AUDIOMETRIC CALIBRATION
Introduction
Audiology focuses primarily on assessing the functioning of different parts of the hearing system and uses different diagnostic instruments like an audiometer, middle ear analyser, electrophysiological tests like ABR, ASSR & OAE etc.
These instruments are sophisticated and demands accurate installment, regular maintenance and calibration.
The audiologist should always ensure that the instrument is in good working condition before testing.
Calibration
Checking the accuracy of the output of a measuring instrument
It is a process of setting equipment to a standard measure (ANSI, IEC, ISO)
Types:
Daily Listening Check
Quarterly / half yearly / annually:
Biologic / Real ear / Subjective
Electroacoustic / Objective
Daily Listening Check
Routine check
Without the help of any other instrument
It is done to ensure:
Audiometer is performing in proper manner
Audiometer calibration is not altered
Audiometers attachments, leads and accessories are free from defect and are safe
Requirement:
Normal hearing sensitivity at least in one ear
Quiet room
Procedure:
Physical inspection
Instrument and all the accessories (earphones, bone vibrator, response button etc.)
Connections, lead wires
Power cord, transducers cords, cushions and headband, controls and all switches
Switch on the instrument
Listening check
Listening to a 1000 Hz tone at 60dB HL, through each transducer
The audiologist should listen for audible mechanical transients through the earphone when the dials or switches are manipulated.
The ANSI S3.6-2010 standard (section 5.4.4) suggests that two normal-hearing listeners should listen at a distance of 1 m from the audiometer with the earphones in place but disconnected and with a proper load resistance (coinciding with the impedance of the earphone at 1,000 Hz) across the circuit while manipulating the presenter/ interrupter switch, and so on, to make sure that there are no audible signals that would inform the subject to the presence of the test signal.
Check for electronic transients.
The audiologist should listen for hum or static with the hearing level dial at a high value, both when a stimulus signal is present and when it is absent. One should not hear static or hum at levels below 60 dB HL on the dial.
“Cross-talk” may occur between earphones, that is, the signal that is sent to one earphone may be heard in the contralateral earphone.
Cross-talk may be detected by unplugging one earphone, sending a signal to that phone, and listening to the other earphone
Attenuator linearity: checking the attenuator linearity is done by setting the HL control to its lowest value and directing a continuous 1000 Hz tone to one earphone and moving the HL control through the entire range.
The clinician should listen to the signal while the attenuation dial is changed from maximum to minimum levels. For instance, a tone may be present at 20 dB HL on the dial, whereas no tone is present at 15 dB HL on the dial.
Frequency: checking frequency is done by setting the frequency control at its lowest setting and directing a continuous 70 dB HL tone to one earphone. The frequency control is slowly moved through the entire range. A non wavering tone should be heard at each frequency in each earphone.
Finally, the threshold of the clinician (or a person with known hearing thresholds) should be checked with the earphones and bone vibrators to make sure that the outputs are approximately correct. If the levels are not within 10 dB of the previous threshold values, the output levels should be checked electronically
Biologic Calibration
Uses human subjects
Air conduction output calibration
Bone conduction output calibration
Air Conduction Output Calibration
Method – 1
Have a record of your thresholds obtained from a calibrated audiometer. This record serves as reference thresholds.
Then check your responses with the audiometer to be calibrated. The threshold at a particular frequency obtained may be better or poorer than your reference threshold level. Thus, you can spot gross deviations in audiometer output for different frequencies.
Correction Rule:
Take the reference value (R, person’s threshold) and the obtained value on the uncalibrated audiometer
The correction factor (CF) = R – O
Eg. If R = 5 dB and O = 15 dB then CF = 5 – 15 dB
CF = – 10 dB
Limitations:
Threshold is not a fixed point, it may vary within the range of 10 dB from day to day (Carhart & Jerger’1959)
Variations in placement of earphones affect thresholds specially at low frequencies.
Subjective bias.
Method – 2
Select about 10 normal hearing adults without any history of familiar hearing loss, otologic problems, noise exposure, tinnitus etc.
