G.R.A.S. Sound & Vibration
Single-channel Low-noise Measuring System consisting of: Type 40HT and Type 12HF - Page 8
2.
Power Module Type 12HF
Since the microphone signal is amplified in the Gain and Filter Unit Type 26HT by 20 dB, 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 Supply is 0.8 V RMS, the microphone
is being subjected to 94 dB re. 20 μPa.
The actual sensitivity is quoted on the individual calibration chart supplied with each Type 40HT.
Important!
The Single-channel Power Module Type 12HF (available from G.R.A.S.) is a dedicated module.
Under no circumstances should any other Power Module (apart from the multi-channel Power
Module Type 12HM - also available from G.R.A.S.) be used with the ½-inch Low-noise Level
Microphone System Type 40HT.
2.1
Front Panel
The front panel has the following features (see Fig. 2.1):
•
Power switch with two LEDs: green “OK”, red “Batt. Low”.
If the power supply is correct, the green LED lights up. If the red LED lights up, either the
batteries are low and should be changed (see section 2.3) or the external DC supply volt
-
age is too low.
•
Two-position switch for selecting frequency response,
Pressure
microphone operation or
Free Field
microphone operation:
Pressure
- to select the output of the compensating filter which gives the system a linear
pressure-frequency response.
Free Field
- to select the output of the compensating filter which gives the system a linear
free-field frequency response at 0 ° incidence.
•
Overload-warning LED synchronised with the one on the Gain and Filter Unit Type 26HT
(see Fig. 1.1).
Fig. 2.1 Front panel of the Power Module Type 12HF
Power switch
Green LED
power OK
Red LED Batt.
Low
Input 7-pin
LEMO socket
Output BNC
socket
Overload LED indicates
when the measured
sound pressure level
overloads the system’s
electronics
Switch for
selecting either
pressure micro-
phone opera-
tion or free-field
microphone
operation
