Electret transducers for measuring acceleration and structure-borne sound

The number of electret microphones produced every year significantly exceeds that of all other microphone types. This is due to the very good acoustical properties and the simple and low-cost design of these electret air-borne sensors. In contrast, most of the discrete structure-borne sound sensors ( or accelerometers) are based on the piezoelectric effect. In the present work, accelerometers utilizing the electret principle were constructed, built and characterized. These electret accelerometers comprise a metallic seismic mass, covered by an electret film, a ring of a soft cellular polymer, and a metallic backplate which replace membrane, spacer, and back electrode, respectively, of the electret microphone. An adjustable static pressure to the seismic mass is generated by one or two metal springs. The dynamic characterization of the electret accelerometers was carried out in the frequency range from 10 Hz to 8 kHz by using an electrodynamic shaker and an external charge amplifier. Sensors with seismic masses from 8 to 25.5 g were investigated. With these masses typical charge sensitivities from 10 to 40 pC/g, voltage sensitivities from 600 to 2000 mV/g, and resonance frequencies from 3 to 1.5 kHz were measured. The relation between sensitivity, seismic mass and resonance frequency was calculated by a simple analytical model and compared with measured results. Good agreement of experimental and calculated data was found. The present work shows that sensitive, lightweight, and inexpensive electret accelerometers can be built for applications in various fields, such as active noise control and adaptronics.