Test one ear of each subject at all frequencies with the audiometer to be calibrated.
Find the average threshold at each frequency.
Correction rule: Average threshold at each frequency is used as CF. eg. At 1000 Hz, if threshold is +10 dB HL, the CF is –10 dB HL. Hence if a patient’s threshold is 50 dB HL at 1KHz, it would be 50 -10 i.e. 40 dB HL.
Limitations:
Adult normal ears threshold can vary from –10 to +15 dB HL range.
Time consuming and less effective.
Method – 3 – Loudness Balance
The output from a calibrated audiometer is matched against the output of an audiometer to be calibrated.
Requirements:
A calibrated audiometer known to be accurate
At least three young subjects with normal hearing and negligible difference in hearing sensitivity between ears.
Detach one earphone of the known audiometer (Calibrated audiometer) and fix it to the headband of the unknown audiometer (audiometer to be calibrated), after removing one of its earphones.
Instruct the subject to match the loudness of the tones
Place this headset on the ears of the subject
Set the frequency selector of each audiometer to l KHz and intensity dial of known audiometer to 40 dB HL and present the interrupted tones.
When the signal is 'off' in the 'known earphone', then signal should be presented in the 'unknown‘ earphone.
While this presentation of signals continues; without looking at the intensity dial, adjust the hearing level of the unknown audiometer until the subject tells you that loudness of the tones are equal, or approximately equal.
Stop presentations and then note down the intensity dial reading of the unknown audiometer.
Similarly, get loudness balance at the other hearing levels. Repeat the procedure at other frequencies. Also perform for other subjects.
Note:
Avoid simultaneous presentation of the signals from both audiometers
The sensitivity of this procedure can be increased by training the subjects to listen carefully and match the tones.
Suppose the dial is set 40 dB HL (A) at 1 kHz and the subject matches the tone of the unknown audiometer when the intensity dial reads at 45 dB (B). Then the difference is (A - B) i.e., 40 dB - 45 dB = - 5 dB
Thus there is difference in output at this frequency.
Then the correction is - 5 dB at I kHz, which should be written on the calibration chart.
Correction Value = A(HL) – B(HL)
Limitations:
It does not permit to find out the attenuator linearity of
the intensity dial of the audiometer. n It requires two audiometers, one of which is
calibrated. n It requires the task of loudness balance which is not
easy.
Method – 4
Objective –
Biologic calibration
n This procedure is both objective and biological. n It makes use of an electronic instrument to elicit the
response without the co-operation of the subject,
hence it is called objective method and employs
human subjects for calibration, hence called
biological calibration.
Requirements:
n Immittance audiometer
n Normal hearing subjects
n Audiometer to be calibrated. n Determine the subject's acoustic reflex threshold
(ART1
) at different frequencies. n Detach the ear phone of the impedance bridge and
replace it by one ear phone of the audiometer, to be
calibrated. n Now, determine the acoustic reflex thresholds (ART2
)
for the same ear, same subject for different
frequencies.
n Find out the correction factor by subtracting ART2
from ART1
for each frequency. n Determine the reflex threshold (ART3 ) for the same
ear, same subject, placing the other ear phone of the
audiometer to the head set of the impedance bridge. n Then find out the correction factor by subtracting
ART3
from ART1
for each frequency.
Limitations:
Does not permit to find out the attenuator linearity of the
intensity dial of the audiometer. n Calibration of output levels at 4kHz and 8 kHz may not be
possible as reflex is usually absent at these frequencies
BONE CONDUCTION OUTPUT
CALIBRATIO
Most helpful and accurate approach in absence of
artificial mastoid. n Select 10 subjects with typical bilateral moderate
sensorineural loss (hearing loss need not be equal in
all ears). n Determine the air conduction (AC) thresholds of both
ears of each subject at 250, 500, I000, 2000, & 4000
Hz using well calibrated air conduction system. n Then Determine the bone conduction (BC) thresholds
for same ear using the bone vibrator to be calibrated. n Find out the average AC thresholds and average BC
thresholds at each frequency. n CF =AC (avg) – BC (av)
Assumption of this method:
n In a typical sensorineural hearing loss subject, the
AC and BC thresholds are equal. The difference is
considered as error in the bone vibrator output.
