Operation; Calibration; Based On System Sensitivity; Pistonphone - G.R.A.S. Type 40HT Instruction Manual

3.

Operation

3.1

Calibration

3.1.1

Based on System Sensitivity

Since the microphone signal is amplified by 20 dB in the Gain and Filter Unit Type 26HT, the
nominal system sensitivity at the output of the Power Module corresponds to 0.8 V/Pa. In other
words when the measured output voltage from the Power Module is 0.8 V RMS, the microphone
is being subjected to 94 dB re. 20 μPa.
The actual system sensitivity is quoted on the individual calibration chart supplied with each
Low-noise Microphone System Type 40HT.
Based on this information, proceed as follows:
1. Connect the Type 40HT via its LEMO plug to the LEMO input socket of the Type 12HF
(see Fig. 1.3).
2. Connect via a suitable cable the BNC output of the Type 12HF (see Fig. 1.3) to the ana-
lyser to be used and switch both Power Module and analyser on.
3. Adjust the analyser to indicate 94 dB re. 20 μPa for an RMS input of S volts; where S is the
system sensitivity of the Type 40HT as quoted on the calibration chart.
3.1.2

Pistonphone

A Pistonphone Type 42AA fitted with a Coupler RA0090 (both available from G.R.A.S.) can be
used to produce 94 dB re. 20 μPa on the microphone of the Type 40HT.
Note: a Pistonphone fitted with a normal ½-inch coupler cannot be used because this will over-
load the system with a level of 114 dB re. 20 μPa.
Proceed as follows:
1 Connect the Type 40HT via its LEMO plug to the LEMO input socket of the Type 12HF
(see Fig. 1.3).
2. Connect via a suitable cable the BNC output of the Type 12HF (see Fig. 1.3) to the ana-
lyser to be used and switch both Power Module and analyser on.
3. Unscrew and remove the normal coupler of the Pistonphone.
4. Screw the Coupler RA0090 to the Pistonphone, see Fig. 3.1.
5. Push fit the ½-inch adapter GR0619 shown Fig. 3.1 to the entrance of the coupler RA0090.
Fig. 3.1
GR0619
Adapter for ½-inch microphone
Pistonphone without its normal coupler and ready to accept the
Coupler RA0090
Single-channel Low-noise Measuring System consisting of: Type 40HT and Type 12HF - Page 11
G.R.A.S. Sound & Vibration