BRUEL & KJAER 4131 - Introduction; Purpose of the Quarter-Inch; Principle of the

0. Introduction
Purpose of the Quarter-inch Microphones.
The B & K Quarter-inch Condenser Microphones are designed for precision
sound pressure measurements. Because of their small dimensions, they leave
the sound fields undisturbed at audible frequencies and their first res · onance
frequency is far above the audible range. They have consequently very flat
frequency responses and excellent responses to sharp pulses as illustrated m
the Appendix. In addition, they allow measurements without distortion of very
high sound pressure levels (up to 180 dB) as missile exhaust noise etc. On
the other hand they are not suited for low sound level measurements (below
70 dB, see Appendix Fig. A.4).
Their most outstanding feature is excellent long-term stability under a great
range of environmental conditions and especially their insensitivity to
temperature variations. They are therefore well suited for field measure-
ments, though the accuracy of calibration matches laboratory standard re-
quirements.
Principle of the Condenser Microphone.
A condenser microphone consists essentially of a thin metallic diaphragm
mounted in close proximity to a rigid back plate . Diaphragm and back plate
are electrically insulated from each other and constitute the electrodes of
a capacitor. See Fig. 0.1. Housing and insulator form with the diaphragm
Insulator
Back
Housing
Hole for
static pressure equalization
Fig. 0.1. Schematic construction of a condenser microphone cartridge.