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5.
Label this line with the Sine Generator output ·level, i.e. 10 mV in the example here,
and the other lines as shown in Fig .4.4
.
Every 2nd line on the recording paper corre-
sponds to a factor of 10 (20 dB)
.
4.2.4. Indirect Calibration of Transducers using an External Sine Generator
It is recommended that a measuring set-up be calibrated directly using a calibrated exci-
tation source, e.g. Sound Level Calibrator, wherever possible as this method calibrates
the entire measuring chain, as used, including the transducer and connecting leads.
However, if it is found necessary to use on indirect calibration method, this may be car-
ried out as follows:
1.
Connect the Preamplifier Type 2642 to the output of the Signal Generator as in Sec-
tion
3 .3.3.
2.
Set the frequency of the Sine Generator to within the frequency range of the 211 6
i.e. 100Hzto 10kHz.
3.
Set the voltage output of the Sine Generator to the sensitivity of the transducer to
be used. Every B & K microphone, accelerometer, or other transducer is delivered
with an individual calibration chart quoting its sensitivity in relevant units
.
4.
Press CALIBRATE on the 2116 so that the pen indicates the input level at the left
margin of the chart.
5.
The calibration potentiometer is now adjusted to place the pen conveniently on the
chart. This procedure is slightly different for microphones than for accelerometers or
other transducers.
a.
Accelerometer voltage sens1t1V1t1es are normally given in mV
/
g or mV
/ ms-
2 ,
the units in which vibration is normally expressed. The pen can therefore be ad-
justed on a line which can then be labelled with the relevant unit i.e
.
1 g or
1 ms
-
2
.
b.
Microphone sens1t1v1t1es are usually expressed in mV
/
Pa ( 1 Pa
=
94 dB), so in
this case the pen should be positioned 4 dB above a convenient line. In the case
of microphone sensitivities between 8 and 80 mV /Pa, the range of the CALI-
BRATION potentiometer enables the pen to be positioned on the 94 dB mark.
For other sensitivities the scale must be renumbered accordingly.
The chart paper is now calibrated.
4.3. GENERAL OPERATING PROCEDURE
20
Carry out all the necessary adjustments as in Section
3
and calibrate the measuring sys-
tem as in Section 4.2.
The test procedure varies slightly depending on whether or not a correction curve is re
-
quired to maintain a constant test environment. If this facility is required, the correction
curve must be recorded in the instrument's memory before any measurements can be
carried out. To do this the rear panel slider switch labelled COMPRESSION must be in
the "In" position. When making measurements at constant voltage, this switch must be
in the "Out" position.
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