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LI0196 – 19 July 2017
Calibration Considerations
Accuracy
42AG is a precision instrument with an accuracy better than ±0.2 dB. Calibration of the sound
calibrator at regular intervals will ensure that drift does not occur and that it performs within
specifications.
If an accuracy better than ±0.2 dB is desired, we recommend using a G.R.A.S. 42AP Intelligent
Pistonphone, Class 0 where an accuracy better than 0.09 dB can be obtained.
Background Noise
It is always good practice to verify that the background noise level is sufficiently below the calibra-
tion level to not influence the calibration. It must be at least 20 dB below the calibration level.
Corrections
Ambient Calibration Conditions
The 42AG Sound Calibrator automatically adjusts the calibration level to the nominal level.
Nevertheless, it is considered good practice to note the ambient calibration conditions as part of
each calibration routine.
Temperature
The temperature may be displayed in either degree Celsius (˚C) or degree Fahrenheit (˚F). ln order
to change the unit, keep the button
Met
depressed during power on.
Load Volume
Various types and makes of microphones may have different effective load volume. Due to its
working principle, the 42AG has a large effective coupler volume. The variations in sound pres-
sure level due to variations in effective load volume among microphones are therefore for most
applications insignificant.
An equivalent microphone volume of 250 mm
3
is used as reference load volume for 1/2” micro-
phones. This corresponds to most 1/2” measuring microphones with protecting grid mounted. lf
no adaptor is used, the nominal reference front volume is 1333 mm
3
. This corresponds to most
1” measuring microphones with the protection grid mounted.
