Main Components of the Hearing Mechanism

 Outer Ear
 Middle Ear
 Inner Ear
 Central Auditory Nervous
System
Structures of the Outer
Ear
 Auricle (Pinna)
 Collects sound
 Localization
 Amplifies sound
(approx. 5-6 dB)
External Auditory Canal:

 Approx. 1 inch in
length
 “S” shaped
 Lined with cerumen
glands
 Outer 1/3 surrounded
by cartilage
 Inner 2/3’s
surrounded by
mastoid bone
Tympanic Membrane:
 Thin membrane
 Forms boundary
between outer
and middle ear
 Vibrates in response
to sound
 Changes acoustical
energy into
mechanical energy
 The Ossicles:
 A: Malleus
 B:Incus
 C:Stapes
 Smallest bones in the body
 Acts as a lever system
 Footplate of stapes enters oval
window of the cochlea
 Stapedius Muscle
 Connects stapes to wall of
middle ear
 Contracts in response to
loud sounds (called the
Acoustic Reflex)
 Eustachian Tube (AKA:
“The Equalizer”)
 Lined with mucous
membrane
 Connects middle ear
to nasopharynx
 “Equalizes” air
pressure
Structures of the Inner Ear

 Cochlea
 Snail shaped organ with a
series of fluid-filled tunnels
 Converts mechanical
energy to electrical energy
 Organ Of Corti:
 The end organ of
hearing
 Contains stereocilia and
hair cells.
 Hair Cells:
 Frequency specific
 High pitches= base of
cochlea
 Low pitches= apex of
cochlea
Vestibular System
 Consists of three
semi-circular
canals
 Shares f luid with
the cochlea
 Controls balance
 Central Auditory System
 VIIIth Cranial nerve or “Auditory Nerve”
 Carries signals from cochlea to brain
 Auditory Cortex
 Temporal lobe of the brain where sound is perceived and
analyzed
 How Sound Travels Through The Ear...

1.Acoustic energy, in the form of sound waves, is channeled into the ear
canal by the pinna
2. Sound waves hit the tympanic membrane and cause it to vibrate,
like a
•drum, changing it into mechanical energy
3.The malleus, which is attached to the tympanic membrane, starts the
•ossicles into motion
4.The stapes moves in and out of the oval window of the cochlea
creating a fluid motion
5. The fluid movement causes membranes in the Organ of Corti to
shear
•against the hair cells
6.This creates an electrical signal which is sent up the Auditory Nerve
Audiomete
r

 An Audiometer is a machine, which is used to

determine the hearing loss in an individual.
 Pure Tone Audiometer works on the principle of
presenting specific pure tone signals to the subject
and determining the intensity at which they can barely
hear these signals .
 They are calibrated in terms of frequency and
output.
Classificatio
Audiometers
n

