IEEE SIGNAL PROCESSING MAGAZINE [102] SEPTEMBER 2010 1053-5888/10/$26.00©2010IEEE

James L. Flanagan[dsp HISTORY]

Digital Object Identifier 10.1109/MSP.2010.937499

This column is from a sympo-sium in tribute to Gerhard Sessler and James West on the occasion of their receiv-ing medals of the Franklin

Institute, Philadelphia, 29 April 2010. I would like to talk about one person’s perception of how the practical electret microphone was created and how it became established. Today, we use them as a commodity, and they appear in near-ly every device that requires changing

aerial acoustic waves into electrical form. The initial incentive was telephony.

GENESISTelephony was conceived as conversion of the sound pressure variations in front of a talker’s mouth into an electrical

Our guest in this column is Dr. James L. Flanagan. Dr. Flanagan holds the doctor of science degree in electrical engineering from the Massachusetts Institute of Technology (MIT), the mas-ter of science degree from MIT, and the bachelor of science degree from Mississippi State University. Dr. Flanagan is current-ly Professor Emeritus at Rutgers University. He was director of Rutgers’ Center for Advanced Information Processing and Board of Governors Professor of Electrical and Computer Engineering. He also was Rutgers University’s vice president for research until his retirement in 2005. Dr. Flanagan spent 33 years at Bell Laboratories before joining Rutgers University. At Bell Labs, he led the Acoustics Research Department and later the Information Principles Research Laboratory. Over the course of his career, Dr. Flanagan has had inventions and contributions to the signal processing field in several areas, including digital speech coding, speech synthesis, auditory information process-ing, array microphones, and acoustic digital-to-analog transduc-ers. Most notably, many of his achievements were reduced to practice with an impact on our current daily lives including speech coding in MP3 and speech recognition. Dr. Flanagan has published approximately 200 technical papers in scientific jour-nals. He is the author of a research text Speech Analysis, Synthesis and Perception (Springer Verlag), which has appeared in five printings and two editions and has been translated in Russian. He holds 50 U.S. patents.

Dr. Flanagan is an IEEE Life Fellow, a long-time member of the IEEE Signal Processing Society, which he served as president in the earlier, formative stages. Among his awards are the IEEE Medal of Honor (2005), and the U.S. National Medal of Science (1996), presented at the White House by the president of the United States. A special pride is the IEEE Signal Processing Society’s creation and sponsorship of the IEEE James L. Flanagan Speech and Audio Processing Technical Field Award. Dr. Flanagan is a member of the National Academy of Engineering and of the National Academy of Sciences.

In the past, Dr. Flanagan has enjoyed deep-sea fishing, swimming, sailing, hiking, and flying as an instrument- rated

pilot. He currently lives in New Jersey with his wife, Mildred, and they have three sons, all married and with families.

On 29 April 2010, the Franklin Institute held its ceremony in Philadelphia, Pennsylvania, to honor Gerhard Sessler and James West with Franklin Institute medals for their inven-tion of the practical electret microphone. The day before the ceremony, a symposium took place at Temple University in which Dr. Flanagan summarized some of the history of the electret microphone. In his talk, Dr. Flanagan shared his perspective on invention of the device that is a commodity nowadays and how the initial incentive was telephony.

I am certain that you, our readers, will appreciate Dr. Flanagan’s organizing his talk as a column in our magazine. It is notable to mention that Dr. Flanagan is the one who initiated the idea of converting his talk into a column, because he wanted to share insights into the invention pro-cess of a very popular device with all of the signal process-ing community. While I was reading the first draft of the column, not only did the column capture my full attention but I also kept thinking of the model a signal processing innovator, Dr. Flanagan, is setting to the rest of us, espe-cially the community’s younger generation. This passion, integrity, and care for high-quality publications are assets that we all work hard to maintain and improve upon. The final sentence in the column suffices to describe the values such a group of innovators maintained: “In truly great measure, we owe these global advances in sound-to-electri-cal transduction to the initiative, diligence, and ingenuity of Gerhard Sessler and James West.” I would like to take this moment to thank Dr. Flanagan for his contributions to our columns.

