Introduction to Acoustics

1036 Part H Engineering Acoustics
Part H 24.4
Table 24.6 Coefficients of the polynomial for calculating the microphone sensitivity pressure corrections using (24.42)or(24.43),
and temperature corrections using (24.44) for specific LS1P and LS2P microphones (ANSI S1.15: 2005)
Coeffi- Pressure correction (24.42) [24.45, 46] Pressure correction (24.43) Temperature correction (24.44)
cients [24.47] [24.47]
Brüel&Kjær Brüel&Kjær G.R.A.S. Brüel & Brüel & Brüel & Brüel &
Kjær Kjær Kjær Kjær
Type 4160 Type 4180 Type 40AG Type 4160 Type 4180 Type 4160 Type 4180
LS1P LS2P LS2P LS1P LS2P LS1P LS2P
a0 –1.625 × 10 −2 –4.446 13 × 10 −3 –3.934 58 × 10 −3 –0.0152 –0.00519 –0.0020 –0.0012
a1 –1.152 × 10 −6 –8.343 14 × 10 −7 –5.687 14 × 10 −7 –0.00584 –0.0304 0.00913 0.00633
a2 1.816 × 10 −9 –3.516 08 × 10 −10 –3.006 41 × 10 −10 0.132 0.5976 –0.245 –0.242
a3 –7.111 × 10 −13 –1.602 78 × 10 −14 –1.439 14 × 10 −14 –0.596 –3.912 1.673 1.656
a4 2.056 × 10 −16 1.763 14.139 –6.058 –6.1833
a5 –2.721 × 10 −20 –2.491 –27.561 11.766 11.81
a6 1.188 × 10 −24 1.581 29.574 –13.11 –12.1366
a7 –0.358 –17.6325 8.5138 6.875
a8 –0.0364 5.4997 –3.0016 –2.0324
a9 0.01894 –0.7017 0.4426 0.2457
(dB/kPa)
0.02
0.01
0.00
–0.01
–0.02
–0.03
LS2P
LS1P
–0.04
0.1 0.2 0.5 1 2 f/f0 5 10
Fig. 24.13 Examples of static pressure coefficient of LS1P
and LS2P microphones relative to the low-frequency value
as a function of the relative frequency f/ f0 (ANSI S1.15:
2005, and IEC 61094-2:1992-03)
at reference conditions to the acoustic impedance at the
relevant static pressure and temperature, respectively.
Both the mass and the compliance of the enclosed air
depend on the static pressure, while the resistance can
be considered independent of static pressure. The static
pressure coefficient generally varies with frequency as
shown in Fig. 24.13. For frequencies higher than about
0.5 f0 ( f0 being the resonance frequency of the microphone),
the frequency variation depends strongly upon
the wave motion in the cavity behind the backplate.
In general, the pressure coefficient depends on constructional
details in the shape of the backplate and
back volume, and the actual values may differ considerably
for two microphones of different manufacture
although the microphones may belong to the same
type, say LS1P. Consequently the pressure coefficients
shown in Fig. 24.13 should not be applied to individual
microphones.
The low-frequency values of the static pressure coefficient
generally lie between −0.01 and −0.02 dB/kPa
for LS1P microphones, and between −0.003 and
−0.008 dB/kPa for LS2P microphones.
For the Brüel and Kjær type 4160 LS1P microphones,
an empirical equation based on precise
measurement of the microphone sensitivity [24.45] at
various static pressures to derive the pressure coefficient,
may be used to correct for microphone sensitivity
variation with static pressure:
∆P =
�
a0 + a1 f + a2 f 2 + a3 f 3
+a4 f 4 + a5 f 5 + a6 f 6�
∆p , (24.42)
where ∆P is the microphone sensitivity level pressure
correction (dB), (to be added to the sensitivity level of
a microphone), a0–a6 are constant coefficients listed
in Table 24.6, f is the frequency (Hz), ∆p = (ps − p0)
is the pressure difference (kPa), ps is the static pressure
during calibration (kPa), and p0 is 101.325 kPa.