BRUEL & KJAER 4188 - Page 186

Chapter 7 — Low-frequency Pressure-field
1
/
2
" Microphone Type 4193
Equivalent Volume and Calibrator Load Volume
Falcon™ Range of Microphone Products
Microphone Handbook
BE1379– 12
7
−
21
The real and imaginary parts of the equivalent volume shown in Fig.7.19 are in
parallel. They are calculated from a simple R–L–C series model of the microphone
which gives the best overall approximation of the microphone’s diaphragm imped-
ance.
The Models
The following equivalent models are valid at 101.325kPa, 23°C and 50%RH:
Model 1
C = 0.062 x 10
-12
m
5
/N
L = 710kg/m
4
R = 119 x 10
6
Ns/m
5
where C = acoustic diaphragm compliance
L = acoustic diaphragm mass
R = acoustic diaphragm damping resistance
Model 2
V
lf
=8.8mm
3
f
0
=24kHz
Q =0.9
where V
lf
= low-frequency volume
f
0
= diaphragm resonance frequency
Q = quality factor
Fig.7.19 Typical equivalent volume (real and imaginary parts) based on mathematical model of
microphone
100 1k 10k 100k
Frequency (Hz)
– 2
0
4
Volume (mm
3
)
940948e
2
6
8
10
12
– V (Im)
V (Re)