We have arranged for the audio demonstration that accompanies this column to be available at http://www. signalprocessingsociety.org/publications/periodicals/spm/ columns-resources/.

Ghassan AlRegib

EDITOR’S INTRODUCTION

A Singular Advance in Conversion of Acoustic Signals to Electrical Form: The Electret Microphone

IEEE SIGNAL PROCESSING MAGAZINE [103] SEPTEMBER 2010

facsimile, which could be transmitted over distance by conducting wires. In the United States, Alexander Bell and his contemporaries strived for practical transducers, ranging through magnetic and chemical contrivances. But at that early time, it remained for Thomas Edison, in 1877, to devise the most attrac-tive method—the carbon button micro-phone (Figure 1).

CARBON BUTTON MICROPHONEThe carbon microphone employed a sus-pended, lightweight diaphragm mechan-ically coupled to a plunger in a miniature container of carbon granules (Edison favored “lamp black,” but later uses included anthracite particles). The diaphragm, exposed to the sound pressure, could vibrate and alter-nately compress and relax the granules, correspondingly chang-ing its electrical resistance [1], [3]. A bias current, typically of order 50 mA and supplied by a battery, flowed through the gran-ules and a fixed series resistor. Variations in the resistance of the granules, evoked by sound pres-sure, developed corresponding voltage variations across the fixed resistor, to provide an electrical output. Typical values of the car-bon resistance were the order of 100 V, and the battery 16 V (later supplied from the central office over the transmission wires), pro-ducing an output signal in the range of 1 V. Hence, the acoustic-to-electrical conversion was accomplished with great power gain. However, the massive mechanical coupling seriously restricted the frequency response, and the carbon granules intro-duced non linearity, intermodula-tion distortion (as much as 10%), and s igni f i cant se l f -no ise . Nevertheless, in the absence of electronic amplification, the great power gain of this device and its robustness overrode the negatives, and the carbon button microphone served telephony for nearly 100 years.

CONDENSER MICROPHONELater, expanding interests in audio and radio stimulated further research, resulting in high-quality conversion but typically expensive and fragile. An exqui-site example is the air condenser micro-phone of Edward Wente (1917) [2], [3]. In this device, the thin metal diaphragm was mounted with small air gap near a conducting back plate (Figure 2). This formed an electrical capacitor (of order 50 m-m F) that could be charged to a high potential (typically 200 V) through a very high resistor (about 20 MV). The charge deposited on the capacitor is the product of its capacitance and the volt-age appearing across it (and remains

sensibly constant because of the long R-C time constant). Vibration of the dia-phragm in response to sound pressure therefore varied the capacitance, caus-ing a corresponding voltage variation across the large series resistor.

By properly selecting the diaphragm mass, its compliance and that of the closed back cavity, and the system damp-ing, the microphone could be made to provide great linearity and flat frequency response over the entire audio range. To a limited extent, trades could be made between frequency range and sensitivity (Figure 3) [3]. But the microphone was delicate and costly to manufacture, and its sensitivity (or output signal) was in

the mV range. Further, voltage of potentially harmful level was required for bias, and the low sen-sitivity required careful electronic amplification. Nevertheless, this microphone was manufactured as the Western Electric 640AA (Figure 4) and used for special high-quality applications [4]. It was so carefully and precisely implemented that it is still used as a secondary standard for acous-tic measurement. My recollection is that the price was about US$500. And while it is no longer available, other highly capable manufacturers now offer compa-rable products.

ELECTRET MICROPHONEThe advent of digital communica-tion in the 1960s, along with the promises of digital voice storage and even human-computer inter-action by voice, sharpened the need for high-quality, low-cost audio conversion. At AT&T Bell Laboratories, Gerhard Sessler and James West sensed these emerging needs and initiated research to preserve the high quality of condenser elements, while seeking ways to lower costs, increase robustness, and eliminate the use of high-voltage external bias. Their result was the electret microphone—where-in, a solid dielectric film is given

[FIG1] Principle of the carbon button microphone [1], [3].

