5
LI0107 - Revision 7 September 2012
Fig 2.1
Pistonphone without its normal coupler
and ready to accept the Coupler RA0090
Calibration
Based on System Sensitivity
Since the microphone signal is amplified by 20 dB in the preamplifier, the nominal system sensitivity
at the output of the preamplifier corresponds to 0.85 V/Pa. In other words, when the measured output
voltage from the preamplifier is 0.85 V RMS, the microphone is being subjected to 94 dB re. 20 μPa.
The actual system sensitivity is given on the individual calibration chart supplied with each Low-noise
Microphone System Type 40HL.
Based on this information, proceed as follows:
1. Connect the Type 40HL via its LEMO plug to the LEMO input socket of the analyzer.
2. Connect via a suitable cable the BNC output of the power module to the analyzer to be used,
and switch both power module and analyzer on.
3. Adjust the analyzer to indicate 94 dB re. 20 μPa for an RMS input of S volts; where S is the
system sensitivity of the Type 40HL as quoted on the calibration chart.
Pistonphone Calibration
Use either Pistonphone Type 42AP, with built-in precision barometer (recommended), or Pistonphone
Type 42AA, fitted with a Coupler RA0090 (all items available from G.R.A.S.) to produce 94 dB re.
20 μPa on the microphone of Type 40HL.
IMPORTANT!
A pistonphone fitted with a normal ½" coupler cannot be used because this will overload the
system with a level of 114 dB re. 20 μPa.
Proceed as follows:
1. Connect the Type 40HL via its LEMO plug to the LEMO input socket of the power module.
2. Connect via a suitable cable the BNC output of the power module to the analyzer to be used,
and switch both Power Module and analyzer on.
3. Unscrew and remove the normal coupler of the pistonphone.
4. Screw Coupler RA0090 to the pistonphone, see Fig. 2.1.
5. Push-fit ½" Adapter RA0181 shown Fig. 2.1 to the entrance of Coupler RA0090.
Adapter for
½" microphone RA0181
