BRUEL & KJAER 4188 - Effect of Ambient Pressure

Chapter 2 — Prepolarized Free-field
1
/
2
" Microphone Type 4188
Effect of Ambient Pressure
Falcon™ Range of Microphone Products
Microphone Handbook
BE1374– 12
2
−
25
2.12 Effect of Ambient Pressure
The microphone’s sensitivity and frequency response are affected by variations in
the ambient pressure. This is due to changes in air stiffness in the cavity behind
the diaphragm, and changes in air mass in the small gap between the diaphragm
and the back plate. The effects are shown in Fig.2.24 to Fig.2.26.
The typical pressure coefficient at 250Hz for Prepolarized Free-field
1
/
2
" Micro-
phone Type 4188 is –0.021dB/kPa, well within the
±
0.03dB/kPa limits required for
Type 1 sound level meters by IEC651.
Fig.2.24 Typical variation in frequency response (normalized at
250Hz) from that at 101.3kPa as a function of change in
ambient pressure
940762e
1k 10k Frequency (Hz)
– 1
0
2
Correction (dB)
1
3
500
50k
– 10kPa
change
– 20kPa
change
– 40kPa
change