Pure tone Speech audiometer

audiometer

Air conduction Speech thresholds

Bone
conduction
AIR & BONE CONDUCTION:
 Air conduction is the transmission of sound through
the external & middle ear to the internal ear.
 Bone conduction refers to the transmission of sound
to the internal ear mediated by mechanical vibration of
the cranial bones & soft tissues.
Threshold of
Hearing
 The threshold pressure level of a sound is the lowest
level at which an observer can discriminate between
the desired sound and the noise background always
present in the auditory sytem.
Audiomete
r
Generally employed transducers in audiometer are the
following:
 Earphone
 Microphone
 Electret microphone
 MEMS based micrphone
 Bone-vibrator
 Loud speakers
Earphones:
 Earphones are usually of the moving coil type and gives
reasonably flat frequency response upto 6 KHz after
which their sensitivity decreases rapidly.
 They are not specially designed for audiometric
applications but for communication
purposes .
 Used in hearing aids in their miniature form.
Microphones:
 These are used to translate wave motion in air into
electrical signal.
 The first one which are carbon button type which
changes resistance with the air pressure.
 The second one is the electrodynamic type in which the
voltage is induced in a coil by its motion relative to a
magnet
 Third type-condenser where capacitance of a condenser
is varied by the vibration of one of the condenser
plates.
 Electret
Microphone
 Electret Condenser
Microphone, as the name
suggests is a parallel
plate capacitor and works on the
principle of a variable capacitance.
It consists of two plates, one fixed
(called the back plate) and the
other moveable (called
Diaphragm) with a small gap
between them. An electric
potential charges the plate. When
sound strikes the diaphragm it
starts moving, thereby changing
the capacitance between the plates
which in turn results in a variable
electric current to flow.
 MEMS based microphone
The ADMP801 is a high quality, ultralow
power, analog output, bottom-ported,
omnidirectional MEMS microphone
designed specifically for hearing aid
applications.
It is fully pick-and-place and reflow
compatible, offering an option to save
on cost using a mechanized assembly
process as compared to ECMs that
require manual assembly processes.
The device offers excellent
environmental and temporal stability,
and multiple ADMP801 MEMS
microphones can be configured in an
array to form a directional response,
facilitating sound of voice localization.
Bone
vibrators
 Bone vibrators is a vibration device which is supposed to be pressed against a
reasonable hard part of the human head, which could be the forehead or more
common the mastoid, a bone right behind the ear and therefore close to the
hearing organs inside the head
 The vibrations will be transmitted through the bones to the inner ear where it
is detected. The bone vibrator should be held firmly into place on the head by
a headband, by glasses or build into a hat/helmet of some sort.
They are of the hearing aid type in which the transduction mechanism changes
the alternating current into a vibratory force through a diaphragm.
 The diaphragm and its basic mechanical parameters like mass, compliance and
resistance are important in establishing its response chs.
 Though convenient it is very ineffecient means of transduction and has arather
limited and peaky frequency response.
 The plane circular contact area of a bone vibrator is recommended to be 175
±25 mm2
 It is heald in position by a headband.
Loudspeaker
sThey are used to deliver auditory stimuli when it is not
possible to have close coupling of the transducer to the
ear.

Ms.Oinam Robita Chanu 27

 Audiometers
A typical
Audiometer:
Identification of
hearing loss.
Screening audiometers :
Used to separate two
groups of people.
Simple
audiometer
 An audiometer will essentially have an
oscillator driving a pair of head phones and is
calibrated in terms of frequency and acoustic
output.
 Pure tone audiometers and speech audiometers
are two main groups of audiometers and are
grouped according to the basis of the stimulus
they provide to evoke audio response.
 The intensity range of most audiometers starts
from approximately 15 dB above normal to 95
below normal over a frequency range from
approximately 500 to 4000 Hz.
Pure tone
audiometer
 A pure tone is the simplest type of
auditory stimulus .
 Generate test tones in octave steps from 125 to
80000Hz, the signal intensity ranging from – 10
dB to + 100 dB.
 Frequency range of 300-3000Hz .
 Changes in threshold sensitivity associated
with various middle ear surgical procedures
can be monitored more accurately with pure
tone than speech tests .
Speech
audiometer
 To carry out tests with spoken voices .
 These tests are particularly important
before prescribing hearing-aids.
CONSTRUCTION:
 A double band tape recorder is preferred to
interface the two channel audiometer units.
 Masking noise is supplied by the noise generator.
 The two channels supply the two head-phones
or the two loud speakers of 25 W each.
MASKING IN AUDIOMETRY
Need for masking:
 In case of monaural & asymmetrical binaural
hearing losses, there is a serious difficulty in
obtaining accurate measures of hearing for
the poorer ear.
 This problem can be overcome by eliminating
responses from the better ear by masking in order,
to shift the threshold to high level, permitting
greater intensities to be presented to the poorer
ear without any danger of cross-over.
Efficiency:
 Masking efficiency depends upon the nature of
masking sound as well as intensity.
 A pure tone can be used to mask other pure
tones.
Nois
e
White noise:
 White noise is a noise containing all frequencies
in the audible spectrum at approximately equal
intensities .
Saw tooth noise:
 Saw tooth noise is a noise in which the basic
repetition rate is usually that of the mains voltage
& contains only those frequencies that are
multiplies of the fundamental .
Bekesy Audiometer System
• Self recording audiometry where various pure tone frequencies
automatically move from low to high while patient controls intensity
through a button.