Acoustical

Signal

Carbon Granules

Electrical

Potential

Current BiasDiaphragm

[FIG2] Principle of the air dielectric microphone [2], [3].

Air Gap

Fixed Back-Plate

Conductor

Electrical

Signal

Bias PotentialConducting

Diaphragm

Acoustical

Signal

[FIG3] Frequency responses for the air dielectric microphone, illustrating exchange between frequency range and sensitivity [3].

–30

–40

–50

–60

–701 10 100 1,000 10,000

Frequency (Hz)

Am

plit

ude

Response (

dB

re 1

V/µ

bar)

100,000

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[dsp HISTORY] continued

a permanent electrostatic charge, met-alized on one side, and arranged in close proximity to a conducting back plate (Figure 5). In effect, they produced a condenser device with permanently charged dielectric, eliminating the need for external bias [5].

The dielectric film is typically a poly-mer, such as Teflon (0.001 in or 25 mmthick). It is given a permanent, embedded charge, either by charge injection from a high-energy electron beam, or by ion deposition from a corona discharge. (Earlier electret methods utilized melting the dielectric and exposing it to a high electrostatic field, order of 30 kV, while allowing it to resolidify.) The resulting electret transducer exhibits sensitivity similar to that of an externally biased air dielectric condenser microphone, i.e., in the mV range. Like the condenser, it requires a high-impedance load and immediate amplification to resist col-lected noise. The back plate of the electret is a convenient mount for an integrated field-effect transistor (FET) preamplifier. And, in alternative construction, posi-tioning the charged dielectric on the back plate rather than on the front diaphragm allows further reduction of its mass.

Sessler and West conducted extensive exploration of materials and charging methods. In this, they benefited substan-tively from periodic collaborations with Dr. Bernhard Gross, an Austrian special-ist in electrostatics, residing in Sao Paulo, Brazil. This work paved the way for the rest of the technical world to embrace their invention. And, today, lit-erally billions of electret microphones are found in devices such as telephones, audio recorders, camcorders, broadcast and public address systems, cell phones, and, in miniature form, in hearing aids. (I count the number of electret micro-phone just in my home—telephones, computers, cameras, cell phones, tape recorders, and even hands-free car phone. I get a total of 18!)

IMPLEMENTATIONSThe initial, overarching incentive for development of the electret microphone was as a replacement for the carbon microphone in the telephone handset.

[FIG4] Commercial air dielectric condenser microphone, W.E.640AA [4].

Pressure-Equalizing Leaks

Slotted Back Plate

Positive Terminal

Lock NutInsulator Clamping Ring

Glass InsulatorKey

Diaphragm Tension

Adjustment and

Back-Plate SupportDiaphragm

Grid

Permanently

Charged DielectricAir Gap

Acoustical Signal

Metalized Foil

Electrical

Signal

Fixed Back-Plate

Conductor

tricAir Gap

Foil

Fixed Back

Conductor

[FIG5] Principle of electret microphone with permanently charged solid dielectric.

[FIG6] Experimental fabrication of electret to replace the telephone carbon microphone [6].

Air Layer

Metal Back Plate

Metal Layer

Electret Foil

Spring Contact

Electrical

Insulation

Air Cavity

Metal Case

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And, some of the first experimental fabri-cations were for this transducer, then designated Western Electric T-1 (Figure 6). To my recollection, these experimen-tal devices were all hand fabricated in the research area and used for demonstra-tion and for trying to overcome the tra-ditional inertia of a large organization and convince the Western Electric engi-neers that the electret was a winner [6], [7]. This was not easy and required about seven years of determined passion and effort by Sessler and West.

