THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

9.3 Microphones 175Figure 9.1. A cut-away view of an electret microphone showing the principal components.A thin electret polymer foil is suspended over a perforated backplate.does not equal the frequency response of a condenser microphone and are notlikely to withstand temperature extremes very well. An alternate form of the electretmicrophone is created by depositing the electret film onto the stationary plateand using the thin metal foil as a moving diaphragm.Condenser microphones are capacitance-varying devices; each one consisting oftwo electrically charged plates with an air gap between them. The thin diaphragmserves as the plate that deflects under the influence of changes in sound pressure,causing the gap to vary. The resulting change in the capacitance is converted intoan electric signal. Figure 9.2 illustrates the principal components of a condensermicrophone. A capillary tube behind the plates provides the air bleed to provide airpressure equalization with the ambient. These devices have relatively low capacitance,require a polarizing voltage supplied by a preamplifier which also providesFigure 9.2. Principal elements of a condenser-microphone cartridge. (Courtesy of Brüel& Kjær.)