Sound is a form of energy produced by vibrating objects that travel in waves. A sound wave consists of alternating compressions and rarefactions of the medium molecules. The document discusses the properties of sound waves including frequency, wavelength, intensity, and pitch. It describes how the outer, middle, and inner ear work together to detect sound and transmit it to the cochlea. The cochlea contains three fluid-filled canals and uses the traveling wave principle to analyze complex sounds into their frequency components.

1. BY
MD.ROOHIA

2.  It is a form of energy
produced by a vibrating
object.
 A sound wave consists of
compression and rarefaction
of molecules of the medium
in which it travels.
 Sound wave shows variations
in pressure of the air, and the
velocity and displacement of
molecules.
 When pressure of wave is at a
maximum, the forward
velocity of air molecules is
also at a maximum.
Graphic representations of a sound wave. (A) Air at
equilibrium, in the absence of a sound wave; (B)
compressions and rarefactions that constitute a
sound wave; (C) transverse representation of the
wave, showing amplitude (A) and wavelength (λ).

3.  Frequency
 Intensity

4.  Frequency is the number of cycles per second.
 The wavelength of sound is the distance between
analogous points of two successive waves.
 Unit of frequency is hertz (Hz).
 If the frequency of a wave is f cycles/s (Hz), then f
waves must pass any point in one sec

5.  Frequencies of 500, 1000
and 2000 Hz are called
speech frequencies as most
of human voice fall within
this range.
 PTA (Pure Tone Average) is
the average threshold of
hearing in these three
speech frequencies.
 Normal hearing frequency
range is 20 to 20,000Hz
 Routine in audiometric
testing only 125 to 8000Hz
evaluated

6.  It is the strength of the sound which determines its
loudness.
 It is usually measured by decibels.

7.  It is 1/10th of a bel.
 It is named after Alexander Graham Bell
 Sound can be measured as power (watts/cm2) or as
pressure (dynes/cm2) or in physical units (N/m2 or
pascals).
 Decibel notation was introduced in audiology to avoid
dealing with large figures of sound pressure level.
 In audiology sound is measured as sound pressure
level (SPL).

8.  The SPL of a sound in
decibels is 20 times the
logarithm to the base 10, of
the pressure of a sound to
the reference pressure.
 The reference pressure is
taken as 0.0002
dynes/cm2or 20µPa for a
frequency of 1000 Hz and
represents the threshold of
hearing in normally
hearing young adults.

9.  A single frequency sound
is called a pure tone.

10.  Sound with more than
one frequency is called a
complex sound.

11.  It is a subjective sensation produced by frequency of
sound.
 Higher the frequency, greater is the pitch.

12.  A complex sound has a fundamental frequency i.e., the
lowest frequency at which a source vibrates.
 All the frequencies above that tone are called
overtones.
 Overtones determine the quality or timbre of sound.

13.  It is the subjective sensation produced by intensity.
 More the intensity of the sound, greater the loudness.

14.  It is defined as an aperiodic complex
sound.
 There are 3 types of noise:
 White noise – contains all
frequencies in audible spectrum. It
is a broad band noise and used for
masking.
 Narrow band noise – white noise
with certain frequencies, above
and below the given noise filtered
out. Frequency range is smaller
than the broad band white noise.
It is used to mask test frequency in
pure tone audiometry.
 Speech noise – noise having
frequencies in the speech range
(300-3000 Hz). All other
frequencies are filtered out.

15.  Audiometric zero is the mean value of minimum
audible intensity in a group of normally hearing
healthy young adults.

16.  It is the sound pressure level produced by an
audiometer at a specific frequency.
 It is measured in decibels with reference to
audiometric zero.

17.  It is the level of sound above the threshold of hearing
for an individual.
 Sensation level refers to the sound which will produce
the same sensation, as in a normally hearing person.

18.  Intensity (Loudness) level of sound that is most
comfortable for the person.

19.  Level of sound which produces discomfort in the ear.
 It is usually 90 – 105 dB SL.
 It is important to find the loudness discomfort level of
a person when prescribing a hearing aid.

20.  ATTENUATION BY
DISTANCE.
 Propagation of sound is
like a ripples on pond.
 Dicreases in amplitude
as they move away from
the source.
 For sound if distance
doubles amp drops by
half.

21.  Transmission between different media
 Air is light and compressible, only small sound pressures
will be needed to give a certain velocity of vibration, and
hence displacement of air molecules.
 In a medium with higher impedance the pressure will be
inadequate to give similar velocities of vibration.
 So when sound in air meets a medium of higher
impedance it can not produce same amount of vibration
in that medium, so the result is much of the sound is
reflected with only small proportion being transmitted.

22.  The analysis of the complex
sound into its constituent
sinusoids is known as
FOURIER ANALYSIS.
 Any realistic waveforms
can be made out of sums of
sinusoids.
 Sinusoidal sound behave in
a simple way in many
complex environments.
 Cochlea itself performs
Fourier Analysis.