• Two tracings one with continuous and other with pulsed tone are
obtained.

• Help to differentiate a cochlear from retrocochlear and organic from

functional hearing loss.

Ms.Oinam Robita Chanu 34

Block diagram

Ms.Oinam Robita Chanu 35

Working

• The motor drive attenuator is controlled by a switch, which is operated

by the patient.
• The patient presses the switch as soon as he hears a sound and releases
it as soon as he stops hearing the sound.
• The audiometer is so programmed that a tracing is recorded only when
the patient presses the switch, the frequency being continually changed
either in the forward or backward manner
• A graphical representation of the patients hearing threshold across the
entire frequency range is thus obtained by the successive crossing and
re-crossing of the hearing threshold in the form of a jugged line.
• Two tracings are recorded for each ear, one by presenting a continuous
tone and other by presenting a pulsed tone

Ms.Oinam Robita Chanu 36

Contd.

ELECTRICAL SECTION MECHANICAL

• Oscillator circuit SECTION
• Modulator circuit. • Carriage device
• Automatic attenuator • Writing system
• Control circuits
• Master clock
generator

Ms.Oinam Robita Chanu 37

ELECTRICAL SECTION

•OSCILLATOR CIRCUITS:

 This oscillator generates test signals with frequencies of 125, 250,

500, 1000, 1500, 2000, 3000, 4000, 6000, and 8000Hz.

 This sequence is first presented to the left ear automatically,

each tone for 30s, and then to the right ear, the shift between the
frequencies being noiseless.

 After both ears have been tested, a 1 kHz tone is presented to

the right ear to provide a useful indication of the test reliability.

Ms.Oinam Robita Chanu 38

Contd.

•Modulators:

 The models of modulators are available “Pulse” or “Cont”.

 In the ‘Pulse’ mode the test signal is modulated giving a
signal which is easily recognized by the patient.
 In the ‘Cont’ mode no modulation is applied, giving a signal
suitable for use, while calibrating the audiometer

Ms.Oinam Robita Chanu 39

Contd.

•ATTENUATOR:

• The attenuation range is 100dB, thereby covering the range of hearing

levels from – 10 to + 90 dB.

• When the test is initiated, the attenuator starts at its top position of –
dB and then increases the level with a rate of 5 dB/s.

• The pen drive is controlled by means of the hand switch operated by

the patient.

• Pressing the switch decreases the output from the potentiometer and
thereby the level in the ear phones
Ms.Oinam Robita Chanu 40
Contd.

HAND SWITCH

• The pen drive is controlled via the logic control circuit by

means of the hand-switch operated by the patient.
• Pressing the switch decreases the output from the
potentiometer and thereby the level in the earphones
• While releasing the switch increases the output both ways
with a speed of 5 dB/s.

Ms.Oinam Robita Chanu 41

Buffer Amplifier and Calibration Circuit:
• From the attenuator the signal is fed via a buffer amplifier to the
hearing level calibration circuit.
• The buffer amplifier isolates the attenuator from the calibration
circuit in order not to affect its output.
• The calibration circuit consists of seven potentiometers, one for each
test frequency.
• During calibration, the potentiometers are adjusted one at a time
until the correct level, measured in a coupler, is obtained in the
earphones.

Ms.Oinam Robita Chanu 42

Contd.

EARPHONES:
• The earphones are a matched pair with distortion, typically less than
1%.

