BRUEL & KJAER 4188 - Sensitivity

Chapter 3 — Prepolarized Free-field
1
/
2
" Microphone Type 4189
Sensitivity
Falcon™ Range of Microphone Products
Microphone Handbook
BE1375– 12
3
−
5
Brüel&Kjær Microphone Viewer must be installed before use (see section 1.3.5).
3.1.4 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
1
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2
" Microphone Type 4189 is very stable over this period
(see section 3.10 to section 3.12). 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.
3.2 Sensitivity
3.2.1 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 251.2Hz and stated on the microphone’s calibration chart (see section 3.1.2) and
data disk (see section 3.1.3). The nominal sensitivity is shown in Table 3.2.
3.2.2 Loaded Sensitivity
When loaded by a preamplifier, the sensitivity of the microphone is given by:
(3.1)
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)
With Microphone Preamplifier Type 2639: G = –0.1dB
With
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" Microphone Preamplifier Type 2669: G = –0.25dB
Nominal open-circuit sensitivity Accepted Deviation
(dB)
mV/Pa dB re 1 V/Pa
50 –26
±
1.5
Table 3.2 Nominal open-circuit sensitivity
S
C
S
O
G
+=