Heyser Memorial Lecture
AES 113th Convention
Los Angeles Convention Center, Los Angeles, CA
Monday, October 7, 6:00 pm
MODERN ELECTRET MICROPHONES AND THEIR APPLICATIONS
Jim West
Abstract:
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 1960’s at Bell Labs.
Others had suggested the use of electrets in transducers (electrical analog of a permanent magnet) in the late 1920’s 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.
James E. West, a Bell Laboratories Fellow and a member of Acoustics and Speech Research Department at Lucent Technologies, where he specialized in electro acoustics, physical acoustics and architectural acoustics. (Retired July 2002) He is now Research Scientist in The Multimedia Technologies Research Lab of Avaya and plans to join the Department of Electrical and Computer Engineering at Johns Hopkins University in September 2002.
His pioneering research on charge storage and transport in polymers (the electrical analogy of a permanent magnet) led to the development of electret transducers for sound recording and voice communication. Almost 90% of all microphones built today are based on the principles first published by West in the early 1960s. This simple but rugged transducer is the heart of most new telephones manufactured by Lucent and other producers of communication equipment. West holds 47 U.S. and over 200 foreign patents on various microphones and techniques for making polymer electrets.
He was inducted into The National Inventors Hall of Fame in 1999 for the invention of the electret microphone.
He has authored more than 100 papers and has contributed to several books on acoustics, solid-state physics and material science.
West is a member of the National Academy of Engineering; a Fellow, and past President of the Acoustical Society of America (1998-99), and a Fellow of the IEEE.
West is a member of the Board of Directors of The National Inventors Hall of Fame, a member of the National Academy of Engineeringπs Committee on Diversity in the Engineering Workforce, Chairman of the First Pan American Meeting of the Acoustical Society of America, and a member of the Scientific Advisory Committee of The International Symposium on Electrets.
West is the recipient of the Callinan Award (1970), sponsored by the Electrochemical Society of America, the Senior Award (1970), sponsored by the IEEE Group on Acoustics, the Lewis Howard Latimer Light Switch and Socket Award (1989), sponsored by the National Patent Law Association, the George R. Stibitz Trophy, sponsored by the Third Annual AT&T Patent Award (1993), New Jersey Inventor of the Year for 1995, The Acoustical Society of America's Silver Medal in Engineering Acoustics (1995), an honorary Doctor of Science degree from New Jersey Institute of Technology (1997), the Golden Torch Award (1998) sponsored by the National Society of Black Engineers, the Industrial Research Institutes 1998 Achievement Award, and The Ronald H. Brown American Innovator Award (1999). The Acoustical Society of America has chosen one of his early papers on Electret Microphones as a Benchmark publication.
