Page 13 - Spring 2009

Fig 17. Ray Stata, Gerhard Sessler and Jim West discuss microphones in handsets. (Source: Analog Devices)
were available at the time. Sessler and West also reflected on the resistance to the technology which now ships in excess of two billion units per year into every segment of the market today. Similarly, Stata reflected on the early days of MEMS accelerometers when no one envisioned that it would become one of the core technologies in cell phones and video games. Companies like Motorola with pressure sensors, Texas Instruments with the digital light processing (DLP) and Analog Devices with accelerometers and gyroscopes were the early pioneers for the technology. Knowles created a major stir when they released the first commercially available MEMS microphones several years ago.
MEMS microphones have had considerable success because they address one of the key weaknesses of electret con-
denser microphones, namely reduced sensitivity through reflow solder tem- peratures. There is continued work in the electret world to create more stable electrets and there has been success reported in the last few years. Sessler commented that he still thinks it is too early to write-off the ECM but agreed that MEMS offers significant potential advantages. Stata stated that he feels that one of the key advantages that MEMS offers is the controllability of the pho- tolithographic process for MEMS creat- ing very stable unit-to-unit perform- ance. He believed that the ability to inte- grate better the underlying electronics
into the package with the transducer is significant. For instance, the inclusion of an analog-to-digital converter and programmable amplifier opens up opportunities to drive fur- ther miniaturization of end-user products while significantly enhancing performance with respect to power supply noise rejection, radio frequency interference (RFI), and electromag- netic interference (EMI) immunity of the digital output sig- nals. All agreed that one major challenge in consumer devices is the continuing drive for higher performance in smaller devices. Jim West commented on the need for microphones to have better performance and more functionality and be able to fit into smaller and smaller products. Jim was especially inter- ested in the use of multiple microphones to reduce back- ground noise and improve audio quality in cell phones and mobile devices.
More than 50 years after the invention of the electret micro- phone the product is still the lowest-cost, highest-volume microphone in use. MEMS microphones are gaining a share in the market addressing some of the new demands of the market not readily met by the electret condenser microphone. It will be very interesting to look back 50 years from now and see what has transpired.AT
References
1 G. M. Sessler and J. E. West, “Self-biased Condenser Microphone with High Capacitance,” J. Acoust. Soc. Am. 34, 1787–1788 (1962).
2 Alexander Graham Bell, U.S. Patent 174465 “Improvement in Telephony” (1876).
3 A. J. Campanella, “Antonio Meucci, The speaking telegraph, and the first telephone,” Acoustics Today 3(2), 37–45 (2007).
4 Thomas A. Edison, U.S. Patent 474230, “Speaking Telegraph” (1877).
5 E. W. Siemens, U.S. Patent 149797, “Improvement in magneto-
electric apparatus” (1874).
6 Edward C. Wente, U.S. Patent 1333744, “Telephone-
Transmitter” (1920).
7 E. Gerlach, U.S. Patent 1557336, “Electrodynamic Loud
Speaking Apparatus” (1925).
8 Harry F. Olson, “A History of High-Quality Studio
Microphones,” J. Audio Eng. Soc. 24(10), 798–807 (1976).
9 Benjamin Bauer, “Uniphase Unidirectional Microphones,” J.
Acoust. Soc. Am. 13, 41–45 (1941).
10 W. Kuhl, G. R. Schodder, F. K. Schroeder, “Condenser transmit-
(Source: Analog Devices) Western Electric 630A microphone (Source: mh acoustics)
THE ORIGIN OF THE MICROPHONE SYMBOL
The Western Electric 630A moving coil microphone was a radical design that was introduced in the early 1930s. The design was a moving coil microphone housed with a “transmitter attachment” which was comprised of a 3 1/4 inch silk screen baffle. The baffle was designed to reduce on-axis acoustic pressure buildup on the relative- ly large microphone diaphragm as well as enhance the response of signals arriving from the rear of the micro- phone. Thus, the baffle acted as a passive acoustic device
1
1 R. N. Marshall, “630A Non Directional Dynamic Microphone and Acoustic Baffle Assembly, “Bell Labs Record (October 1935).
that gave the microphone a nondirectional response. This microphone became known as the “8 ball” micro- phone and its unique shape became the basis for the stan- dard microphone symbol.
Reference
12 Acoustics Today, April 2009
ters and microphones with solid dielectric for airborne ultrason-