Transcript Audiometer

AUDIOMETRY
An Audiometer is a machine, which is used to
determine the hearing loss in an individual .
Classification
Audiometers
Pure tone audiometer
Air conduction
Bone conduction
Speech audiometer
Speech thresholds
Audiometer
• Generally employed transducers in audiometer
are the following:
• Earphone
• Microphone
• 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 specially designed for audiometric
applications rather than for communication
purposes .
Microphones:
• These are used to translate wave motion in
space into electrical signal.
• Two types which are carbon button changes
resistance with the pressure.
• The second one is the electrodynamic type in
which the voltage is induced in a coil by its
motion relative to a magnet
Simple audiometer
• An audiometer will essentially have an oscillator along
with a pair of head phones.
• 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
elicit 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
• 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
• 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 .
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.
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.
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
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.
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 .
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 .
• 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.
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.
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 .