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4.
Adjust the SENS. potentiometer beside in the input socket used, to obtain the re-
quired deflection.
Using the Internal Reference Voltage of the 2010
If an Accelerator Calibrator is not available, the
201 0
may
.
be calibrated for vibration
measurements by means of its internal reference voltage of 50 mV RMS. In this case,
set the controls as above except that CALIBRATION is set to "Ref. 50 mV. Refer to the
Calibration Chart supplied with the accelerometer to find its sensitivity, and use this to
determine the acceleration level which would produce a voltage of 50 mV at the output
of the accelerometer preamplifier. Adjust the SENS. potentiometer beside the input
socket used, to obtain a deflection equal to this acceleration level.
The remarks above under "Using the Type
4291"
about choice of scales and units apply
equally to this method of calibration.
3.2.4. Insert Voltage Calibration
When the Type
2627
Preamplifier is used in conjunction with the
2010, 1
inch micro-
phones 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 ap-
pears 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 imped-
ance of an amplifier.
The principle of the method may be explained with reference to Fig.3.3.
The condenser microphone, capacitance C
M,
is first subjected to a known sound pres-
sure 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 acousti-
cal impedance. Therefore, any couplers, etc. used to connect the sound source to the
microphone, should not be removed.
b
v
171276
Fig. 3. 3.
Principle of Insert Voltage calibration
29
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