2. Sound is vibratory energy that is transmitted
from the source through surrounding media
in the form of pressure waves.
It cannot travel through a vacuum; a physical
medium is required to convey the vibratory
energy.
3.
4. frequency
• linked to the
perception of pitch:
Frequency (f) is the
number of cycles per
second (usually
expressed in Hertz
(Hz)
intensity
• linked to the
perception of
loudness.
• The energy of sound
and how this is
transferred
(dissipated)is
referred to as the
intensity of sound.
duration
• The duration of a sound
interacts with frequency and
intensity to influence pitch
and loudness sensations. For
example, a sound must
exceed a certain minimum
duration before its pitch
characteristics become
apparent. Below this point,
about 200 ms, the sound will
have a click-like sensation
regardless of its duration.
5. The Resistance offered by a medium to sound
waves.
Depends on stiffness and density of the
medium. AIR<FLUID i.e. same pressure in fluid
will produce a smaller velocity of movement.
The outer and middle ears match the acoustic
impedance of the air in the external
environment and the fluid of the cochlea,
allowing efficient transmission of sound.
8. There are numerous anatomical ridges and convolutions in the pinna and pinna
acts like a funnel
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
9. Because of its shape, the pinna shield the sound from rear end, change timbre,
and helps to localize sound from in front or back
THE HEAD SHADOW:Cues for sound localization from right/left
1. Sound wave reaches the ear closer to sound source before it arise in farthest ear
2. Sound is less intense as it reaches the farthest ear because head act as barrier
Interaural time and interaural intensity differences generated by the same
acoustic signal impinging on the pinna with different temporal and intensity
values at each side to be finally perceived by the auditory cortex as directionality
of sound or a spatial representation of sound
A pathology in the pinna may therefore interfere with this localization and
directionality process
10. (a) Sound coming from a loudspeaker off to
the right side arrives differently at the two
ears.
(b) An acoustical shadow (the head shadow
effect) occurs for high-frequency sounds
because their wavelengths are small
compared with the size of the head.
(c) Because of their large wavelengths, low-
frequency sounds are not subjected to a
head shadow because they are able to bend
around the head
11. I. FUNNEL to collect sound
II. LOCALISATION OF SOUND : as discussed previously
III. REFLECTOR : best above 1kHz
IV. PINNA NOTCH: Some of the reflected waves enter the canal and
may interfere with the original waves if they are out of phase,
noted particularly between 7 kHz and 10 kHz where they produce
a Pinna notch.
V. The frequencies at which the reflected and incident waves boost
each other are 2–3 kHz, roughly at frequencies which possess
wavelengths of four times the length of the pinna.
12.
13. The most important physiological function of the EAM
and the EAC is to ‘transfer’ the acoustic signal from a
sound field to a much smaller area, i.e. the tympanic
membrane.
TOTAL GAIN:The resonant frequency which enhances the
incident sound by 10–20 dB.It generally 2.7 kHz with a
range of 2–7 kHz. (corresponding to natural resonant
freq of the EAC)
14. I. EAC size variation up to 7-9 yrs : hearing aids
must be calibrated accordingly
II. Change in resonance at old age
III. Eac disorders hamper resonance gain- conc
around max resonant freq ie 2.7khz and this
must also be accommodated in occlusive
hearing aid moulds
IV. Complete occlusion – upto 40db hearing loss
15. A standing wave is
created when two sounds
of the same frequency
travel in opposite
directions in the canal
(e.g. when the same
sound is reflected by the
tympanic membrane) and
cancel each other out.
For sounds of high
frequencies {8kHz}, a
standing wave may be
produced which might
generate a spurious
hearing deficit
16. The middle ear in the human acts as an
efficient passive and linear
transformer to conduct acoustic
energy from the tympanic membrane
to the stapes footplate at the oval
window and to the cochlea.
Its fundamental function is to provide
critical modifications to the sound
energy by providing mechanical
advantages to overcome the
impedance encountered by the
acoustic signal when it is conducted
from an air-filled medium (the middle
ear cavity) to a fluid-filled medium (the
perilymphatic and the endolymphatic
fluid in the cochlea.This is called
IMPEDENCE MATCHING.
