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Sound results when the particles of amedium are set into vibration. Thedisplacement is around a mean position andthere is no net flow of air in the direction of
motion. A sound wave has 2 basic properties:-
- Intensity - It is the power transmittedby the wave through a unit area. It is a
subjective correlate of loudness.- Frequency It is the number of cycles
per second of back and forth motion of airparticles and is subjective correlate of pitch.
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Pure tone :single frequency sound
Complex sound: sound with more than one
frequency
Overtones: complex sound has a fundamentalfrequency ie. the lowest frequency at which
a source vibrates.All frequencies above that
tone are called overtones.
Impedance: it is a measure of opposition of asystem to movement and is a function of the
medium in which sound is traveling and its
surroundings.
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Fouriers analysis: Analysis of a complex sound intoits constituent sinusoids. It is one of the essentialtools for those attempting to understand theworkings of auditory system.
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Periodic sound: These are complex soundswith all the frequencies being multiples ofthe lowest or fundamental frequency
Such components at exact multiples of a
frequency are called harmonics.
Non-periodic sound
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Root Mean Square value is used to measure
the intensity of sound in air. It is useful
because the same relation between intensity,
pressure and velocity holds over all shapes ofwaveform.
It is common to use a logarithmic scale to
grade sound pressures because:-
- Ears sensitivity to pressures varies bymore than a million times.
- Human ear can discriminate fractional
changes in pressure.
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Formula for calculating decibel
Number of dB = 10 Log actual sound intensity
10 reference intensity
The decibel (1/10th of a bel) is a logarithmicmeasure of relative energy where 10 db (1
bel) represents an increase over a given
reference energy level of first order of
magnitude. Reference sound intensity 0.0002dynes/cm2
Corresponds to threshold of hearing in
normal subjects at 1000hz
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Human ear can hear frequencies from 20 Hz
to 20,000 Hz.
Subjects threshold is by definition 0 db
sensation level.
14 - 20 db- just audible whisper
40 - 60 db- normal conversation
60 - 90 db- noisy room.
90 - 120db- loud music.
130db Pain threshold.
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Sound conducting mechanism: Transmissionof sound waves through External auditorycanal, tympanic membrane and theossicular chain.
-- acoustically from sound in the middle earthat reaches the window of the cochleadirectly
--bone conduction
Perceptive neural mechanism: Transductionin the cochlea , auditory division of the 8th
nerve and its central connections.
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1.Pinna
2.Ear canal also known as external auditory meatus
3.Eardrum also known as tympanic membrane.)
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The external ear has 2 main influences on theincoming sound
It increases pressure at tympanic membranein a frequency sensitive way thus
emphasizing certain frequencies in the input. It increases pressure in a way that depends
on the direction of the sound sourcetherefore it aids in sound localisation.
This acoustic function of theexternal ear is also known as external ear gainand is dependent on frequency and directionof sound.
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Total effect of reflections from head,pinna and
various external ear resonances is to add 15 to 20
db to the sound pressure over the frequency rangefrom 2 to 7 kHz.
As a sound source is moved around the head, starting
in front and moving round to the side, the main
change produced is an attenuation of upto 10 db inthe frequency range of 2 to 7 kHz, therefore changes
in this frequency range could indicate whether the
source was in front of the subject or behind.
In the horizontal plane, determining the direction ofsound depends on the difference in time between
the arrival of stimulus in the two ears .
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Impedance of the tympanic membrane in
man seems to be 3-4 times that of the air in
the EAC over a wide frequency range of 1
kHz. This leads to some 50% of energy being
reflected back into the meatus.
Resonance in EAC at a frequency of about 3
kHz adds 10-12 dB at the TM
Resonance at concha at 5 kHz adds 10 dB
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Couples sound energy to cochlea
Acoustic transformer- Reduces the
impedance mismatch --auditory meatus air
cochlear fluids
--(low impedance) (high impedance)
Provides physical protection to cochlea.
Couples sound preferentially to only onewindow of cochlea required for movement
of cochlear fluids.
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Bekesy suggested thatTM moved like a stiffplate up to 2k Hz andfound that inferior
edge was flaccid andhence the movementswere greatest at thatpart.
Khanna & Tonndorf
suggested twomaxima of vibration,one on either side ofthe manubrium .
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Axis of rotation of the ossicles and the axis of
suspension by their ligaments nearly coincide
with their center of rotational inertia. Hence
the bones are able to vibrate with very little
loss through the suspending ligaments. At
low frequencies where mass effects are
small, ligaments play important role in
maintaining position of ossicles. Also the
coincidence of the center of inertia of the
ossicles with their center of rotation will
help reduce the perception of bone
conducted sound.
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When a sound wave meets a medium of
higher impedance from a lower impedance
medium, much of the sound energy is reflected.
The middle ear by acting as an acoustic
impedance transformer reduces this attenuation
substantially. An effective impedance transformer
will change the low pressure high displacement
vibrations of air into high pressure low
displacement vibrations suitable for drivingcochlear fluids.
According to Aibara the impedance of
cochlear fluid is 5.8*10 4 N.sec/m3.
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Area of tympanic membrane is larger than that of stapesfootplate. The forces collected over the TM are
concentrated on a smaller hence increased pressure at
oval window.
T f A i f Middl E
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Transformer Action of Middle Ear
Lever Action
Fulcrum Effect pressure gain: 2.1 times
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Impedence transformer ratio calculated as
follows.
Areal ratio= vibratory area of TM (60sqmm)
stapes footplate area(3.2sqmm)
= 18.75
Lever (ossicles) ratio- The arm of incus isshorter than that of malleus and this
produces a lever action that increases the
force and decreases the velocity at stapes.
