G.R.A.S. Sound & Vibration
Pistonphone Type 42AC - Page 14
3.5.1 Calibrating Hydrophone Type 10CT
A special adapter RA0043, see Fig. 3.8, is needed in place of the standard coupler (Fig. 3.7)
delivered with the Pistonphone.
Fig. 3.8 Adapter RA0043
Futhermore, when the RA0043 is fitted to the Pistonphone, a reduction in “coupler” volume is
effected. This will result in a corresponding increase in sound pressure level (≈ 8 dB) within the
RA0043 when callibrating a Hydrophone Type 10CT.
The exact increase can be determined via a reference microphone, e.g. any G.R.A.S. ½" micro
-
phone and preamplifier.
Fig. 3.9 The three steps of calibrating a given Hydrophone Type 10CT
1) first calibrate the reference microphone in the normal way
2) with the reference microphone and hydrophone mounted, use the refer
-
ence microphone to measure the SPL applied to the hydrophone
3) future calibration checks knowing the SPL applied to the given hydro
-
phone
Fig. 3.9 shows the three steps of calibrating a given Hydrophone Type 10CT.
In step 1, check the reference microphone in the normal way as described in section 3.2.2.
In step 2 replace the pistonphones coupler with the RA0043. Push both reference microphone
and hydrophone all the way in to their respective stops before measuring and noting the SPL in
the Adapter RA0043.
In step 3, the reference microphone is not needed but seal the microphone hole as shown (push
the plug in all the way) when making future calibrations of the given hydrophone. The hydro
-
phone will be subjected to the same SPL as measured in step 2.
Stand
-
ard ½"
coupler
Reference
micro
-
phone
Hydrophone Type 10CT
Reference microphone,
e.g. a G.R.A.S. ½" mic.
and preamp.
Adapter RA0043
Plug in place
for future
calibration
checks
Detachable
½" plug
1)
2)
3)
