GRAS 42AA - Inch Microphones

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G.R.A.S. Sound & Vibration

Pistonphone Types 42AA/42AA-S1 - Page 12

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

given as the sum of the Pistonphone’s nominal sound pressure level L

(found on the piston-

phone’s calibration chart as “Sound Pressure Level”), the static pressure correction L

and the

volume correction L

, i.e.:

3.3.4 ⅛-inchMicrophones

To calibrate a ⅛-inch measurement microphone, rst loosen the microphone retention collar as

shown in Fig. 3.2. Then insert the ⅛-inch microphone adapter (RA0069) into the ½-inch coupler

as shown on Fig. 3.4. Make sure that the adapter is all the way in, then tighten the microphone

retention collar so that the adapter is held rmly in place. Insert the ⅛-inch microphone into the

⅛-inch 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 specied frequency. If the LED shows red or ashing red, the Pistonphone is not

operating at the specied frequency and the batteries should be changed (see section 3.2).