural differences
Left
ear
Right
ear
time
sound
onset
left right
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Schematic illustration of interaural differences
Left
ear
Right
ear
time
sound
onset
arrival time
difference
Department of Electrical & Computer Engineering
Schematic illustration of interaural differences
Left
ear
Right
ear
time
sound
onset
ongoing time
difference
Department of Electrical & Computer Engineering
Schematic illustration of interaural differences
intensity difference
Left
ear
Right
ear
time
sound
onset
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Thresholds
Interaural time differences (ITDs)
•
Threshold ITD  10-20 ms (~ 0.7 cm)
Interaural intensity differences (IIDs)
•
Threshold IID  1 dB
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D
Interaural time differences (ITDs) Low frequencies
Ongoing disparities can only be detected for frequencies up to around 1500 Hz
sensitivity declines rapidly above 1000 Hz
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Auditory system assumes that the smallest phase difference corresponds to the
true ITD
P
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E
X
For frequencies below 700 Hz, this strategy will always give the correct answer
Interaural intensity differences (IIDs)  High Frequencies
The amount of attenuation varies across frequency
•
below 500 Hz, IIDs are negligible (due to diffraction)
•
from 2 – 4 kHz, IIDs of 10 dB occur for sources located at 90º
•
U
IIDs can reach up to 20 dB at high frequencies
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Pinna /Auricle
Outer Ear
Auditory Canal
Horizontal
localization
Sound Localization
Vertical
localization
Is sound above or below?
Pinna Directional Filtering
•Pinna amplifies sound above and below differently
•Curves in structure selective amplifies certain parts of the sound spectrum
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Sound localization of Barn Owls and cats
In a Barn Owl, the left ear left opening is higher than the right - so a sound
coming from below the Owl's line of site will reach the right ear first.
Hearing sensitivity comparison of Barn
Owls, Cats & Humans
•Both the cat and the Barn Owl have much
more sensitive hearing than the human in the
range of about 0.5 to 10 kHz.
•The cat and Barn Owl have a similar sensitivity
up to approximately 7 kHz.
• Beyond this point the Barn Owl's sensitivity
declines sharply.
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Project1:
Using Head-Related Transfer Functions to deliver speech for
virtual reality applications
•The simplest spatial audio systems are limited to localizing in azimuth only.
•To go beyond the limited capabilities of these approaches, we need to use Head-Related
Transfer Functions (HRTF's).
•The impulse response from the source to the ear drum is called the Head-Related Impulse
Response (HRIR), and its Fourier transform H(f) is called the Head Related Transfer
Function (HRTF)
•It accounts for diffraction around the head, reflections from the shoulders and most
significantly, reflections from the pinnae.
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Project 2
Beamforming and Direction of Arrival
Frequency Dependent
Frequency Independent
Most DOA algorithms apply Eigen Decomposition for the Spatial correlation
matrix and noise subspace eg. MUSIC, ESPRIT
More biologically inspired DOA algorithm should do better
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Outer Ear
Pinna /Auricle
Auditory Canal
•Auditory canal length 2.7cm
•Can model the canal as a ¼ wave resonator
•Resonance frequency ~3Khz
•Boosts energy between 2-5Khz upto 15dB
•Correspondingly, the hearing curves show a
significant dip in the range 2000-5000 Hz with a peak
sensitivity around 3500 -4000 Hz.
•High sensitivity region at 2-5kHz is very important
for the understanding of speech.
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Eardrum
Middle Ear
Ossicles
Oval window
The tympanic membrane or "eardrum" receives vibrations
traveling up the auditory canal and transfers them through the
tiny ossicles to the oval window.
Functions of Inner Ear
Impedance matching
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–
Between vibrations in air and the liquid medium in the inner ear.
Acoustic impedance of the fluid is 4000 x that of air. => All but 0.1%
would be reflected back.
Stapedius reflex (explained later)
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Eardrum
Middle Ear
Ossicles
Oval window
Eardrum MalleusIncusStapesOval Window
Ossicles: 3 bones Malleus (Hammer), Incus (Anvil), Stapes (Stirrup)
An amplification by lever action < 3x
Area amplification 15x
• Large area of ear drum ( 55mm2), small area of
stapes (3.2 mm2)
• Increases effective Force/Unit area.
Stapedius Reflex:
Protection against low frequency sounds
Tenses muscles stiffens vibration of Ossicles reduces sound transmitted (20dB)
Reflex is triggered by loud sounds
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Inner Ear
Semicircular
Canals
Cochlea
•The semicircular canals are the body's
balance organs.
•Hair cells, in the canals, detect movements of
the fluid in the canals caused by angular
acceleration
•The canals are connected to the auditory
nerve.
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Semicircular
Canals
Inner Ear
Cochlea
The inner ear structure called the cochlea is a snail-shell
like structure divided into three fluid-filled parts.
Two are canals (Scala tympani and Scala Vestibuli) for the
transmission of pressure and in the third is the sensitive
organ of Corti, which detects pressure impulses and
responds with electrical impulses which travel along the
auditory nerve to the brain.
This mid-modiolar section shows the
coiling of the cochlear duct (1) the scala
vestibuli (2) and scala tympani (3).The
red arrow is from the oval window, the
blue arrow points to the round window.
Within the modiolus, the spiral ganglion
(4) and auditory nerve fibres (5) are
seen.
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Inner Ear
Semicircular
Canals
Cochlea
•The organ of Corti can be thought of as the body's microphone.
