The EL2 electret transmitter: Analytical modeling, optimization, and design

We describe here the development of an el2 Electret Transmitter that provides desirable attributes for use in hands-free-answer telephony and future electronic and special-purpose sets. The el2 has lower sensitivity to spurious electromagnetic and mechanical signals than do existing magnetic transmitters. It offers lower dc power consumption, smaller size, and lower intrinsic noise and distortion than the carbon microphone. Formulation of an electro-mechano-acoustic electret model allows parameter optimization, in which side conditions on electrostatic stability and a prescribed transmit frequency response are adhered to. We show that higher sensitivities are possible with larger air film thickness, until the decreasing source capacitance becomes a limiting factor. Multiple diaphragm supports allow decreased film stress with no change in the stable electret charge or sensitivity. We describe theoretically a thermal stabilization procedure that minimizes long-range stress relaxation effects by accelerating viscoelastic changes. Based on film data, we project nominal sensitivity variations within ±1 dB over 20 years of service. In the design, metallized poly(tetrafluoroethylene) electret film is tensioned, supported, and clamped above a selectively metallized stationary electrode forming three cells acoustically and electrically in parallel. A preamplifier completes the subassembly, which is housed in a rectangular aluminum enclosure shielding the transducer. Typical EL2 parameters are −32 dBV/N/m
²
sensitivity at 1 kHz, 1 KΩ output impedance, 3.2 kHz response resonance frequency, and 2 to 16 V required dc supply.