G.R.A.S. Sound & Vibration
Pistonphone Type 42AC - Page 12
3.3.5 1" Microphones
To calibrate a 1" measurement microphone, the standard ½" coupler has to be replaced by
the optional 1" coupler
1
(RA0023), see Fig. 3.6. 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 1" coupler (RA0023)
onto the Pistonphone body. Then insert the 1" microphone into the 1" coupler. Make sure that
the microphone 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 3.2).
Wait approximately for 15 seconds for the static pressure in both the Pistonphone and the
microphone to stabilise, and for the microphone itself to stabilise within the coupler.
The static pressure within the coupler volume is equalised via an air-equalisation tube located
under the cap which protects the pistons and retention spring shown in Fig. 2.1.
With the Pistonphone switched on, the microphone is subjected to a sound pressure level
L
C
given as the sum of the Pistonphone’s nominal sound pressure level
L
N, the static pressure cor
-
rection
L
B and the volume correction
L
V, i.e.:
Fig. 3.6 1
"
and ½
"
couplers
1
The 1" coupler is individual calibrated with the pistonphone, and these must be considered as a pair
L
C
= L
N
+ L
B
+ L
V
(7)
