Download AUDIOMETERS AND HEARING AIDS

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 fluid 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 to the
brain
The brain interprets it as sound!
Audiometer
 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.
Classification
Audiometers
Pure tone audiometer
Speech audiometer
Air conduction
Bone conduction
Speech thresholds
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.
Audiometer
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
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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.
Loudspeakers
 They are used to deliver auditory stimuli when it is not
possible to have close coupling of the transducer to the
ear.
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.
Noise
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
OPERATION:
 The instrument generates a pure-tone signal which is
presented to him through an air-conduction earphone.
 The subject is asked to press a switch when the tone is
heard and to release the switch when it is not heard.
 A pen connected to the attenuator traces a continuous
record of the patient’s intensity adjustments on an
audiogram chart, producing a graphic representation .
Bekesy Audiometer System
Bekesy Audiometer System
ELECTRICAL SECTION
 Oscillator circuit
 Modulator circuit.
 Automatic attenuator.
 Control circuits
 Master clock
generator
MECHANICAL SECTION
 Carriage device
 Writing system
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.
Electrical section
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 .
Electrical section
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 .
Earphones
 The earphones are a matched pair with distortion which
is typically less than 1%.
 Mechanical section:
 Mechanical carriage with the writing system is driven by
a stepping motor via a toothed belt.
Earphones
 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.
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 hands witch operated by the patient.
 Limit switches are also included with the pen drive.
Audiogram Chart
The audiogram is printed in
standard 5 format (148*210mm).
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.
MASKING IN AUDIOMETRY
 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.
Evoked response audiometer
Five major subsystems
 The tone generator
 EEG amplifier
 The programmer
 Signal averaging
computer
 Chart recorder