18. HYDRAULIC EFFECT : area ratio
between theTM and the stapes
footplate. The humanTM has a
surface area approximately 20
times larger than that of the
stapes footplate (69 mm2 vs 3.4
mm2, respectively). If all the
force applied to theTM were to
be transferred to the stapes
footplate, the force per unit area
would be 20 times larger (26 dB)
on the footplate than on theTM.
19. CURVED MEMBRANE
EFFECT :The incident
sound from the EAC
hits the membrane and
sets up a travelling
wave, which is mainly
collected at the rim of
the membrane.This is
then conducted to the
umbo and coupled to
the manubrium of the
malleus.
20. In disorders of the tympanic
membrane (e.g. tympanic
atelectasis or perforations) the
multiple lever action is
compromised, leading to a hearing
loss, and a myringoplasty cannot
restore this feature of the
membrane in all cases
Tympanosclerosis : hearing is
preserved in spite of a loss of action
of the membrane, presumably
because the preferential
distribution of the acoustic energy
to the oval window from the
middle ear is intact due to
preservation of some structural
integrity
21. when the rim of the membrane is
intact (i.e. central perforations), hearing
is better preserved than when the
margins are affected as energy transfer
from the canal to the membrane is via
the rim of the membrane
the volume velocity of the sound
waves (the volume of vibrating air
particles conducting thesound per unit
time) is not preferentially distributed to
the stapes–oval window interface a part
of it is absorbed or shunted by the
middle ear cavity
The main reason for a hearing loss is
the reduction in pressure across the
tympanic membrane due to this shunt
and preclusion of the multilever action
22. LEVER RATIO: which refers to the
difference in length of the
manubrium of the malleus and the
long process of the incus.
lever ratio is about 1.31 to 1 (2.3 dB).
The combined effects of the area
ratio and the lever ratio give the
middle ear output a 28 dB gain
theoretically; in reality, the middle
ear sound pressure gain is only
about 20 dB.This is mostly due to
the fact that theTM does not move
as a rigid diaphragm.
23. Most efficient at 2kHz
higher frequencies the motion becomes
more complex, allowing some slippage
between the two to conduct higher-
frequency sounds more efficiently. The
stapes shows a similar pattern of movement,
i.e. simple piston-like movements at low
frequencies and more complex spatial
movements in higher frequencies
24. DISCONTINUITY
• preferential distribution of sound to
the oval window is lost
• Cochlear partition pressure between
the round and the oval windows,
which drives the cochlear travelling
wave, is compromised
• total expected conductive loss is
about 60 dB, but much less if there is
incomplete discontinuity or when
there is a pathological bridge
between the ossicles (e.g.
cholesteatoma)
FIXATION
• E.g. : Chronic adhesive otitis media
• Joints/ligaments maybe involved
especiallyAnt Malleolar ligament
• High impedance in the system
• Hearing loss is lesser than
discontinuity
• OTOSCLEROSIS
• Fixation concentrated on the stapes
footplate affect forward propagation to
the cochlea, thereby generating a
significant conductive hearing loss
proportional to the mobility of the
stapes affected by the disease
25. Maintain air pressure levels
across theTM
Evolution theory- high
freq(6-8KHz)- most
consonants- req most
effective conduction – ET
opens – Max air entry
26. In Eustachian tube dysfunction where the
middle ear cavity is deprived of air, as in early
stages of otitis media with effusion (OME), the
stiffness of the system is augmented, which
leads to a low-frequency hearing loss.(Natural
resonant freq of middle EAR-800 Hz and of
ossicles is 500-2000Hz)
With further progression, fluid collection in the
middle ear cavity increases the mass of the
system, which generates a high-frequency
hearing loss
27. TENSORTYMPANI
MUSCLE ATTACHESTO
THE HANDLE OF
MALLEUS.IT PULLSTHE
DRUM MEDIALLY.(CN-
V)(LESS ROLE)
STAPEDIUS MUSCLE
ATTACHESTOTHE
POSTERIORASPECT OF
STAPES(CN-VII)(MAX
ROLE)
28. At the 80–90 dB SL intensity
range, the reflex is elicited
causing bilateral contraction
of the stapedial muscles
Stapedius contraction is
responsible for the stapedial
reflex which moves the
stapes about 50 microns,
increases the stiffness of the
ossicular chain and can
attenuate the sound
transmission to the inner ear
by as much as 30 dB
29. Protection of the cochlea to high-
intensity sounds to preserve a dynamic
range of the auditory system.