MASTER CLOCK GENERATOR:

• A stable clock generator supplies the necessary signals for the control
of motor speed, attenuator speed, frequency shift, modulation and
other timing functions.
• The system independent of variations in line voltage and frequency

Ms.Oinam Robita Chanu 43

Block diagram

Ms.Oinam Robita Chanu 44

Mechanical section:

 Mechanical carriage with the writing system is driven by a stepping

motor via a toothed belt.
 The speed and direction of rotation of the motor are automatically
controlled via the logic control system.
 When the test is initiated and the patient indicates that he hears the
signal by pressing hand switch, the carriage moves along the X-axis
(Frequency axis) of the audiogram in tune with the frequency of the test
signal.
 When the complete test is finished the carriage and writing system
returns to the start position.

Ms.Oinam Robita Chanu 45

Writing System

 Operation: operated by the pen drive, which is driven by a

stepping motor.
 The pen drive moves the pen, and with it the wiper of the
automatic attenuator, along the Y-axis (hearing level axis) with
a constant speed corresponding to the change in attenuation of
5dB/s.
 The direction of movement of the pen is determined by the
position of the hand switch operated by the patient.
 Limit switches are also included with the pen drive.

Ms.Oinam Robita Chanu 46

AUDIOGRAM CHART

• The audiogram is printed in standard A5

format (148 x 210 mm). The recording
space is large, 0.8 dB/mm, to enable easy
reading.
• Space is provided on the audiogram side
for registration of information on the
patient, audiometer, operator, etc.
• While the other side has space for
recording the patient’s medical and
occupational history.
• Four holes in the chart give precise and
automatic location of the audiogram on
the chart bed.

Ms.Oinam Robita Chanu 47

EVOKED RESPONSE AUDIOMETRY
SYSTEM

Brainstem Auditory Evoked Potentials

• Following a transient acoustic stimulus, ear and parts of the nervous

system generate a series of electrical signals with latencies ranging
from milliseconds to hundreds of milliseconds
• Recorded from electrodes placed on the skin
• To evaluate noninvasively the function of the ear and portions of the
nervous system activated by the acoustic stimulation

Ms.Oinam Robita Chanu 48

Contd.

• Generated by an anatomically distinguishable neuronal subsystem for

sound localization within the brainstem
• BAEPs can be used only to assess the status of the ear, auditory
nerve, and brainstem auditory pathways up through the level of the
mesencephalon

Ms.Oinam Robita Chanu 49

AUDITORY PATHWAY

• Ascending projections from the

cochlear nucleus are bilateral but
are more extensive
contralaterally than ipsilaterally
• Despite this anatomic
asymmetry, the BAEPs appear to
reflect predominantly activity in
the ipsilateral ascending
pathways

Ms.Oinam Robita Chanu 50

 Contd.
• Short-latency components,
with latencies of under 10
msec in adults
• Long-latency AEPs, with
latencies exceeding 50 msec
• Middle-latency AEPs, with
intermediate latencies

Ms.Oinam Robita Chanu 51

Block diagram

Ms.Oinam Robita Chanu 52

 Working
• The system basically comprises a conventional wide range pure-tone audiometer, which
operates under the control of an automatic programmer and provides a series of auditory
stimuli to the subject via either a loudspeaker or standard earphones.

• The EEG signal is picked up by standard electrodes placed in contact with the subjects scalp.

• One electrode is usually placed on the vertex, one at the post auricular area, and a third
(ground) on the earlobe or forehead.
• The instrument stores and evaluates that part of the EEG signal, which follows each
individual stimulus presentation.
• At the end of the programmed series of stimuli, it writes out on a paper
chart a waveform that is the average response to stimuli.
• The presence of characteristic amplitudes and latencies in this waveform give an indication
that the test intensity exceeded the subject’s threshold at the test frequency.
• Similar trials at other intensity levels and other frequencies establish
the threshold contour.

Ms.Oinam Robita Chanu 53

FIVE MAJOR SUBSYSTEMS

• Tone Generator
• EEG Amplifier
• Programmer
• Signal Averaging Computer
• Chart Recorder

Ms.Oinam Robita Chanu 54

Modern EVOKED RESPONSE
AUDIOMETRY SYSTEM

Ms.Oinam Robita Chanu 55