Limitation
A slight conductive component might add to the air - bone differences in thresholds. Hence to that extent,
accuracy of this method is limited
Calibration
Types:
n Daily Listening Check
n Quarterly / half yearly / annually:
n Biologic / Real ear / Subjective
n Electroacoustic / Objective
Basic Equipment for calibration
INTENSITY:
Oscilloscope
n Voltmeter
n Multimeter
n Condenser microphone
n Spectrometer
n Graphic level recorder
n Sound level meter
n SLM filter
n Acoustic coupler (NBS 9-A Or IEC 318;HA-1 Or HA-2, or
Occluded Ear Simulator-711)
n Mechanical coupler (artificial mastoid)
FREQUENCY:
Oscilloscope
n Electronic counter
n Frequency analyzer
n Distortion meter
n Spectrum analyser
TIME:
Oscilloscope , Electronic counter , Graphic level recorder
Electroacoustic Calibration
1. Measurement
of output sound
pressure levels
for pure tones
.Through
headphones
• Through
bone
vibrator
2. Frequency
calibration for
pure tones
3. Determination
of attenuator
linearity
Preliminary check-up
The working voltage is within permissible limits
n All the jacks, cords and plugs are in place and there
are no loose connections. n There is no visible damage on the audiometer, its
controls and its accessories. Damage, if any, should
be rectified before checking the calibration. n The earphones and the vibrator are of the type that
match the audiometer. n The headbands have adequate tension. n The instrument is not delivering any electric s
CALIBRATION OF
AUDIOMETRIC OUTPUT
INTENSITY VIA HEADPHONES
Purpose:
To measure the output sound pressure level of an audiometer at
various frequencies and to check the attenuator linearity via the
earphone
Requirements
Audiometer to be calibrated
n Artificial ear (NBS 9A Coupler)
n Condensor microphone
n Sound level meter (SLM)
n Microphone adaptor
n Octave filter set (OFS)
Procedure
Phase
I
• Mounting of microphone and the ear phone on artificial
ear
Phase
II
•
Measurement of output sound pressure level
Phase
III
•
Adjustment of the output
Phase
IV
• Linearity check
Phase I
1. Unscrew the coupler from the artificial ear
2. Unscrew the protection grid from the microphone. Without touching the diaphragm fit the microphone into
the socket inside the artificial ear.
3. Replace the coupler on the artificial ear.
4. Remove the earphone to be calibrated from its headband
clamp. Place it on the coupler so that the earphone
perforations face the coupler cavity.
5. Unscrew the adjustable clamp on the artificial ear slightly
and adjust the weight on the earphone to 0.5 Kg or as
per specification provided by the manufacturer.
6. Connect the artificial ear and the octave filter set to the
SLM
Phase II
1. Switch on the Audiometer
2. Set the attenuator to 60 dB / 70 dB (depending upon the
manufacturers specifications) frequency dial to 125 Hz or
250 Hz as required.
3. Set the output selector to right/ left depending on the
earphone placed on the artificial ear
4. The octave filter set is connected to the SLM. Set it to the
required frequency i.e., the same frequency as that
selected on the Audiometer
5. Switch on the SLM and set it to "slow response" and to
"external filter"
6. Adjust the tone switch on the audiometer so that the
signal is continuously on
7 Set the attenuator on the SLM to 60 dB / 70 dB,
depending on the intensity selected in the
audiometer. If the needle deflects to either extreme,
vary the attenuator setting suitably so that the
needle deflects to the centers of the meter.
8. Note down the combined reading from the attenuator
and the meter
9. Repeat the procedure for each of the test
frequencies. Remember to vary the frequency
setting on the octave filter set
10. Repeat the entire procedure with the other
earphone.
