The electret microphone

A new kind of electrostatic microphone which uses a foil electret between the electrodes Is described. The foil electret Is a solid dielectric material, metallized on one side, which has been prepolarlzed In an electrostatic field at an elevated temperature. The use of an electret eliminates the need of an external de blas. The capacitance of the electret microphone (about 60 pF cm²) Is considerably higher than that of air gap systems. Consequently, for the same frequency response, an amplifier with lower Input Impedance can be used for the electret microphone. The sensitivity of the microphone depends largely on the surface charge of the electret. Sensitivities of typical systems are between −50 and −60 dBV for a sound pressure level of 1 ubar. The free field frequency response Is within ±3 dB from 50 to 15000 cps If the microphone Is terminated by 5 MΩ. The sensitivity of the electret microphone has been measured for a period of more than a year. Over this period, the sensitivity has not changed more than ±1 dB.