G.R.A.S. Sound & Vibration
Pistonphone Types 42AA/42AA-S1 - Page 7
A change in the total volume of the coupler will cause a change in the sound pressure level
generated within the coupler. A change in the sound pressure level ∆ P, in decibels, for a change
in coupler volume ∆ V is given by:
Where:
V = 15540mm
3
, including the effective load volume of 40AG
∆ V is the equivalent volume correction
The Pistonphone is calibrated using a G.R.A.S. ½-inch Microphone Type 40AG. Since all
G.R.A.S. ½-inch microphones have the same grid volume, the only correction necessary when
calibrating the various types is the correction for the various equivalent volumes*.
Table 2.1 shows the equivalent volumes of G.R.A.S. ½-inch microphones and the correspond-
ing Pistonphone corrections.
* The G.R.A.S. Microphone Type 40AG is equivalent to the Brüel & Kjær Type 4134 and the volume correction for these are
0 dB. As a matter of interest, the volume correction for a G.R.A.S. Type 40AU and a Brüel & Kjær Type 4180 is 0.077 dB.
The equivalent volume uncertainty is ±7mm
3
. The correction uncertainty is ±0.004 dB.
No equivalent volume correction is needed for calibration of ¼-inch microphones (G.R.A.S. type
40BD, 40BE, 40BF, 40BP) using the adaptor RA0049, nor 1-inch microphone (G.R.A.S. type
40EN) using the adaptor RA0023.
When calibrating 1/8-inch microphones (G.R.A.S. type 40DD, 40DP) using the adaptor RA0069
the equivalent volume correction is +19mm
3
and requires a correction -0.011 dB.
∆ P = 20 ⋅ Log ⋅ ( )
V
V + ∆ V
(2)
Table 2.1 Corrections for G.R.A.S. ½-inch microphones
re. the G.R.A.S. Microphone Type 40AG
G.R.A.S. Equiv-volume Correction
Mic. Type correction (mm
3
) (decibels)
40AC – 20 0.011
40AD 20 – 0.011
40AE 25 – 0.014
40AF 30 – 0.017
40AN 25 – 0.014
40AP 25 – 0.014
40AQ 15 – 0.008
40AR 25 – 0.014
40AU – 137 0.077
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