Download S 319

S 319
< Auditory system >
• 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
•
Outer ear
Middle ear
Inner ear
Four major divisions of auditory system - Anatomy
1.
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.
4.
The inner ear
- cochlea (hearing)
- vestibular system (balance)
The central auditory system
Four major divisions of auditory system – Function
(CD, Figure 3.4.1)
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
I. Outer ear:
(1) Pinna (Binaural cue to sound source
location)
t Right
t Left
Right
ear
Left
ear
* Different distances from source to each ear
=> different arrival times (Interaural time-difference)
and different sound level (interaural level-difference)
Sound
I. Outer ear:
Outer ear gain
(1) Pinna (Spectral cue to sound source location)
+15°
0°
-15°
The spectral feature of sound is changed depending on the sound elevation
=> Head Related Transfer Function (HRTF)
I. Outer ear
(1) Pinna: Cases of abnormal pinna
• Anotia
• Microtia (Grade I)
• Microtia (Grade II)
• Microtia (Grade III)
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
I. Outer ear:
(2) Transfer function of ear canal
From Gelfand (1998)
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.a
c.jp/FEM_mid-e.html#fig1
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
II. Middle ear (3) Eustachian Tube
• Comparison of Eustachian tubes In adults and children
: shorter, smaller, less steep eustachian tube in children
=> Hard to be drained away from middle ear
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)
=> Impedance matching
II. Middle ear (CD, Figure 3.4.9)
• Three mechanisms for
impedance matching
1) Area ratio of the ear drum to the
stapes footplate (20:1)
=> 20 log (20/1) = +26dB SPL
* Basic concept: p = f/a
2) Lever action of the ossicles (1.3:1)
=> 20 log(1.3/1) = +2 dB SPL
3) Buckling of ear drum
( x 2 pressure increase
=> 20 log(2/1) = +6dB SPL
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.wadalab.mech.tohoku.ac.jp/F
EM_mid-e.html#fig6
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
III. Inner ear – Cochlea
endolymph
perilymph
perilymph
III. Inner ear – Resonance of Basilar membrane
Figure 15.32
III. Inner ear – Cochlea
Figure 15.28
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
III. Inner ear – Sound transduction
IV. Central auditory system (CD, Figure 3.4.14)
•
Tonotopy!
IV. Central auditory system
- Auditory pathway to brain
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.
QUESTIONS?