Optimization of a Ridge Backplate for Electret Microphones

The desired properties for most microphones are high sensitivity, flat frequency response, insensitivity of response to vibration and temperature fluctuations, and long life. The life of an electret is particularly important since the self polarization has a natural decay. It is possible, however, to design an electret microphone that has little variation in sensitivity even though the charge density of the trapped charges diminishes to 14 of its original value. This is possible by controlling every contributing parameter. The parameter that is hardest to control is the tension of the polymer diaphragm and the problem has been resolved by choosing a rectangular backplate with a diaphragm tensioned across parallel ridges. Stray capacitance and electrical load are among the parameters that affect the stability of the microphone, especially when the microphone becomes small as for a lightweight operator's headset. The results of computations and experiments are given.