Phase III
1. If the switch is available on the audiometer,
If measured output is not as per expected level then
turning the attenuator dial either clockwise or
anticlockwise adjust the output and press the
mentioned switch to store the changes in the
memory.
2. The difference should not exceed ± 3 dB at
frequencies from 250 Hz to 4000 Hz, ± 5 dB at 6000
and 8000 Hz (IS9098 - 1979).
Phase IV
1. The procedure is similar to that in phase III. However, linearity need be checked at only one
frequency
2. Follow the steps from 1 to 6 in phase II except step 2
where the intensity dial should be set at maximum
level, frequency dial should be set at 1000 Hz.
3. Set the attenuator to the SLM at a level that
corresponds to the maximum level on the
audiometer
4. Decrease the attenuator setting on the audiometer in
5 dB step and note down the corresponding reading
on the SLM.
5. Repeat step 3 till the audiometer reads 30 or 40
dB H Record the level on the SLM. If with every
decrease in the dial reading the SLM indicates a
corresponding reduction then the intensity variation
is linear.
CALIBRATION OF
AUDIOMETRIC OUTPUT
INTENSITY VIA BONE VIBRATOR
Purpose:
To measure the output sound level of an audiometer at various
frequencies via bone conduction vibrator.
Requirements
Audiometer to be calibrated
n Artificial Mastoid
n Sound Level Meter (SLM)
n Adaptor
n Octave Filter set (OFS)
n Other accessories:
n Spring balance
n Level indicator
Procedure
Phase I
• Mounting of bone conduction vibrator
on the artificial mastoid to the SLM
Phase II
• Measurement of output sound
pressure level
Phase III
• Adjustment of the output
Phase I
1. Ensure that the artificial mastoid is placed on a
horizontal plane
2. Detach the BC vibrator from the headband and place
it on the artificial mastoid
3. Note the level, with the help of the level indicator
and remove the BC Vibrator
4. With the help of the spring balance, check the weight
(500 gm) on the clamp / arm of the artificial mastoid
and readjust the level of the clamp with reference to
the level indicator
5. Remove the spring balance and level indicator
6. Connect the adaptor to SLM
7. Plug in the output jack of the artificial mastoid to the
adaptor and thus the artificial mastoid is connected
to the SLM
8. Connect the OFS to the SLM Audiometer BC Vibrator Artificial Mastoid SLM + OFS
Phase II
1. Switch on the audiometer
2. Set the intensity dial at 20 / 40 dB (depending upon
the instructions in the manual) and the frequency
dial 250 Hz.
3. Set the output selector to bone
4. Set the OFS to the required frequency
5. Switch on the SLM and set it to slow response and
to external filter
6. Set the tone switch on the audiometer so that the
signal is continuously on 30
7. Set the attenuator on the SLM depending on the
expected output reading. If the needle deflects to
either extremes, change the attenuator setting
suitably so that the needle deflects to the central
area of the meter
8. Note down the combined reading from the attenuator
and meter
9. Repeat the procedure for each of the test
frequencies. Also adjust the frequency setting on the
OFS to correspond with the test frequency
Phase III
Same as for air conduction.
FREQUENCY CALIBRATION
Purpose:
To determine the deviation of the output frequency from that of the
dial reading Audiometer to be calibrated
n Digital Frequency Counter Audiometer
Frequency
Counter
1. Remove one of the earphone jacks from the output
socket
2. Insert a spare ear phone jack in to the socket
3. Connect the spare jack to the input terminals of the
frequency counter with a wire
4. Switch on the Audiometer and the Frequency
counter
5. Set the Audiometer frequency dial to 125 Hz, as the
case may be
6. Set the output intensity to maximum
7. Adjust the tone switch to continuously 'on' position
8. Turn the function selector on the frequency counter
to frequency
9. Note down the reading on the frequency counter. Adjust the sensitivity of the counter to obtain a stable
value
10. Compute the deviation of the frequency generated
by the audiometer from the expected frequency
11. Repeat the procedure for other test frequencies
12. Whenever the deviations exceed ± 3 % (IS9098- 1979) internal calibration is required
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