G.R.A.S. Sound & Vibration
Pistonphone Type 42AC - Page 13
3.4
Calculation of Microphone Sensitivity
The sensitivity of a microphone under test can be calculated via a measurement of the micro
-
phone’s output voltage. If the measured output voltage is
V
o, and the sound pressure level in
the Pistonphone’s coupler is
L
C (decibels), the microphone sensitivity
S is given by:
The value
20 μP
a is the standard reference sound pressure. The result here includes the load
-
ing effect of the preamplifier’s input impedance as well as the gain or attenuation within the
preamplifier. To measure the “Open Circuit Sensitivity” of the microphone (i.e. when the micro
-
phone’s output is not affected by the load of a preamplifier), a special preamplifier, for example
the G.R.A.S. Type 26AG (which has insert voltage calibration capability), should be used.
3.5
Calibrating Hydrophones
For information about accessories for calibrating hydrophones, contact your G.R.A.S. represent
-
ative or send an email to [email protected].
The Adapter GR0398 shown in Fig. 3.7 is available for hydrophone couplers. Unscrew the ½"
coupler from the Pistonphone body. The pistons and retention spring shown in Fig. 2.1 are
protected so there is no risk of accidentally damaging these parts when removing the coupler.
Screw the Adapter (GR0398) onto the Pistonphone body. The Pistonphone is now ready to
accept hydrophone couplers. When fitted with the appropriate hydrophone coupler, insert the
hydrophone into coupler. Make sure that the hydrophone is all the way in.
Switch the Pistonphone on via the on/off
(I/
0) button. The LED above the on/off button is a
dual-colour LED for showing red or green. The LED shows green if the Pistonphone is operating
properly at the specified frequency. If the LED shows red or flashing red, the Pistonphone is not
operating at the specified frequency and the batteries should be changed (see section 3.1).
Wait approximately for 15 seconds for the static pressure in Pistonphone to stabilise and for the
hydrophone to stabilise within its coupler.
Fig. 3.7 Adapter GR0398 for hydro-
phone couplers
S =
(8)
V
o
20 μPa
⋅
10
L
c
20
