Modern Electret Microphones and Their Applications – James West (The Johns Hopkins University)
It is well known that condenser microphones are the transducer of choice when accuracy, stability, frequency characteristics, dynamic range, and phase are important. But conventional condenser microphones require critical and costly construction as well as the need for a high DC bias for linearity. These disadvantages ruled out practical microphone designs such as multi- element arrays and the use of linear microphones in telephony. The combination of our discovery of stable charge storage in thin polymers and the need for improved linearity in communications encouraged the development of modern electret microphones. Modern polymer electret transducers can be constructed in various sizes and shapes mainly because it is a simple inexpensive transducer. Applications of electret microphones range from very small hearing aid microphones to very large single element units for underwater and airborne reception of very low frequencies. Because the frequency and phase response of electret microphones are relatively constant from unit to unit, multiple element two-dimensional arrays have been constructed using 400 electret elements that cost about $1.00 each. The Internet Protocol (IP) offers the bandwidth needed to further improve audio quality for telephony, but this will require broadband microphones and loudspeakers to provide customers with voice presence and clarity. Directional microphones for both hand-held and hands free modes are necessary to improve signal-to-noise ratios and to enable automatic speech recognition. Arrays with dynamic beam forming properties are also necessary for large conference rooms. Signal processing has made possible stereo acoustic echo cancellers and many other signal enhancements that improve audio quality. I will discuss some of the current work on broadband communications at Avaya Labs.