BRUEL & KJAER 4188 - Sensitivity

Chapter 2 — Prepolarized Free-field
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" Microphone Type 4188
Sensitivity
Falcon™ Range of Microphone Products
Microphone Handbook
Brüel & Kjær
2
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4
The right-hand curve on the rear side of the calibration chart is the open-circuit
random-incidence response for the microphone with the supplied Random Incidence
Corrector DZ9566.
Each microphone’s individual lower limiting frequency is measured to ensure that it
is within the specified tolerances (see Fig.2.3).
2.1.3 Recommended Recalibration Interval
With normal handling of the microphone and any associated instrument,
Brüel&Kjær recommends that the microphone be recalibrated every 2 years.
Prepolarized Free-field
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2
" Microphone Type 4188 is very stable over this period
(see section 2.10 to section 2.12). Improper handling is by far the most likely cause
of change in the microphone’s properties. Any damage which causes improper oper-
ation can probably be detected using a sound level calibrator. In many cases, the
damage can be seen by carefully inspecting the protection grid and diaphragm.
2.2 Sensitivity
2.2.1 Open-circuit Sensitivity
The open-circuit sensitivity is defined as the sensitivity of the microphone when not
loaded by the input impedance of the connected preamplifier (the termination is
described in IEC1094–2). The sensitivity is measured for the individual microphone
at 1000Hz and stated on the microphone’s calibration chart (see section 2.1.2). The
nominal sensitivity is shown in Table 2.1.
2.2.2 Loaded Sensitivity
When loaded by a preamplifier, the sensitivity of the microphone is given by:
(2.1)
where S
C
= overall sensitivity of microphone and preamplifier combination
S
O
= open-circuit sensitivity of microphone
G = voltage gain of microphone and preamplifier combination (in dB)
Nominal open-circuit sensitivity Accepted Deviation
(dB)
mV/Pa dB re 1 V/Pa
31.6 –30
±
2
Table 2.1 Nominal open-circuit sensitivity
S
C
S
O
G
+=