The physics behind hearing

1. PHYSIOLOGYOF HEARING
Dr Seema S
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2. Outline
 Introduction to sound
 Ear
 Function as a transducer
 Parts and how it functions
External ear
Middle ear
Internal ear
 Electrical potential
 Auditory path way
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3. SOUND
 A form of energy
 propagates in the form of waves
 The speed of sound depend on the medium
through which the wave pass
 air - 343m/s
 water - 1482m/sec
 Audible frequencies t for humans 20 to 20,000
cycles per second (cps, Hz).
 It can detect the difference between two sounds
occurring 10micro seconds apart in time
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4. EAR AS A TRANSDUCER
SOUNDENERGY
MECHANICAL
ENERGY
ELECTRICAL
ENERGY
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5. Technical jargons
• Strength of the sound
• Loudness denotes the appreciation of sound
intensity
• Expressed in decibel (dB)
Amplitude/loudness
• Number of cycles per second
• Pitch /Tone denotes the appreciation of
frequency
• Expressed in Hertz(Hz)
Frequency/Pitch/Tone
• Resistance offered by a medium to sound
waves Impedence
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6. • Resonance is the tendency of a system
to oscillate with larger amplitude at
some frequencies than at others
RESONANCE
• Attenuation is a general term that refers
to any reduction in the strength of a
signal
ATTENUATION
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7. Values of hearing:
 15-25dB —Whisper
 35dB —Background noise
 40-60dB —Background noise ( home )
 65—70dB –- normal speaking voice
 130dB —painful noise
 140-180dB —jet air craft engine noise

8. Natural resonant frequency
EXTERNAL AUDITORY CANAL--------------- 3000Hz
TYMPANIC MEMBRANE----------------------- 800-1600Hz
MIDDLE EAR--------------------------------------- 800Hz
OSSICULAR CHAIN--------------------------- 500-2000Hz
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9.  External ear
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10. Functions of EXTERNAL EAR
Sound collection
Increasing pressure on the tympanic
membrane in a frequency sensitive way
Sound localization
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11. Sound collection
 Pinna- concha system catches sound over
large area and concentrate it to smaller area
of ext. auditory meatus.
 This increases the total energy available to
the tympanic membrane
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12. Pressure increase by EAC
 If a tube which is closed at one end and open
at other is placed in a sound field then
pressure is low at open end and high at closed
end.
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13. Sound localization
 Because of its shape, the pinna shield the
sound from rear end,change timbre,and helps
to localize sound from infront or back
 Cues for sound localization from right/left
 Sound wave reaches the ear closer to sound source before it arise
in farthest ear
 Sound is less intense as it reaches the farthest ear because head
act as barrier
 Auditory cortex integrates these cues to
determine location.
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14.  Middle ear
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15. FUNCTIONS OF MIDDLE EAR
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16. Impedence mismatch
IF THERE WAS NO MIDDLE EAR SYSTEM
,99% OF SOUND WAVES WOULD HAVE
REFLECTED BACK FROM OVAL WINDOW
MIDDLE EAR BY ITS IMPEDENCE
MATCHING PROPERTY ALLOWS 60% OF
SOUND ENERGY TO DISSIPATE IN
INNER EAR
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17. “Impedance Matching” by the
middle ear System
(a) Area of tympanic membrane relative to oval window
(b)The lever action of middle ear ossicles
(c)The shape of tympanic membrane
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18. (a) HYDRAULIC ACTION OFTYMPANIC MEMBRANE
 Total effective area of tympanic membrane
45mm2
 Area of stapes footplate is 3.2mm2
 Effective areal ratio is 14:1
 Thus by focusing sound pressure from
large area of tympanic membrane to small
area of oval window the effectiveness of
energy transfer between air to fluid of
cochlea is increased
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19. (b) Lever action of ossicles
 Handle of malleus is 1.3 times
longer than long process of
incus
 Overall this produces a lever
action that converts low
pressure with along lever
action at malleus handle to
high pressure with a short lever
action at tip of long process of
incus
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20. (c) Action of tympanic membrane
 Eustachian tube equilibrates the air
pressure in middle ear with that of
atmospheric pressure, thus
permitting tympanic membrane to
stay in its most neutral position.
