S 319 < Auditory system >

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S 319 lt Auditory system gt

• Focus
• - Anatomy and function of the ear
• - Sound transfer function of ear
• When sound is conducted into the ear, how
  sound is affected by that process

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Outer ear Middle ear Inner ear
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Four major divisions of auditory system - Anatomy

• The outer ear
• - pinna
• - ear canal
• - eardrum
• 2. The middle ear
• - three ossicle bones
• (malleus, incus, stapes)
• - two major muscles
• (stapedial muscle, tensor
• tympani)
• - Eustachian tube
• 3. The inner ear
• - cochlea (hearing)
• - vestibular system (balance)

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• Four major divisions of auditory system
  Function
• (CD, Figure 3.4.1)

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I. Outer ear (CD, Figure 3.4.2)

• Three parts of outer ear
• 1) Pinna
• 2) Ear canal
• 3) Ear drum
• Major function of outer ear 1) protection
• 2) amplification
• 3) sound localization

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I. Outer ear (1) Pinna (Binaural cue to sound
source location)
Sound
t Right
t Left
Left ear

Right ear
Different distances from source to each ear gt
different arrival times (Interaural
time-difference) and different sound level
(interaural level-difference)
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I. Outer ear (1) Pinna (Spectral cue to sound
source location)
Outer ear gain

The spectral feature of sound is changed
depending on the sound elevation gt Head Related
Transfer Function (HRTF)
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I. Outer ear (1) Pinna Cases of abnormal pinna

• Anotia

• Microtia (Grade I)

• Microtia (Grade II)

• Microtia (Grade III)

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I. Outer ear (2) Ear canal (CD, Figure 3.4.4)

• Three parts of outer ear
• 1) Pinna
• 2) Ear canal
• 3) Ear drum
• Major function of outer ear 1) protection
• 2) amplification
• 3) sound localization

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I. Outer ear (2) Transfer function of ear canal
From Gelfand (1998)
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I. Outer ear (3) ear drum (CD, Figure 3.4.5)
Major function of outer ear As a boundary
between outer and middle ear Vibrates in
response to sound Three-dimensional finite
element method (FEM) analysis of the middle ear
http//www.wadalab.mech.tohoku.ac.jp/FEM_mid-e.htm
lfig1

.
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II. Middle ear

• Three main parts of middle ear
• (1) Three Ossicle bones
• - Malleus(1), Incus(3), Stapes(6)
• Function) Impedance matching
• (2) Two muscles
• - Stapedial muscle(5)
• - Tensor tympani(9)
• Function) Protection
• (3) Eustachian tube(8)
• Function) Equalizer of air pressure

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II. Middle ear (3) Eustachian Tube

• Comparison of Eustachian tubes In adults and
  children
• shorter, smaller, less steep eustachian
  tube in children
• gt Hard to be drained away from middle ear

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II. Middle ear (CD, Figure 3.4.7)

• Middle ear cavity

• Function of ossicles
• - 99.9 sound is reflected due to high
  impedance of fluid in the cochlea (0.1 sound is
  only passed - 30 dB sound loss from air - fluid
  impedance mismatch)
• - Middle ear bones overcome the loss of
  sound by increasing sound pressure (34dB)
• gt Impedance matching

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II. Middle ear (CD, Figure 3.4.9)

• Three mechanisms for impedance matching
• 1) Area ratio of the ear drum to the stapes
  footplate (201)
• gt 20 log (20/1) 26dB SPL
• Basic concept p f/a
• 2) Lever action of the ossicles (1.31)
• gt 20 log(1.3/1) 2 dB SPL
• 3) Buckling of ear drum
• ( x 2 pressure increase
• gt 20 log(2/1) 6dB SPL

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II. Middle ear - Impedance matching
In total, 20 x 1.3 x 2 increase in pressure by
middle ear and ear drum ( 34 dB SPL). It works
for mismatched impedance (99.9 sound loss apx
- 30 dB) Three-dimensional finite element method
(FEM) analysis of the middle ear http//www.wadala
b.mech.tohoku.ac.jp/FEM_mid-e.htmlfig6

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III. Inner ear (CD, Figure 3.4.11)

• Twp parts of inner ear
• 1) Cochlea (Hearing)
• - Scala vestibuli
• - Scala media
• - Scala tympani
• 2) Vestibular system (balance)
• Major function of inner ear
• 1) Hearing
• (It transmits sound to
• neural impulse and gives
• resonant frequency)
• 2) Balance

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III. Inner ear Cochlea
endolymph
perilymph
perilymph

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III. Inner ear Resonance of Basilar membrane
Figure 15.32
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III. Inner ear Cochlea
Figure 15.28
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III. Inner ear Inner hair cells (IHC) Outer
hair cells (OHC)
Inner hair cells produce sensation of
hearing Outer hair cells modify BM response and
act as amplification system
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III. Inner ear Sound transduction

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IV. Central auditory system (CD, Figure 3.4.14)

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IV. Central auditory system - Auditory pathway to
brain
Tonotopy!

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Overall, how sound travels through the ear...
Outer ear Acoustic energy, in the form of sound
waves, passes pinna, ear canal. Sound waves hit
the ear drum, causing it to vibrate like a drum.
Middle ear It sets three ossicle bones
(malleus, incus, stapes) into motion, changing
acoustic energy to mechanical energy. These
middle ear bones mechanically amplify sound and
compensate mismatched impedance. Inner ear and
Central auditory nervous system When the stapes
moves in and out of the oval window of the
cochlea, it creates a fluid motion, hydrodynamic
energy. It causes membranes in the Organ of Corti
to shear against the hair cells. This creates an
electrochemical signal which is sent via the
auditory nerve to the brain.
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QUESTIONS?