Introduction to Acoustics

The variation of the pressure sensitivity correction
with frequency is shown in Fig. 24.14. At a particular
frequency, if the difference between the standard
pressure and the barometric pressure during calibration
is known, the sensitivity pressure correction can
be obtained from the above equation. Over a pressure
range 94–106 kPa, if (24.42) is used for sensitivity level
pressure correction for a type 4160 microphone, the
maximum deviation between the corrected microphone
sensitivity level and the corresponding measured microphone
sensitivities level is within 0.0085 dB over the
frequency range 250–8000 Hz. For a wider frequency
range of 63–10 000 Hz, the corresponding deviation is
within 0.013 dB. Similar corrections can be obtained for
LS2P (half-inch) microphones by applying the coefficients
[24.46]giveninTable24.6.
Users of condenser microphones, particularly for
very precise measurements such as during aircraft certification
that involves huge resources, should be fully
aware of the fact that the sensitivities of condenser
microphones change with frequencies (frequency response)
and these changes are functions of barometric
pressure ([24.45], Fig. 3). Therefore for two sets of
measurements at different barometric pressures, pressure
corrections, such as with (24.43), can be applied at
each frequency to arrive at more-precise measurements.
For LS2P microphones Brüel and Kjær type 4180
and GRAS-type 40AG, the pressure correction curves
similar to that shown in Fig. 24.14 have been published
[24.46]. With (24.42) their corrections can be
computed from the coefficients shown in Table 24.6.
Alternatively, a similar equation for pressure correction
developed at a later date also based on
measurement [24.47], may be used
δp = a0 + a1x + a2x 2 + ...+ a9x 9 , (24.43)
where δp is the microphone sensitivity pressure correction
(dB/kPa), a0–a9 are the constants listed
in Table 24.6, x = f/ f0 is the frequency normalized
to the resonance frequency f0 of the individual microphone,
and f is the frequency (Hz).
With (24.43), it is necessary to determine the resonant
frequency f0 of the microphones. With the
assumption of a mean value for f0, the numerical values
obtained with (24.43) and(24.42) differ by a few
thousandths of a decibel per kPa.
24.4.6 Temperature Correction
Both the mass and the resistance of the enclosed air
depend on the temperature, while the compliance can
0.000
–0.005
–0.010
–0.015
–0.020
–0.025
–0.030
Microphones and Their Calibration 24.4 Corrections 1037
Pressure coefficient (dB/kPa)
S/N 907055
S/N 907045
S/N 907039
50 100 200 500 1000 2000 5000 10000 20000
Frequency (Hz)
Fig. 24.14 Variation of the slopes of sensitivity correction curves
with frequency for three Brüel and Kjær type 4160 microphones.
The curve is obtained with an empirical equation for the computation
of microphone sensitivity pressure correction. See (24.42). Similar
corrections can be obtained for Brüel and Kjær Type 4180 and
GRAS 40AG microphones with the coefficients shown in Table 24.6
(after [24.48])
be considered independent of temperature. The dependence
on temperature is of secondary when compared
with the pressure dependence. The resulting frequency
dependence of the temperature coefficient is shown
in Fig. 24.15.
In addition to the influence on the enclosed air, temperature
variations also affect the mechanical parts of the
microphone. The main effect will generally be a change
in the tension of the diaphragm and thus a change in its
(dB/K)
0.02
0.01
0.00
–0.01
LS2P
LS1P
–0.02
0.1 0.2 0.5 1 2 f/f0 5 10
Fig. 24.15 General frequency dependence of the part of the
temperature coefficient for LS1P and LS2P microphones
caused by the variation in the impedance of the enclosed
air (ANSI S1.15: 2005, and IEC 61094-2:1992-03)
Part H 24.4