G.R.A.S. Sound & Vibration
Power Module Type 12HM - Page 9
3. Operation
Note:
this chapter assumes that the Type 12HM is being used with either of the following
G.R.A.S. preamplifiers (no others can be used):
•
1-inch Preamplifier Type 26HF with 1-inch Microphone Type 40EH
•
½-inch Preamplifier Type 26HH with ½-inch Microphone Type 40AH
3.1
Power Supply
Before connecting the Type 12HM to an AC mains/line supply and swithcing it on, make sure
the
Mains Voltage Selector
on the rear panel (see Fig. 2.4) matches the local AC voltage
supply. Select:
•
230
for an AC mains/line power supply of nominally 230 V AC, or
•
115
for an AC mains/line power supply of nominally 115 V AC
Check also that the correct fuse is in place and not blown (see Fig. 2.4).
3.2 Calibration
3.2.1 Based on Channel Sensitivity
Since the microphone signal is amplified in the Preamplifier by 20 dB, the nominal sensitivity at
the output of the channel in question corresponds to:
•
1.1 V/Pa with the 1-inch preamplifier and microphone, or
•
0.9 V/Pa with the ½-inch preamplifier and microphone
Correspondingly (if
Level Adjust
is swithched
Off
- see Fig. 2.2), when the measured RMS
output voltage from the channel is 1.1 V or 0.9 V, the microphone in question is being subjected
to 94 dB re. 20 μPa.
Based on this information, proceed as follows:
1. Connect the channel output to the analyzer to be used and switch both units on.
2. Adjust the analyzer to indicate 94 dB re. 20 μPa for an RMS input of
S
volts; where
S
is the
sensitivity (1.1 V/Pa or 0.9 V/Pa, see above) of the channel in question.
3.2.2 Pistonphone
A G.R.A.S. Pistonphone Type 42AA fitted with a G.R.A.S. Coupler RA0090 can be used to pro-
duce 94 dB re. 20 μPa on a microphone.
Note: a Pistonphone fitted with a normal 1-inch coupler (RA0023) cannot be used because this
will overload the channel with a level of 114 dB re. 20 μPa.
Proceed as follows:
1. Connect the channel output to the analyzer to be used and switch both units on.
2. Unscrew and remove the normal coupler of the Pistonphone.
3. Screw the Coupler RA0090 to the Pistonphone, see Fig. 3.1.
4. Insert the microphone of the channel in question in the Coupler as shown in Fig. 3.2 and
switch the Pistonphone on. Note: for a ½-inch microphone, use the adapter GR0619 sup-
plied with the Coupler RA0090.
5. Adjust the analyzer to indicate 94 dB re. 20 μPa.
3.3 Measurements
1. Connect the preamplifier (with microphone) to the LEMO EGA input of the channel in ques-
tion.
2. Connect the output from the channel to an analyzer
