G.R.A.S. 42AC, Instruction Manual

Page: 11 / 18

G.R.A.S. Sound & Vibration
Pistonphone Type 42AC - Page 11
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.:
3.3.4 ⅛
"
Microphones
To calibrate a ⅛
"
measurement microphone, first loosen the microphone retention collar as
shown in Fig. 3.2. Then insert the ⅛" microphone adapter (RA0069) into the ½" coupler as
shown on Fig. 3.5. Make sure that the adapter is all the way in, then tighten the microphone
retention collar so that the adapter is held firmly in place. Insert the ⅛" microphone into the ⅛"
adapter.
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. 3.5 ⅛ " microphone adapter inserted
in the coupler
L
C
= L
N
+ L
B
+ L
V
(5)
L
C
= L
N
+ L
B
+ L
V
(6)