23.  Sound just a
combination of sine
functions.
 We can assign each sinefunction,&therefore the
original sound, to a
distinct energy or power
spectrum which gives us
the
energy/amplitude/freq.
 This process called FT.

24.  Open on one end only.
 The impedance of ear
drum is around 3 to 4 times
more than air.
 30% of incident sound
energy is reflected from
external canal.
 It is efficient in conducting
sound in frequency range
of 3 to 5 kHz.
 It cuts off unwanted
frequencies helping in
better speech
discrimination.

25.  It acts as a resonator.
 It increases the pressure at the ear drum in a frequency
sensitive way.
 Helps in localization of sound.
 Its length is 28mm.

26.  If a tube of one quarter wave length long and one end
is open and the other end is blocked with hard
termination, the pressure will be low at the open end
and high at the closed end when the tube is placed in a
sound field.
 This phenomenon is seen in human external meatus at
frequency of 3 kHz, resonance adds 10 to 12 dB at the
tympanic membrane.

27.  Both sound pressure
levels and phase of
acoustic waves are
important factors in
sound localization.
 Maximum time
difference (phase
difference) between two
ears is 750 milliseconds.

28.  It couples sound energy to
cochlea.
 It serves as an acoustic
transformer to match the
impedance of air to
cochlear fluids.
 It couples sound
preferentially to only one
window, thus producing a
differential pressure
between the windows
required for movement of
cochlear fluids.

29. 1)




CATENARY LEVER(ear
drum)
Buckling mechanism of
TM
Force is transmitted from
centre of TM.
TM memb doesn’t move
as a plate.
This causes high pressure
with low displacement.

30. 2.OSSICULAR
LEVER(lever ratio):
 Length of the handle of
malleus 1.3 times longer
than long process of
incus.

31. 3.HYDRAULIC
LEVER(areal ratio):
 Average area of TM is
larger(60mmsq0 than
foot plate
area(3.2mmsq)(OW).
 Effective vibratory area of
TM 65% that is 45mmsq.

33.  Only 65% of sound
energy from TM gets
absorbed and
transmitted to the
cochlea.
 Without middle ear only
1% of the sound energy
will be absorbed by the
cochlea.

34.  Tensor tympani attaches to
the handle of malleus. It
pulls the drum medially.
 Stapedius muscle attaches
to the posterior aspect of
stapes.
 Contraction of these
muscles increases the
stiffness of ossicular chain
thus blunting low
frequencies.
 These muscles decreases a
person’s sensitivity to their
own speech.

35.  Stapedius contraction can
reduce transmission up to
30dB for frequencies less
than 1 to 2 kHz. For higher
frequencies it is limited to
10dB.
 Only stapedius muscle
contracts in response to
loud noise in humans.
 The whole stapedial reflex
arc has 3 to 4 synapses.
 Stapedial reflex latency is 6
to 7 ms.

36.  Damaged middle ear can cause loss of transformer
mechanism.
 Differential pressure levels between the two windows
could not be maintained.
 Scala vestibuli is more yielding than scala tympani.
Differential movements of fluid with in the cochlea is
still possible.
 Small compliance of annular ligament in comparison
to much larger compliant round window could again
cause differential pressure.

37.  Normal route for hearing
one’s own voice.
 Useful in cases of severe
conductive losses.
 Can be used as a
diagnostic tool.

38.  Intrinsic detection of




distortional vibrations of
cochlear bone.
Differential distortion of bony
structures of cochlea (scala
vestibuli is larger than scala
tympani) could cause
movement of cochlear fluids.
Direct vibration of osseous
spiral lamina.
Direct transmission of
vibrations from the skull via
CSF to the cochlear fluids.
Leaving one window open
improved sound conduction.

39.  Vibrations of the skull gets
faithfully transmitted to
the ossicles of middle ear
cavity.
 Inertia of the middle ear
ossicles doesn’t coincide
with their points of
attachments.
 Middle ear acts as a band
pass filter with peak
transmission around 1kHz.
 This accounts of carhart’s
notch though at a slightly
higher frequency.

40.  Bone vibrations are
conducted through the
external canal and the air
within it.
 Vibrations can escape
externally if the canal is
open.
 Occlusion of external ear
increases bone conduction.
 External radiation of sound
is best for low
frequencies, hence change
with occlusion is greatest
for these frequencies.

41.  Scala vestibuli and scala tympani




contains perilymph.
Scala media contains
endolymph.
Perilymph space opens into CSF
via cochlear aqueduct.
Endolymphatic space joins the
endolymphatic sac by
endolymphatic duct.
Scala vestibuli is separated from
scala media by reissner’s
membrane. It is very thin and
does not obstruct the passage of
sound from s. vestibuli to s.
media. They may even be
considered to be a single
chamber.

42.  Formed by stria vascularis.
 Endolymphatic sac
maintains homeostasis of
endolymph.
 It has high potassium and
low sodium content.
 Endolymph has positive
potential gradient +50 to
120mV (endocochlear
potential).
 Na K ATPase is responsible
for this gradient.