Reduction of output of the middle ear
from high intensity low frequency
sounds – these high intensity low-
frequency sounds may mask the high-
frequency speech consonant sounds at
the base of the cochlea due to the
forward propagating cochlear wave
thereby aiding speech recognition.
Modulation of self-monitoring of voice
and preserving high-definition, high-
frequency ambient sounds when one is
speaking or by self-generated noise.
30. The pressure difference
between the oval and the
round windows is
fundamental for the cochlear
travelling wave which drives
cochlear function
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
31. Disorders of the oval window or in the
vicinity (e.g. stapedial or cochlear
otosclerosis), congenital absence of the
oval window will lead to a loss of this
preferential distribution and have a
significant effect on auditory sensitivity.
32. A pathological third window
which can be :
a real one (real connection
between the inner and middle
ear) or a
virtual one i.e. no direct
communication between the
inner and middle ear) absorbs or
shunts part of preferential stapes-
oval window sound distribution.
The cochlea might in part be
directly stimulated by the
vibrations in the third window
itself.
REAL
Perilymph
fistula(perilympha
tic and ME)
X-linked
Gusher
(perilymphatic and
SA space)
VIRTUAL
Dilated
vestibular
aqueduct
Semicircular
canal
dehiscence
33. Measured air-conduction
thresholds therefore
decrease and bone-
conduction (BC) thresholds
increase, generating a
spurious conductive
hearing loss or ‘false’ or
air–bone gap.The
improved BC can also be
explained by the fact that
the third window generates
a pressure drop across the
two natural windows.
34. INTRALABYRINTHINE
BONY SKELETON
• vibration-induced
compression and distortion
of the cochlear space
consisting of the
intralabyrinthine bony
skeleton which causes
distortion of the BM due to
the flexibility of the
vestibular aqueduct
INERTIA OFTHE
OSSICLES
• vibration-induced inertia of
the ossicles transferring to
the bony labyrinth
VIBRATIONS OF
EAC,TYMPANIC
SULCUS
• directly transmitted to the
cochlea
35. The middle ear has a resonant
frequency around 1–3 kHz
considering both air and bone
conduction.
If this resonance is reduced or
damped by a mechanical problem
in the ossicular chain (e.g.
adhesion, fixation or sclerosis), the
ossicular component of bone
conduction jeopardized and
generates a drop in the pure-tone
thresholds as measured by BC of
about 5–10 dB.This classically
appears at 2 kHz as Carhart’s
notch in the BC thresholds
36. In any pathology involving the ossicles, a
Carhart’s effect may be present between 1
kHz and 4 kHz with dips in BC thresholds
37.
38. As sound energy travels through the external
and middle ears, it causes the stapes footplate
to vibrate.The vibration of this footplate results
in a compressional wave on the inner ear fluid.
Because the pressure in the scala vestibuli is
higher than that in the scala tympani, this sets
up a pressure gradient that causes the cochlear
partition to vibrate as a traveling wave..
39. The basilar membrane varies in its stiffness and mass along its length, it is able to act
as a series of filters that respond to specific sound frequencies at specific locations
along its length
The travelling wave propagates aided by the gradual diminution of the thickness and
stiffness of the basement membrane from base to apex.
BASE-THICK/STIFF-REQ HIGHER FREQ FORVIBRATION
APEX-THIN/MORE PLIABLE-CAN BEVIBRATED BY LOW FREQVIBRATION
40. 3500 in number
Arranged in 1 row
True sensory end organs
the AP conducted to the type 1
spiral ganglion cells(myelinated
and well developed
synapses)(95% of auditory
nerve)
cells respond to the velocity
rather than displacement of
basilar membrane & receive the
output from the OHCs through
the modified movement of the
BM.
12000 in number
Arranged in 3 rows
outer hair cell acts as a cochlear
“amplifier” that augments the
signals transmitted into the inner
ear (MOLECULAR MOTOR-
PRESTIN)
AP to type 2 spiral ganglion(non
myelinated) with reciprocal
synapses.
