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FREQUENCY RESPONSE
"Linear"
Depress REF. 50 mV push-button.
5. From the calibration chart of the accelerometer, calculate the acceler-
ation level which corresponds to a voltage output from the preampli-
fier of 50 mV RMS. For example, if a B & K Type 4343 Accelerome-
ter of sensitivity 10. 1 pC/g is used with a Type 2624 Charge Amp I ifier
with a gain setting of 1 mV /pC, then a voltage level of 50 mV RMS is
produced by a signal of 4.95 g RMS.
6. Set the INPUT SECTION ATTENUATOR so that the acceleration
level calculated in point 5 will appear on-scale. For the example given,
the meter scale indication should be set for a full scale deflection of
10 g.
7. Adjust the SENS. potentiometer beside the input socket in use so that
the acceleration level calculated in point 5 is correctly indicated on
the meter.
3.2.4. Insert Voltage Calibration
When the Type 2627 Preamplifier is used in conjunction with the 2010,
1 inch microphones can be calibrated by means of the Insert Voltage or
substitution method. This is a method for determining the open circuit
sensitivity of the microphone. The open circuit voltage of a microphone is
defined as the voltage at a single given frequency which appears at its
terminals when the microphone is working into an infinite electrical imped-
ance.
This open circuit voltage can be determined by the insert voltage
technique even when the microphone is terminated in a finite electrical
impedance, such as the input impedance of an amplifier. The principle of
the method may be explained with reference to Fig.3.2.
The condenser microphone, capacitance CM, is first subjected to a
known sound pressure level, (position "a" on the figure). The open circuit
voltage produced is E
0
,
and the voltage at the input to the amplifier is V. The
sound source is then stopped. However, for the validity of this method, the
microphone should be terminated in the same acoustical impedance. There-
fore any couplers, etc. used to connect the sound source to the microphone,
should not be removed.
27
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