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b~l
0
0
Ec
v
Eo
Fig.3.2. Insert Voltage Calibration of a Microphone
A source of known and adjustable voltage, Ec, is then connected in series
with the microphone (position"b" on the figure). This insert voltage is
adjusted until the input voltage to the amplifier is the same (V) again. Then
E
0
=
Ec and the open circuit voltage sensitivity can be calculated as follows:
Open Circuit Voltage Sensitivity
=
---,dE-=c _ _ _
soun pressure
When the 2627 is used with the 2010 the insert voltage Ec can be
supplied in two ways. The internal reference oscillator can be used to give a
voltage at a fixed frequency of 1 kHz, or the B.F.O. section of the 2010, or
an external generator, can be used to give a signal at any other required
frequency.
It is also necessary to have a sound source of known pressure level and
frequency. This can be provided in two convenient ways:
28
a) The B & K Sound Level Calibrator Type 4230 produces a signal at
1 kHz, 94 dB SPL, and may be used when the insert voltage is
supplied by either the internal reference oscillator, or by the B.F.O.
section, or by an external generator.
b) The B & K Pistonphone Type 4220 produces a signal at 250Hz,
124 dB SPL, and may be used when the insert voltage is supplied by
the B.F.O. section or by an external generator. Using the normal
voltage scale supplied with the 2010 (SA 0051), recommended pro-
cedures are as follows:
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