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Under
OSHA 29 CFR 1910.95, audiometers used in occupational hearing conservation
programs need to be checked before each day’s operation.
The exact wording is:
5(h)(5)(i)
The functional
operation of the audiometer shall be checked before each day's use by testing a
person with known, stable hearing thresholds, and by listening to the
audiometer's output to make sure that the output is free from distorted or
unwanted sounds. Deviations of 10 decibels or greater require an acoustic
calibration.
The
functional check, when using a person with known stable hearing thresholds,
requires establishment of their hearing thresholds as a baseline reference, and
then comparing subsequent test results to this baseline reference to determine
if any changes have taken place. If,
when comparing subsequent tests to the baseline reference, a change of 10dB or
more occurs at any frequency for either ear, then the audiometer cannot be used
for testing and the system needs further evaluation, repair and/or calibration.
Note
that the OSHA law states the requirement is for a person with known stable
hearing thresholds. The law does
not state that a person must have a specific threshold at each frequency.
It does not state that all thresholds for every frequency must be the
same. In fact, it is unlikely that within your facility you will
find very many employees with hearing thresholds that are all 0’s or 5’s or
10’s or 20’s for both ears at every frequency. The law states that the
thresholds must be stable; i.e. they do not vary from day to day by more than
5dB. In most facilities where a
person is used as the daily biological reference, an employee with good hearing
is selected and their thresholds are recorded accordingly.
Recognizing
that the law is concerned with a stable hearing reference, and recognizing that
humans are susceptible to temporary hearing losses from allergies, colds,
excessive noise exposure, etc., the preferred instrument for performing the
functional check is an electro-acoustic ear; an electronic device that triggers
a response at a pre-established sound pressure level on a consistent basis.
The
OSCAR electro-acoustic ear operates using two independent microphones to convert
earphone sound pressure levels into AC voltages.
These voltages are then amplified and sent to rectifiers for AC to DC
conversion. The DC voltage enters
its respective detector. When
sufficient signal is present at the earphone coupler, the appropriate indicator
light (RIGHT OR LEFT in the Oscar 7 or RESPONSE in an Oscar II, III or IV) will
light and the response “output transistor” will activate.
Once this occurs, the earphone signal must be decreased by at least 10 dB
(hystersis) to deactivate the indicator light and the response “output
transistor” is deactivated. Activation causes the OSCAR to respond through its
phone connector exactly the same as the corresponding handswitch.
The audiometer, with its testing algorithm will then determine the
appropriate hearing threshold.
OSCAR
electro-acoustic ears are activated by sound pressure levels.
Audiometers are calibrated in Hearing Threshold Level; i.e. sound
pressure levels that are adjusted at each frequency to replicate the non-linear
response of the human ear. For
example, for TDH39 earphones, the Sound Pressure Level at 500 HZ is 81.5 dB when
the audiometer output is 70dB HTL. The
appendix E-1 found in 1910.95 is shown below for all required test frequencies.
TABLE E-1 - REFERENCE THRESHOLD LEVELS FOR TELEPHONICS -
TDH-39 EARPHONES
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Reference
threshold Sound
level for level
Frequency, Hz TDH-39 meter
earphones, reading,
dB dB
_____________________________________________________________
500 .................................. 11.5 81.5
1000 ................................. 7 77
2000 ................................. 9 79
3000 ................................. 10 80
4000 ................................. 9.5 79.5
6000 ................................. 15.5 85.5
_____________________________________________________________
Since
OSCAR electro-acoustic ears respond to SPL, not HTL, and since the microphones
themselves have pre-existing response characteristics, the OSCAR will generally
respond to slightly different HTL levels at specific frequencies.
So, for example, if an OSCAR is set so that it responds at 70 dB HTL at
1000 Hz., it may respond to 2000 Hz at a lower level, say 65 dB HTL because:
1. The
sound pressure level at 2000 Hz. 70 db HTL is actually 79 dB SPL ( 2 dB higher
than 1000 Hz.) and
2.
The
sensitivity of the microphone is probably greater at 2000 Hz. than 1000 Hz.
In
fact most OSCARS respond to slightly different HTL levels simply because of
their microphone characteristics. Remember,
audiometer calibrations are performed with microphones that are classified as
type II. These microphones
generally cost from $400.00 to $1500.00 because they are designed to be very
linear in response from 125 to 8000 Hz. - typically well within + or – 3dB.
OSCAR
microphones, while reasonably linear, are
required to be primarily stable. They
are chosen for their stability properties. A microphone in an OSCAR that
responds at 65 dB HTL at 2000 Hz. today, is designed to respond to 65 dB HTL
tomorrow and the day after, etc. In fact, it is designed to deviate by no more
than 5 dB as required by law, for many years.
To
insure that OSCAR electro-acoustic ears are responding properly, and to document
any new permanent changes in microphone characteristics, OSCAR electro-acoustic
ears are calibrated annually. Our
office performs these calibrations by setting the OSCAR to respond to a level of
70 dB HTL at 1000 Hz. This
calibration level is based on the assumption that audiometers are calibrated at
70 dB and that 1000 Hz. represents the level that has the smallest correction
from HTL to SPL (7.0 dB). Knowing that all of our customers have OSCARS that
trigger at 70 dB HTL at 1000 Hz. enables us to immediately diagnose the possible
cause of the malfunction when customers indicate that their OSCAR is not working
properly. An OSCAR that triggers at
80 dB or 90 dB at 1000 Hz. HTL usually has a different problem than one that
triggers at 45 dB or 50 dB.
Some
of our customers test in locations where there is significant low frequency
noise and for these customers, we set the reference level higher, 75 or 80 dB
SPL so that low frequency SPL levels do not interfere with test levels.
Our loaner OSCARS include both types so, on occasion, a customer may
receive a loaner OSCAR that triggers at higher levels.
Tremetrics
uses a slightly different reference level for their OSCARS. A new OSCAR or and
OSCAR that has been returned from the factory for repair may have a level that
is slightly different. In any case,
once a customer has received a calibrated OSCAR the levels initially obtained
immediately upon use become the new reference levels and they should not be
modified unless instructed by the manufacturer or servicing distributor.
Since
OSCARS do not have filter and detector circuits for each frequency in the
electro-acoustic circuit, one overall microphone gain adjustment is made to set
calibration levels. Once the
microphone gain is set at 1000 Hz. the other frequencies are automatically
preset. The typical OSCAR will trigger at 70 dB at 500 and 1000 Hz.,
about 65 dB at 2000 Hz, and about 60 or 55 dB at the remaining frequencies.
Regardless of the trigger level, once the electro-acoustic ear is set,
each frequency will ALWAYS trigger at that level plus or minus 5dB. Again, these
deviations are usually the result of variances in the microphones.
In
conclusion, it is incorrect to assume that OSCAR electro-acoustic ears are
pre-set or calibrated to respond to exactly the same HTL level at EVERY
frequency. Similarly, it is
incorrect to assume that the daily biological subject is required to have
hearing that is exactly the same HTL level at EVERY frequency.
OSHA and audiometer manufacturers require a stable reference for each
frequency and they do not require that these reference levels be identical for
each frequency in both ears.
A
stable reference is essential for checking the daily operation of your
audiometer.
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