43.  Secretes Endolymph.
 Superficial dark staining
marginal cells.
 Lightly staining basal
cells.
 Marginal cells are
secretory in nature.

44.  Site of production is




controversial - ? CSF
Occupies perilymphatic space.
Continuous between vestibular
and cochlear divisions.
Ionic concentration resembles
extracellular fluid.
Perilymph from s. vestibuli
originates from plasma, while
perilymph from s. tympani
originates from both plasma and
CSF.
Electrical potential from s.
tympani is +7mV and from s.
vestibuli is +5mV.

45.  It separates s. media from s.




tympani.
Length’s of basillar membrane
increases from oval window to the
apex (0.04mm near oval window
and 0.5mm at helicotrema) 12 folds
increase.
Diameters of basilar fibers decrease
from oval window to helicotrema.
The stiff short fibers near the oval
window vibrate best at very high
frequency, while long limber fibers
near the tip of cochlea vibrate best at
a low frequency.
It is known as tonotopic
presentaion.

46.  By the movement of
ossicles sound wave
reaches through oval
window to cochlea.
 Here the fluid in sv &st set
in motion as well as BM.
 BM moved by travelling
wave.
 Location of max amplitude
of this wave depends on
freq of incomming sound
signal,here freq analysis
take place.

47.  BM movts leads to
stimulation of nerve cells
In OC, & send electrical
impulses to brain&
sound percieved.
 BM movt is amplified by
OHC called active
amplification.
 Low input signals evoke
larger BM displacements
than high sound levels.

48.  When the steriocilia are
deflected in the direction
of the tallest steriocilia,
the links are stretched
opening up calcium
channels.

49.  Makes large number of synaptic






contact with afferent fibers of
auditory nerve.
95% of afferent auditory nerves
make contact with inner hair cells.
Detects basillar membrane
movements.
Tips of inner hair cells are not
embedded in the tectorial
membrane as outer hair cells.
They fit loosely into a groove called
“Henson’s stripe”.
The cilia are driven by vicious drag
of endolymph.
Inner hair cells respond to the
velocity rather than displacement.

50.  Very few outer hair cells




synapse with auditory
nerves.
Inside of outer hair cells
have -70mV.
They serve to amplify
basillar membrane
vibration.
They increase the
sensitivity and selectivity
of cochlea.
Cochlear microphonics are
derived from these cells.

51.  Cochlear microphonics –
A/C
 Summating potential –
D/C
 Negative neutral
potentials – N1 & N2

52.  Inner hair cells excite auditory nerves.
 Single auditory stimulus is always excitatory.
 Sound stimulus, transmitter release and action
potential generation occur in synchrony (Phase L
 ocking). Commonly seen in low frequency.
 Timing AP in the nerve is able to signal details of the
temporal properties of the sound wave form is called
TEMPORAL CODING.
 Coding based on frequency selectivity is called PLACE
CODING.

54. AUDITORY PATHWAY

55.  Signals from both ears are
transmitted to both sides
of the brain.
 Preponderance of
transmission in
contralateral pathway.
 Three cross over points are:
 In the trapezoid body.
 In the commisure between
the two nuclei of lateral
lemnisci.
 In the commisure
connecting the two
inferior colliculi.

56.  Sound localization and lateralization
 Auditory discrimination
 Temporal aspects of audition including
 Temporal resolution
 Temporal masking
 Temporal integration
 Temporal ordering
 Auditory performance with competing acoustic signals
 Auditory performance with degraded signals.

58. 1st GROUP THEORIES
 Telephonic Theory Of Rutherford(1880)
 Volley Theory of Waver & Bray(1949)
2nd GROUP THEORIES
 Resonance Theory of Helmholtz(1883)
 Place Theory
 Travelling Wave Theory VonBekesy(1960)

59.  Rutherford proposed that the entire cochlea responds
as a whole to all frequencies instead of being activated
on a plate.
 Here the sounds of all frequencies are transmitted as
in a telephone cable and frequency analysis is
performed at a higher level (brain).
 Damage to certain portions of the cochlea can cause
preferential loss of hearing certain frequencies i.e., like
damage to the basal turn of cochlea causing inability
to hear high frequency sounds.
 This can not be explained by telephonic theory.

60.  Proposed by Wever &Bray(1949)
 Volleys means groups
 Impulses of frequency above 1000cyc/sec were
transmitted by diff group of nerve fibres

61.  Basilar memb acts as series
of tuned resonators as in
piano string
 Each pitch vibrate BM
particular to its own place.
 High freq at basal region,
loe at apical region.
 Individual resonators not
found in cochlea so its
modified to place theory.

62.  According to Helmholtz basillar membrane has
different segments that resonated to different
frequencies.
 Particular nerve fibre gives information frm org of
corti to regarding region to brain.
 Eg: boiler maker’s disease

63.  Proposed by Bekesy.
 This theory proposes
frequency coding to take
place at the level of
cochlea.
 High frequencies are
represented towards the
base while lower
frequencies are closer to
apex.

64. By MD.ROOHIA
MODERATOR
DR.JANARDHAN