Page 5 - Spring 2009

A HISTORY OF CONSUMER MICROPHONES: THE ELECTRET CONDENSER MICROPHONE MEETS MICRO-ELECTRO-MECHANICAL-SYSTEMS
GaryW.Elko
mh acoustics Summit, New Jersey 07901
and
Kieran P. Harney
Analog Devices, Micromachined Products Division Cambridge, Massachusetts, 02139
“Sessler and West were on the trail when they metalized charged Teflon with a thin layer of aluminum and created the modern electret microphone.”
Introduction
On a bright fall morning last
October there was a historic
meeting among three industry
stalwarts that was of particular interest
to the acoustics community. Gerhard
Sessler and James West returned to New
Jersey, the home state of Bell Labs where
they both started their professional
careers, to be inducted into the “New
Jersey Inventors’ Hall of Fame.” Ray
Stata, founder of Analog Devices and a
pioneer of Micro-Elecro-Mechanical-
Systems (MEMS) devices, was passing through on business. The three got together at the beautiful Reeves-Reed arbore- tum in Summit, New Jersey for a discussion on the evolu- tion of consumer microphones towards MEMS (see Fig.1).
Drs. Sessler and West invented the Electret Condenser Microphone (ECM) at Bell Labs in the early 1960’s.1 The low cost and small size of the ECM has enabled the production of modern consumer devices such as cell phones, headsets, and video cameras. As a result, in excess of 2 billion ECM micro- phones shipped in 2008. In 1965, Ray Stata founded Analog Devices, a world leader in semiconductor converter and amplifier microchips today. In the early 1990’s Ray Stata played a key role in evangelizing MEMS technology, enabling
the manufacture of small, low cost accelerometers and gyroscopes for auto- motive and consumer markets. More recently, he, with his company, devel- oped world-leading MEMS microphone technology. In the 1980’s Dr. Sessler did much of the early academic research in MEMS microphones at the University of Darmstadt, Germany. Dr. Sessler is rec- ognized as one of the earliest advocates for research in the field of silicon micro- phones.
Amid the beauty of the autumnal foliage of the Reeves-Reed Arboretum, Sessler, West, and Stata sat down to discuss the evolution of microphones and
consider what the future might hold.
Sessler, West and electret condenser microphones (ECMs)
Gerhard Sessler and Jim West both started their profes- sional careers at AT&T Bell Laboratories in the 1950’s. Jim arrived first in 1955 as a summer intern from Temple University. Gerhard arrived soon after in 1959 as a fresh Ph.D. from the famous University of Göttingen Laboratory of Professor Erwin Meyer. Both began working in the already well-known Acoustics Research Department, a department that had a deep and rich history in contributions to the field of acoustics and to the founding of the Acoustical Society of America. This historical link is so important that we felt that some of the highlights should be mentioned.
A brief history of the evolution of ECMs
Although the word microphone can be found in refer- ences back to the late 1600’s, it was not until the 1870’s that Alexander Graham Bell realized that time varying electrical signals were a direct analog of acoustic pressure variations
2
that could be used to transmit speech on electrical wires. With this novel insight, Bell began working on the transduc- ers that would be required for telephony. A moving-armature microphone and receiver were constructed (albeit with some help from Joseph Henry) and these transducers formed the basis of Bell’s patent of the telephone in 1876. Figure 2 shows the now famous “Fig. 7” from Bell’s 1876 patent entitled “Improvement in Telephony,”2 where the moving armature
Fig. 1. Kieran Harney, Gary Elko, Jim West, Gerhard Sessler, Ray Stata. (Source: Analog Devices)
4 Acoustics Today, April 2009