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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.

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meena ramani 04 10 06

EEL 6586

Automatic Speech Processing

Meena Ramani

04/10/06

topics to be covered
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
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
Why study hearing?
  • Best example of speech recognition
    • Mimic human speech processing
  • Hearing aids/ Cochlear implants
  • Speech coding
slide5

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
Specifications

Frequency range: 20Hz-20kHz

Dynamic range: 0-130 dB

JND frequency: 5 cents

JND intensity: ~1dB

Size of cochlea: smaller than a dime

slide7

A

N

A

T

O

M

Y

outer ear

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
slide9

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 Intensity Difference (IID)

interaural differences
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)

slide11

left

right

Illustration of interaural differences

Left

ear

Right

ear

time

sound

onset

slide12

Illustration of interaural differences

Left

ear

Right

ear

time

sound

onset

arrival time

difference

slide13

ongoing time

difference

Illustration of interaural differences

Left

ear

Right

ear

time

sound

onset

slide14

Illustration of interaural differences

Left

ear

intensity difference

Right

ear

time

sound

onset

slide15

Thresholds

Interaural time differences (ITDs)

Threshold ITD  10-20 ms (~ 0.7 cm)

Interaural intensity differences (IIDs)

Threshold IID  1 dB

slide16

D

U

P

L

E

X

T

H

E

O

R

Y

Interaural time differences (ITDs)  Low frequencies

  • Up to around 1500 Hz; sensitivity declines rapidly above 1000 Hz
  • Smallest phase difference corresponds to the true ITD

Interaural intensity differences (IIDs)  High Frequencies

  • 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
outer ear1

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 structureselective amplifies certain parts of the sound spectrum
outer ear2

Pinna /Auricle

Outer ear

Auditory Canal

  • Closed tube resonance: ¼ wave resonator
  • Auditory canal length 2.7cm
  • Resonance frequency ~3Khz
  • Boosts energy between 2-5Khz upto 15dB
slide19

A

N

A

T

O

M

Y

middle ear

Eardrum

Middle Ear

Ossicles

Oval window

Impedance matching

  • 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)

Pressure variations are converted to mechanical motion

Eardrum OssiclesOval Window

Ossicles: Malleus, Incus, Stapes

slide21

A

N

A

T

O

M

Y

inner ear

Semicircular Canals

Inner Ear

Cochlea

  • Body's balance organs
  • Accelerometers in 3 perpendicular planes
    • Hair cells detect fluid movements
    • Connected to the auditory nerve
slide23

Semicircular Canals

Inner Ear

Cochlea

  • Cochlea is a snail-shell like structure 2.5 turns
  • 3 fluid-filled parts:
    • Scala tympani
    • Scala Vestibuli
    • Cochlear duct (Organ of Corti)
  • Organ of Corti
  • Scala tympani
  • Scala vestibulli
  • Spiral ganglion
  • auditory nerve fibres
inner ear1

Semicircular Canals

Inner Ear

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
slide25
The cochlea works as a frequency analyzer

It operates on the incoming sound’s frequencies

place theory

4mm2

1mm2

32-35 mm long

Place Theory
  • 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
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 Neuron

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 Area of Brain

auditory neurons adaptation
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
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
Threshold of Hearing

Hearing area is the area between the Threshold in quiet and the threshold of pain

slide32

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

4mm2

1mm2

32-35 mm long

Threshold variation with age
  • Presbycusis
  • Hearing sensitivity decreases with age especially at High frequencies
  • Threshold of pain remains the same
  • Reduced dynamic range
equal loudness curves
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
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
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