Electret microphones with stiff diaphragms

Electret microphones with stiff plates instead of flexible diaphragms are described. The stiff plate and a backplate, separated by a soft cellular polymer spacer ring, yield a capacitance that is varied by the incoming sound wave; thus a voltage change in the plates is induced. Various such plate microphones were built and characterized. Sensitivities of more than 10 mV/Pa and equivalent noise levels of 23 dB(A) are obtained. An analytical model for the sensitivity of plate microphones was developed. Advantages of the plate microphones are high mechanical robustness, low harmonic distortion, and no risk of membrane collapse.