Download PHY238Y Lecture 11 - University of Toronto Physics

PHY238Y
Lecture 11
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The human ear: amplification of the outer/inner ear
Acoustic impedance. Physics of hearing (III)
Protection mechanisms of the human ear
References:
Hallett et al.: Physics for the life sciences, 4th ed., Ch.2 (2.6)
K. Bogdanov: Biology in Physics: is Life Matter, Acad. Press 2000, Ch. 6
Hyper Physics: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/ear.html#c1
Thanks to dr. Rod Nave for the permission to use the above resource
PHY238Y
Lecture 11
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The middle ear
The tympanic
membrane
or eardrum receives
vibrations traveling up the
auditory canal and
transfers them through the
tiny ossicles to the oval
window, the port into the
inner ear. The eardrum is ~
fifteen times larger than the
oval window, giving an
amplification of about
fifteen compared to the
oval window alone.
PHY238Y
Lecture 11

The middle ear:
ossicles
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The three tiniest bones in the
body couple the vibration of the
eardrum and the forces exerted
on the oval window of the inner
ear
The ossicles form a compound
lever which achieves a
multiplication of force.
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PHY238Y
Lecture 11
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The middle ear: Ossicle
vibration
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Ossicles achieve an
amplification by lever action.
The lever is adjustable under
muscle action and may actually
attenuate loud sounds for
protection of the ear.
PHY238Y
Lecture 11
The outer- middle ear amplification
Funnel
effect and
resonances
of the
auditory
canal
Tympanic
membrane
(eardrum)
area vs.
oval window
area
Outer ear
~2x
Tympanic
membrane
~15x
Ossicles
(hammer,
anvil and
stirrup)
contribute
by a levertype
amplification
when
listening to
soft sounds.
Ossicles
~2x
(soft sounds only)
PHY238Y
Lecture 11
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Why is the amplification needed?
Outer and middle ear are filled with air at normal pressure; inner ear is filled with a
liquid.
The oscillation of ossicles causes a vibration of the oval window, but part of the
acoustic wave is reflected from the interface due to the difference in acoustic
impedance between the middle and inner ear.
We define the acoustic impedance Z and we use the definition of the wave velocity v:
B
Z
v
but : v 
 Z  B
B

for a longitudin al wave
PHY238Y
Lecture 11
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Ear protection to loud
sounds
PHY238Y
Lecture 11
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Ear protection
mechanisms
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In response to loud sounds, the
tensor tympani muscle tightens
the eardrum and through the
tendon between the hammer and
anvil and shifts the stirrup
backward from the oval window of
the inner ear. This shifting of the
ossicles reduces the transmitted
force to the inner ear, protecting it.
However, it is a relatively slow
action and cannot protect the ear
from sudden loud sounds like a
gunshot. The process is less
effective in older ears