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LI0151 – 4 July 2017
Calibration
An acoustic calibration with a pistonphone or a sound calibrator requires that windscreen and top
cone are dismantled. How to do this is shown in Fig. 7a. Calibration can be performed using a
Sound Calibrator or a Pistonphone. The G.R.A.S. 42AP Intelligent Pistonphone is recommended.
The Sound Calibrator/Pistonphone must be fitted with a coupler for ½-inch microphones.
Refer to the manual for your sound calibrator or pistonphone for further information.
Remote Check Signal Waveform and Level
The signal should be a single tone, a 1 kHz sine wave is recommended. The precision of the test
signal should reflect your requirements for accuracy.
The signal will be attenuated by about 40 dB when applied to the RC network built into the
preamplifier. Roughly speaking, a RemoteCheck signal of about 5 V RMS will result in a response
signal corresponding to a sound level of about 94 dB SPL.
When setting up your measurement system, we recommend that you – after the acoustic
calibration of the microphone – calibrate the RemoteCheck test signal. When calibrating the
RemoteCheck signal, you can adjust the signal from your generator to correspond to a desired
reading, for example 94 dB SPL.
Analysis of the Remote Check Signal
RemoteCheck verification is based on comparison of historical data from frequent RemoteCheck
measurements. Depending on the background noise level, the expected accuracy of the verifica-
tion signal will be within 0.15 dB to 0.5 dB. If the background noise is about 60 dB SPL, the
accuracy will be about 0.15 dB, if it is 70 dB SPL it will be about 0.5 dB.
To get the best possible accuracy, you must therefore monitor the background noise and select
your RemoteCheck measurement values from samples obtained in situations with low levels of
background noise. If background noise is still a problem, it can be minimized by using a larger
RemoteCheck signal and/or use a narrow band analysis around the test frequency, and in this way
you can isolate the test signal response from the background noise. In practice this should not be
necessary.
Maintenance
Depending on local conditions, the windscreen must be cleaned or replaced. How often must be
determined by visual inspection and knowledge of local weather conditions.
O-ring OR2038 (see page 5) must be lubricated with silicon grease at regular intervals.
Without lubrication it must be replaced once a year.
When under power, the preamplifier emits heat. This heat is part of the 41AC’s protection against
moisture, and therefore the power to the 41AC should be turned on permanently.
