Electret microphones with backplate electrets

Present electret transducers utilize either an electret diaphragm or an electret coated back electrode. Transducers based on the latter principle allow the freedom to use a variety of materials as the membrane. For example, proper membranes can be placed under high tension resulting in an increase in resonance frequency. Investigations of the average membrane deflection, and thus the sensitivity, of circular electret microphones show that the radiation impedance damps the resonance peaks and shifts them to lower frequencies. The phase shift between the incident sound wave and the ac output voltage is affected in a similar way. The influence of different surrounding gases on the frequency response can be calculated by changing the gas constants. Several gases, for instance helium, neon, CO2, and ethane, have been investigated. Studies of the vibration sensitivity of electret microphones, which is controlled by the mass of the membrane, the mass loading of the surrounding gas, and the sensitivity to airborne sound, are also presented.