Malleus is 2.1 times longer than incus, solever action multiplies force 2.1 times but
velocity decreases 2.1 times. Thus
impedance ratio is increased by 4.4 times.
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Final transformer ratio is calculated as ratio
of specific impedances, obtained by
multiplying these 2 factors together,
as 18.75*4.4 = 82.5
The result of the transformer action of
middle ear(combined with effect of external
ear) is that upto 50% of the incident energyis transmitted to cochlea as against 3%(a 15
db loss) expected in absence of middle ear
transformer.
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Entire TM does not move as a rigid body.
at low freq-entire tm moves with the samephase with varying magnitude
at freq above 1000Hz-more complicatedpattern of vibration
Energy used up in middle ear to stretch tmand ossicular ligaments
Middle ear air spaces load the motion of TM
Slippage in the ossicular system at freqabove 1000 to 2000hz
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Effective stimulus to the inner ear is adifference in the sound pressure between theround and oval windows
Middle ear maximizes pressure diff between
the two windows by:a) A tympano-ossicular system whichpreferentially increases pressure at ovalwindow.
b)Round window shielding effect by a intact
tympanic membrane which reduces soundpressure in middle ear by 10-20 db comparedto EAC.
c) Presence of air in middle ear around theround window allows its movement.
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Influence of middle ear muscles
Tensor tympani-pulls malleusmedially
(T.T. attached to
manubrium ofmalleus)
Stapedius- pullsstapes posteriorly
(stapedius insertsto post aspect ofstapes)
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Contraction of both muscles- increases
stiffness of ossicular chain- influences
transmission
It can change the direction of vibration of
ossicles n thus lead to less effective coupling
wit the cochlea.
Middle ear muscle reflex has various
functions:
1. Protection from loud noise( esp long
lasting)
2. Selective attenuation of low frequencystimulus components improves
intelligibility of speech.
3. Reduces influences of some of the
unwanted resonances in middle ear
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INNER HAIR CELLS( single row)
make synaptic contact with approx 95% of afferentfibres of auditory nerve
IHC detect basilar membrane movement , transfer
it to auditory nerve
Response can be divided into a osscilating ACresponse f/b DC depolarisation
OUTER HAIR CELLS( three rows)
More sensitive, more susceptible to damage.
amplifies basilar membrane vibration, generates
cochlear microphonics
produce only AC response
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Inner ear: hair cells
Outer hair cells 30,000 in number
3-5 rows
Cylindrical in shape
50-150 stereo cilia arranged in2-3 rows that assume V or Wshape
Tallest tips are embedded inoverlying membrane
Those in apex are longer 8mmthan in base 2mm
Innervated by type 2 auditorynerve fibers
Efferent nerve fibersterminate directly onto cells
Inner hair cells 10,000 in number
Single row
Flask shaped cells
Stereo cilia in 3-4 ascendingrows assuming a flattened Ushape or st line
No such difference in height
Same dimension throughoutentire length of cochlea
Innervated by type 1 auditorynerve fibers
Efferent nerve fibersterminate on dendrites ofnerve fibers
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The mechanical travelling wave incochlea forms the basis of frequencyselectivity of whole organ and in addition isthe basis of our extreme sensitivity to sound.
Cochlea travelling wave was originallydescribed by Bekesy.
According to him response is large for only avery narrow range of sound frequencies and if
the sound freq is changed the response dropssharply. As a wave moves up the cochleatowards its peak it encounters a region inwhich the membrane is mechanically active.
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Sterocilia - present on apical surface of hair cell. Theyare mechanically rigid, cross linked to each other.
Stimulated by shear or relative motion betweentectorial membrane and reticular lamina
Moves away from and towards modiolus If they are deflected in the direction of tallest sterocilia
there is associated opening of ionic channel
K+ ions from endolymph move inside the cell
Energy for this whole process comes from striavascularis, which by ion pumping , stores energy inbattery ofendolymph
This is the battery or resistance modulation theory ofDavis
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Travelling wave
Von Bekesy noted that the motion of the basilar
membrane was in the form of a travelling wave, like the
one that occurs when you flick a rope.
The wave oscillates at the frequency of stimulation, but it
is not a sinusoidal wave.
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Travelling wave characteristics
Always starts at the base ofthe cochlea andmoves toward the apex
Its amplitude changes as ittraverses the length of thecochlea
The position along thebasilar membrane at whichits amplitude is highestdepends onthe frequency of the
stimulus
All of these characteristicsdepend on the change instiffness along the length ofthe basilar membrane.
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PHASE LOCKING
FREQUENCY SELECTIVITY (PLACE CODING)
TEMPORAL CODING OF STIMULI
PHASE LOCKING
neurotransmitter- released in synapses at the base of IHC as a result of
depolarisation of IHC-gives rise to AP in auditory nerve fibres
synchrony between sound stimulus,transmitter release and AP generation in
individual cycles of stimulus is known as phase locking.
FREQUENCY SELECTIVITY (PLACE CODING)
each nerve fibre has a frequency of stimulus for which it is most sensitive
it is possible to determine the frequency of stimulus depending upon which fibres
were activated
TEMPORAL CODING OF STIMULI
for stimulus above 5kHz the timing of AP in nerve is able to signal details oftem oral ro erties of sound wave form
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Helmholtz theory: (Place theory) The
selectivity of cochlea and pitchdiscrimination is based on the place of
displacement of BM.
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Volley theory
As Wever pointed out, if different nerve fibers
respond on different cycles and the brain has a way
of adding up the responses of all the neurons,
then it would have a perfect representation of the
frequency of the tone. This is called the volley
theory. It is a combination of the above two
theories.
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