Respond to displacement of the
membranes
46. From the superior olivary
nucleus,three fiber tracts project
auditory information to the
contralateral inferior colliculus: the
dorsal stria, also called the stria of
Monaco; the intermediate stria,also
called the stria of Held; and the
ventral stria, also known as the
trapezoid body.These fiber tracts
collectively form the lateral
lemniscus
47. Some of the fibres terminate in the
nucleus of lateral lemniscus ,but
many bypass this nucleus and travel
on to the inferior colliculus,where all
or almost all the auditory fibres
synapse
From there the pathway passes to the
medial geniculate nucleus,where all
the fibres do synapse Plays an
important role in sound localization
and processing of complex vocal
communications,such as human
speech
Finally the pathway proceeds by way
of the auditory radiations to
auditory cortex.
48.
49. The auditory association cortex
is also known as area A2 and
corresponds to Brodmann
areas 22 and 42.
Located lateral to the primary
auditory cortex and is part of a
language reception area
known as the Wernicke area
Important role in speech
perception
The primary auditory cortex
is located on the superior
surface of the temporal
lobe (Heschl gyrus); this is
also known as area A1, which
corresponds to Brodmann
area 41.
Tonotopically tuned, with
high frequencies being
represented more medially
and low frequencies more
laterally.
Involved with integrating
and processing complex
auditory signals, which
includes language
comprehension.
Editor's Notes
As the diaphragm of the loudspeaker moves to the right of its centre position, the air molecules at the surface of the diaphragm are displaced to the right. This movement causes the air molecules to be pushed closer to adjacent air molecules that are further over to the right. Relative to the ambient pressure, a localized pressure increase is produced and this pressure increase is known as compression. Next, when the diaphragm
of the loudspeaker moves back through its centre position and over to the left, those air molecules that were displaced to the right are now drawn to the left. When the displaced molecules reach their centre (equilibrium) position, the pressure will momentarily be equal to ambient pressure, but as the molecules move further to the left they are drawn increasingly further apart from their adjacent air molecules. The pressure will now
drop below the ambient pressure and this decrease in pressure is called rarefaction.
Each of these three dimensions of a sound can be
detected and processed by the cochlea.
Sound waves from either the horizontal or the vertical direction are reflected from these ridges and enter the ear canal with the original non-reflected incident sound
There is also a destructive interference due to reflection of the sound waves by the membrane that creates a pressure difference across the membrane
The reflectance of the membrane is high at the lower frequencies below 1 kHz while it is lowest between 1 kHz and 4 kHz, which means that between these frequencies maximum energy is delivered to the cochlea and the middle ear is most efficient
The degree of the loss is proportional to the size of the perforation and inversely proportional to the frequency, i.e. the greatest loss is at lower frequency.because reflectance of the membrane, which is an important factor for the pressure difference across the membrane, is lost or diminished at lower frequencies
They both participate in a reflex contraction, thought in part to protect the delicate sensory epithelium of the cochlea by attenuating high amplitude sounds.
(A) Schematic cross-sectional view of the human cochlea. The scala media (cochlear
duct) is filled with endolymph, and the scala vestibuli and tympani are filled with perilymph. The endolymph
of the scala media bathes the organ of Corti, located between the basilar and tectorial membranes and
containing the inner and outer hair cells. A relatively high concentration of potassium in the endolymph of
the scala media relative to the hair cell creates a cation gradient maintained by the activity of the epithelial
supporting cells, spiral ligament, and stria vascularis. (B) Cells contain stereocilia along their apical surface
and are connected by tip links. The potassium gradient is essential to enable depolarization of the hair cell
following influx of potassium ions in response to mechanical vibration of the basilar membrane, deflection of
stereocilia, displacement of tip links, and opening of gated potassium channels. Depolarization results in
calcium influx through channels along the basolateral membrane of the hair cell, which causes degranulation
of neurotransmitter vesicles into the synaptic terminal and propagates an action potential along the auditory
nerve. Gap junction proteins between the hair cells (potassium channel, yellow) and epithelial supporting cells
(connexin channels, red) allow for the flow of potassium ions back to the stria vascularis, where they are
pumped back into the endolymph.
It is thought that as
the stereocilia are deflected in the direction of the tallest row, it causes the tip links to stretch. The stretch of
the tip links then causes stretch-sensitive cationic channels located on the stereocilia to open and to release
a large influx of cationic current, which leads to hair cell depolarization. As the stereocilia are deflected away
from the tallest row, it causes a relaxation of the tip links, which decreases the probability of ion channel
opening and leads to hyperpolarization of the hair cell.
