B&K 2627 Instructions And Applications page 21

b) The B & K Piston phone Type 4220
produces a signa I at
250
Hz,
124
dB SPL, and may be used when the insert voltage is supplied by
an external generator.
Recommended procedures are as follows:
Using the Internal Reference Oscillator facility
1.
Fit the microphone to the
2627
Preamplifier and connect to the
"PREAMP." input of the measuring instrument.
2.
Fit the Sound Level Calibrator
4230
on to the microphone and press
the ca I ibrator button.
3. Note the voltage V indicated on the measuring instrument.
4. Wait till the Sound Level Calibrator switches itself off, but do not
remove it from the microphone. This is to ensure the microphone is
terminated in the same acoustical impedance. Then on the Measuring
Amplifier depress the push-buttons "Ref.,
50
mV RMS" and "Insert
Volt. Cal. Int.". Then adjust the GAIN CONTROL until the voltage V
is the same as before.
5.
Then, the calibration voltage from the reference oscillator is
equivalent to the open circuit voltage E
0
of the microphone. This
calibration voltage Ec can be measured by releasing the "Insert Vol.
Cal., Int." button and reading the indicating meter without re-
adjusting the GAIN CONTROL.
Then:
Ec
Open Circuit Voltage Sensitivity=
Sound pressure
In this case, the sound pressure produced by the Sound Level Calibrator
is 94 dB SPL = 1 N/m2.
Note: When point 5 is carried out, the change in voltage indicated on the
meter will be very small (within
0.2
dB). This is because of the very low
attenuation due to the preamplifier.
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