The EL2 Electret Transmitter: Analytical Modeling, Optimization, and Design

Electromagnetic transducers based on the modulation of a biasing magnetic field in an air film appeared as microphones (often referred to as transmitters in telephony), receivers, and loudspeakers very early in the development of telephony.1,2 Today, this mechanism is employed in general-purpose u- and L-type telephone receivers in the Bell System. A smaller electromagnetic transducer, the AFI, was used as the electroacoustic transmitter in speakerphone modules until mid1978. However, because of its operating principle, in recent years it was recognized to be inherently susceptible to an increasing incidence of spurious electromagnetic signals at customer locations. At the same time, interest was growing in a new telephone transmitter for electronic and special-purpose residential sets as well as for hands-free-answer features in business sets. The former residential applications generally require lower dc power consumption and smaller size than are typical of the variable-resistance granular carbon transmitter3 such as the Ttype used in general-purpose sets. Moreover, the carbon transmitter, being dependent upon periodic mechanical agitation, is not suitable as the stationary transmitter in hands-free-answer applications. These events encouraged Bell Laboratories development efforts4 and the subsequent design of the EL2 Electret Transmitter. Compared to the AFI, the electret promised a greatly reduced sensitivity to electromagnetic as well as mechanical vibration interference.