•Perception of pitch and perception of loudness is connected with this organ.
• It is situated on the basilar membrane in the cochlea duct
• It contains inner hair cells and outer hair cells.
•There are some 16,000 -20,000 of the hair cells distributed along the basilar membrane.
•Vibrations of the oval window causes the cochlear fluid to vibrate.
•This causes the Basilar membrane to vibrate thus producing a traveling wave.
•This causes the bending of the hair cells which produces generator potentials
•If large enough will stimulate the fibers of the auditory nerve to produce action potentials
•The outer hair cells amplify vibrations of the basilar membrane
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The cochlea works as a frequency analyzer
It operates on the incoming sound’s frequencies
Frequency Theory
Place Theory
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Frequency Theory
BM vibrates in synchrony with the sound entering the ear, producing
action potentials-- in auditory nerve cells -- at the same frequency
(e.g., 50 Hz sound -> 50 APs/sec).
Limitations: max APs/sec = 200 Hz.
Use this theory for Frequencies <100Hz
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Place Theory
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4mm2
1mm2
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32-35 mm long
High frequency sounds selectively vibrate the BM
of the inner ear near the oval window.
=>Each position along the BM has a characteristic
frequency at which it has maximum vibration.
Lower frequencies travel further along the
membrane before causing excitation of the
membrane.
The place along the basilar membrane where
maximum excitation of the hair cells occurs
determines the perception of pitch
At the base, the basilar membrane is stiff and
thin (more responsive to high Hz)
At the end or “apex”, the basilar membrane is
wide and floppy (more responsive to low Hz)
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Tuning curves of auditory nerve fibers
Tonotopic map on Cochlea: Cells in different spots on the cochlea
respond to different frequencies, with high frequencies near the base,
and low frequencies near the apex.
Method to verify
•Apply 50ms tone bursts every 100ms
•Increase sound level until discharge rate
increases by 1 spike
•Repeat for all frequencies
Response curve is a BPF with almost
constant Q(=f0/BW)
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Auditory Neuron
The auditory nerve takes electrical
impulses from the cochlea and the
semicircular canals
Makes connections with both auditory
areas of the brain.
Auditory Area of Brain
Information from both ears goes to both
sides of the brain - binaural information
is present in all of the major relay
stations.
----- Left ear information
___ Right ear information
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Auditory Neurons Adaptation
•When a stimulus is suddenly applied spike rate
of an auditory neuron fiber increases rapidly
•If the stimulus remains (a steady tone for eg.)
the rate decreases exponentially
•Spontaneous rate: neuron firings in the
absence of stimulus.
Neuron is more responsive to changes
than to steady inputs
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Perception of Sound
Threshold of hearing
– How it is measured
– Age effects
Equal Loudness curves
Bass loss problem
Critical bands
Frequency Masking
Temporal Masking
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Threshold of Hearing
Hearing area is the area between
the Threshold in quiet and the
threshold of pain.
Note:
Shift in threshold of quiet for
those who listen to loud music
The sound intensity required to be heard is quite different for different frequencies.
Threshold of hearing at 1000 Hz is nominally taken to be 0 dB.
Marked discrimination against low frequencies so that about 60 dB is required to be
heard at 30 Hz.
The maximum sensitivity at about 3500 to 4000 Hz is related to the resonance of the
auditory canal.
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Bekesy Tracking
Used to measure Threshold in
quiet or JNL of a test tone
STEPS:
•Play a tone
•Vary its amplitude till its
audible
•Then tone’s amplitude is
reduced to definitely inaudible
and the frequency is slowly
changed
•Then increase the SPL till you
can hear and so on.
Whole recording will last atleast 15minutes
Change in level at fine steps <2dB else clicks become audible and act as a
cue to listener
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Threshold in Quiet variation with age
•Hearing sensitivity decreases
with age especially at High
frequencies
•Note we also loose the
sensitivity at 3.5-4Khz
•Presbycusis: hearing loss
because of age
•Hair cells which process HF
are closest to the oval window
and are often the first to be
damaged.
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Equal Loudness Curves
Loudness is not simply
sound intensity!
Subjective term describing
the strength of the ear's
perception of a sound.
Have to include the ear's
sensitivity to the particular
frequencies contained in the
sound as in the equal
loudness curves.
Sound must be increased in
intensity by a factor of ten
for the sound to be
perceived as twice as loud.
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The Bass Loss Problem
For very soft sounds, near the threshold of
hearing, the ear strongly discriminates against
low frequencies.
For mid-range sounds around 60 phons, the
discrimination is not so pronounced
For very loud sounds in the neighborhood of
120 phons, the hearing response is more nearly
flat.
Eg. Rock music
Too lowno bass
Too hightoo much bass
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Ohms law of hearing
The sound quality of a complex tone depends ONLY on the
amplitudes
and NOT relative phases of its harmonics.
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Elephants
Sound Production
A a typical male elephant’s rumble is around an average minimum of 12 Hz, a
female's rumble around 13 Hz and a calf's around 22 Hz.
Produce sounds ranging over more than 10 octaves, from 5 Hz to over 9,000 Hz
Produce very gentle, soft sounds as well as extremely powerful sounds. (112dB
recorded a meter away)
Hearing
Wider tympanic membranes
Longer ear canals (20 cm)
Spacious middle ears.
Low frequency detection
Department of Electrical & Computer Engineering