 A buckling motion of tympanic
membrane result in an increased
force and decreased velocity to
produce a fourfold increase in
effectiveness of energy transfer
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21. Total gain
 Total transformer ratio=14x1.3x4=73:1
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22. Attenuation reflex
 When loud sounds are transmitted through the
ossicular system and from there into the central
nervous system, a reflex occurs after a latent period
of only 40 to 80 ms to cause contraction of the
stapedius muscle and the tensor tympani muscle
The tensor tympani muscle pulls the handle of the
malleus inward while the stapedius muscle pulls the
stapes outward. These two forces oppose each other
and thereby cause the entire ossicular system to
develop increased rigidity, thus greatly reducing the
ossicular conduction of low frequency sound
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23. Function of attenuation
reflex
 To protect the cochlea from damaging
vibrations caused by excessively loud sound.
 To mask low-frequency sounds in loud
environments. This usually removes a major
share of the background noise
 To decrease a person’s hearing sensitivity to
his or her own speech
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24. PHASE DIFFERENTIAL EFFECT
 Sound waves striking the tympanic membrane
do not reach the oval and round window
simultaneously.
 There is preferential pathway to oval window
due to ossicular chain.
 This acoustic separation of windows is achieved
by intact tympanic membrane and a cushion of
air around round window
 This contributes 4dB when tympanic membrane
is intact
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25.  INNER EAR
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26. COCHLEA ---TWO FUNCTIONS….
 A TRANSDUCER that translates sound energy
into a form suitable for stimulating the
dendrites of auditory nerve.
 AN ENCODER that programs the features of
an acoustic stimulus so that the brain can
process the information contained
instimulating sound.
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27. Electrical potential of
cochlea and CN VIII
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28. Endocochlear potential
 An electrical potential of
about +80 millivolts exists all
the time between endolymph
and perilymph, with positivity
inside the scala media and
negativity outside.
 This is called the endocochlear
potential, and it is generated
by continual secretion of
positive potassium ions into
the scala media by the stria
vascularis
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29. Cochlear microphonic
 When basilar membrane move in response to
sound stimulus electrical resistance at the tip
of hair cells change allowing flow of K+
through hair cells and produce voltage
fluctuations called cochlear micro phonic.
 This is AC potential
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30. Summating potential
 Produced by hair cells
 DC potential superimposed on VIII nerve
action potential
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31. Compound action potential
 All or none response of auditory nerve fibres
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32. Central auditory pathway
• nerve fibers from the spiral
ganglion of Corti enter the dorsal
and ventral cochlear nuclei
• second-order neurons pass
mainly to the opposite side of
the brain stem to terminate in
the superior olivary nucleus
• the superior olivary
nucleus,the auditory pathway
passes upward through the
lateral lemniscus.
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33. Some of the fibers terminate in the
nucleus of the lateral lemniscus, but many
bypass this nucleus and travel on to the
inferior colliculus, where all or almost all
the auditory fibers synapse
From there, the pathway passes to the
medial geniculate nucleus, where all the
fibers do synapse
Finally, the pathway proceeds by way
of the auditory radiation to the
auditory cortex, located mainly in the
superior gyrus of the temporal lobe.
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34. Pecularities of auditory
pathway
 First,signals from both ears are
transmitted through the pathways of both
sides of the brain, with a preponderance
of transmission in the contralateral
pathway
 Second, many collateral fibers from the
auditory tracts pass directly into the
reticular activating system of the brain
stem
 Third, a high degree of spatial orientation
is maintained in the fiber tracts from the
cochlea all the way to the cortex
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35. Function of auditory cortex
 Perception of sound
 Judging the intensity of the sound
 Analysis of different property of sound
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