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EEL 6586
Automatic Speech Processing
Meena Ramani
04/10/06
Topics to be covered
Lecture 1: The incredible sense of hearing 1
Anatomy
Perception of Sound
Lecture 2: The incredible sense of hearing 2
Psychoacoustics
Hearing aids and cochlear implants
Lecture 1:
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]
Why study hearing?
• Best example of speech recognition
– Mimic human speech processing
• Hearing aids/ Cochlear implants
• Speech coding
Interesting facts
• The stapes or stirrup is the smallest bone in our body.
– It is roughly the size of a grain of rice ~2.5mm
• Eardrum moves less than the diameter of a hydrogen atom
– For minimum audible sounds
• Inner ear reaches its full adult size when the fetus is 20-22 weeks old.
• The ears are responsible for keeping the body in balance
• Hearing loss is the number one disability in the world.
– 76.3% of people loose their hearing at age 19 and over
Specifications
Frequency range: 20Hz-20kHz
Dynamic range: 0-130 dB
JND frequency: 5 cents
JND intensity: ~1dB
Size of cochlea: smaller than a dime
A
N
A
T
O
M
Y
Pinna /Auricle
Outer ear
Auditory Canal
Focuses sound waves (variations in pressure) into the ear canal
Pinna size:
• Inverse Square Law
• Larger pinna captures more of the wave
• Elephants: hear low frequency sound from up to 5 miles away
Human Pinna structure:
• Pointed forward & has a number of curves
• Helps in sound localization
• More sensitive to sounds in front
Dogs/ Cats- Movable Pinna => focus on sounds from a particular
direction
Pinna /Auricle
Outer ear
Sound Localization
Auditory Canal
Horizontal
localization
Vertical localization
Is sound on your right or left side?
Interaural Time Difference (ITD)
Interaural Intensity Difference (IID)
Interaural differences
- 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)
Illustration of interaural differences
Left
ear
Right
ear
time
sound
onset
left right
Illustration of interaural differences
Left
ear
Right
ear
time
sound
onset
arrival time
difference
Illustration of interaural differences
Left
ear
Right
ear
time
sound
onset
ongoing time
difference
Illustration of interaural differences
intensity difference
Left
ear
Right
ear
time
sound
onset
Thresholds
Interaural time differences (ITDs)
Threshold ITD  10-20 ms (~ 0.7 cm)
Interaural intensity differences (IIDs)
Threshold IID  1 dB
D
U
P
Interaural time differences (ITDs)  Low frequencies
L
•
Up to around 1500 Hz; sensitivity declines rapidly above 1000 Hz
E
•
Smallest phase difference corresponds to the true ITD
X
Interaural intensity differences (IIDs)  High Frequencies
T
•
The amount of attenuation varies across frequency
•
below 500 Hz, IIDs are negligible (due to diffraction)
•
IIDs can reach up to 20 dB at high frequencies
H
E
O
R
Y
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
Pinna /Auricle
Outer ear
•Closed tube resonance: ¼ wave resonator
•Auditory canal length 2.7cm
•Resonance frequency ~3Khz
•Boosts energy between 2-5Khz upto 15dB
Auditory Canal
A
N
A
T
O
M
Y
Eardrum
Middle Ear
Ossicles
Oval window
Pressure variations are converted to mechanical motion
Eardrum OssiclesOval Window
Ossicles:
Impedance matching
Malleus, Incus, Stapes
– Acoustic impedance of the fluid is 4000 x that of air
– All but 0.1% would be reflected back
Amplification
– By lever action < 3x
– Area amplification [55mm2  3.2mm2] 15x
Stapedius reflex
– Protection against low frequency loud sounds
– Tenses muscles stiffens vibration of Ossicles
– Reduces sound transmitted (20dB)
A
N
A
T
O
M
Y
Inner Ear
Semicircular
Canals
Cochlea
Body's balance organs
Accelerometers in 3 perpendicular planes
•Hair cells detect fluid movements
•Connected to the auditory nerve
Inner Ear
Semicircular
Canals
Cochlea
Cochlea is a snail-shell like structure 2.5 turns
3 fluid-filled parts:
•Scala tympani
•Scala Vestibuli
•Cochlear duct (Organ of Corti)
(1) Organ of Corti
(2) Scala tympani
(3) Scala vestibulli
(4) Spiral ganglion
(5) auditory nerve fibres
Inner Ear
Semicircular
Canals
Cochlea
Organ of Corti
Basilar membrane
Inner hair cells and outer hair cells (16,000 -20,000)
IHC:100 tiny stereocilia
The body's microphone:
•Vibrations of the oval window causes the cochlear fluid to vibrate
•Basilar membrane vibration produces a traveling wave
•Bending of the IHC cilia produces action potentials
•The outer hair cells amplify vibrations of the basilar membrane
The cochlea works as a frequency analyzer
It operates on the incoming sound’s frequencies
Place Theory
4mm2
1mm2
32-35 mm long
• Each position along the BM has a characteristic frequency for
maximum vibration
• Frequency of vibration depends on the place along the BM
• At the base, the BM is stiff and thin (more responsive to high Hz)
• At the apex, the BM is wide and floppy (more responsive to low Hz)
Tuning curves of auditory nerve fibers
To determine the tonotopic map on Cochlea
•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)
Auditory Neuron
Auditory Area of Brain
Carries impulses from both the cochlea and the
semicircular canals
Connections with both auditory areas of the brain
Neurons encode
–
–
Steady state sounds
Onsets or rapidly changing frequencies
Auditory Neurons Adaptation
•At onset, auditory neuron fiber firing
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
Perception of Sound
Threshold of hearing
– How it is measured
– Age effects
Equal Loudness curves
Bass loss problem
Critical bands
Frequency Masking
Temporal Masking
Threshold of Hearing
Hearing area is the area between the Threshold in quiet
and the threshold of pain
Bekesy Tracking
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
•Continu\e
Threshold variation with age
•Presbycusis
•Hearing sensitivity decreases
with age especially at High
frequencies
•Threshold of pain remains the
same
•Reduced dynamic range
4mm2
1mm2
32-35 mm long
Equal Loudness Curves
Loudness is not simply
sound intensity!
Factor of ten increase in
intensity for the sound to be
perceived as twice as loud.
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
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