Monday, October 7 6:00 pm 8:00 pm
RICHARD C. HEYSER MEMORIAL LECTURE featuring JAMES E. WEST
The lecture will be followed by a reception hosted by the Technical Council.
The Distinguished Richard C. Heyser Lecturer is professor
Jim West of Avaya Labs
and the John Hopkins University, co-inventor of the electret condenser
microphone. His lecture is titled: MODERN ELECTRET MICROPHONES AND
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 advanced 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 in the early 1960s at Bell Labs.
Others had suggested the use of electrets in transducers (electrical analog of a permanent magnet) in the late 1920s but these and subsequent efforts all suffered from the insufficient stability of wax electrets under normal environmental conditions. Water molecules from atmospheric humidity were the main depolarizing factor in Carnauba and other wax electrets. The first broad application of wax electret microphones was discovered in captured World War II Japanese field equipment. Because of the decay of the polarization of the electret, these microphones had a lifetime of about six-months.
Modern polymer electret transducers can be constructed in various sizes and shapes mainly because of the simplicity of the transducer. All that is needed in the mechanical system is a thin (25microns) charged polymer, a small (irregular) air gap and a back plate. An impedance converter is necessary and is provided by a FET transistor to better match conventional electronic equipment. Applications of electret microphones range from very small hearing aid microphones (a few square millimeters) to very large single element units (20cm diameter) 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 can be constructed. We have constructed a two dimensional 400-element array for Arnold Auditorium at Bell Labs with electret elements that are available for under $1.00 each.
Telephone bandwidth and frequency characteristics have remained constant for the past 30 years while entertainment has brought high fidelity including surround sound into most homes through out the world. People are accustomed to good quality sound and expect it in communication systems. The Internet Protocol (IP) offers the needed bandwidth to 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 required especially for 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.