I had the opportunity, in a small way, to help with acceptance of the new device: Our Bell Labs president at the time was Ian Ross, and his office was in the beautiful laboratory in Holmdel—the last building designed by the famous Swedish architect, Eero Saarinen. During one of Ross’s absences, I went to his office and told his secretary I wanted to improve his telephone. She watched me suspiciously while I disassembled his desk handset and installed the experi-mental electret. I believe she thought I was bugging his phone!

Later, upon his return, I called Dr. Ross on some pretext. And, as we ended the conversation, I remarked on the unusually high quality of his voice trans-mission. He asked why? I said “I think it’s your handset microphone.” It seemed he immediately took his handset apart and found the electret. I trust he got good comments on other calls. [There are various ways for overcoming inertia in a large corporation. But, you’d better be right! (Another instance of overcom-ing inertia was getting the * and # keys added to the Touch Tone dial, but this is a separate story.)]

My recollection is that the then-pop-ular Model 500 desk set cost about US$17 and the carbon T-1 transmitter (amortized over almost a century of refinement) cost US$0.98, a relatively low-cost target. Eventually, the promise of higher quality and lower cost was compelling, and Western committed to some initial development (still with a good bit of hand fabrication). This resulted in the first Western Electric commercial electret elements known as EL-2 and EL-3, both of which included

integrated FET preamp/impedance con-verters on their back plates.

Meanwhile, others were sensitive to the advantages of electrets and to their marketing opportunities. This was espe-cially true in Japan, where the Sony company and others took up interest. Notably, a Matsushita (Panasonic) fac-tory was established, having absolutely exceptional capabilities for automated fabrication. On one occasion Vice President Yasuhiro Riko took me to his factory outside of Tokyo. He proudly showed me the component materials being loaded into magazines (or canis-ters) on one side of the building, and, totally untouched by human hand, the packaged, tested, and calibrated micro-phones issuing from the other side.

Also, the Primo company in Japan and the Knowles company in the United States joined the commercial efforts. In particular, the Knowles work included a miniature unit for hearing aids. (Figure 7, display circa 1990 by Robert Kubli). The left column from top to bottom shows the hand fabricated T-1, the EL-2, and the EL-3. The right column shows the Primo cardioid, the Primo omni-direc-tional, the Panasonic gradient, the Knowles omni, and the miniature Knowles hearing aid microphone. One result of these diverse efforts was a dramatic reduction in cost for a high-quality electret micro-phone, typically US$0.25 in vol-ume, including the integrated preamplifier.

Additionally, these successes, occurring in the early days of inte-grated electronics, stimulated important interest in microelec-tromechanical systems (MEMS) implementations, which could include electret transducers fabri-cated on silicon chips. Pioneering advances in this sector subse-quently were made in Germany, and by the Analog Devices com-pany and the Agere company in the United States, among others. At the bottom of Figure 7 is a magnifying glass over a one-cent coin, on the right edge of which are two integrated electrets pro-

duced by Prof. Sessler at the Technical University of Darmstadt [8]. Research in the MEMS area continues apace, as does the study of piezo- and ferro-electric films [9]–[11].

APPLICATIONWhile today’s applications of electret microphones are vast and diverse, one early research application has some unique features. It is rendered practical by low-cost, high-quality electret trans-ducers and by economical computing

[FIG7] Display, circa 1990, of different implementations of electret microphones. (Courtesy R.A. Kubli.)

[FIG8] Microphone array of 401 electret elements. The array is 3 m 3 3 m, and designed with three harmonically nested subarrays to provide constant beam width over the telephone bandwidth. Pointing direction is controlled by signal processing hardware and a desk top computer [3], [12], [13].

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and digital signal processing. This is an autodirective array for sound capture in large meeting spaces, or auditoria [12], [13].

Often in large group meetings held in large enclosures, a person in the audience wishes to speak so all can hear, or question-and-an-swer sessions are desired between audience and speaker on a stage or podium. Further, teleconfer-encing for large distant groups aspires to provide an acoustic environment approximating that as though all participants were seated in the same room. In all cases, sound reinforcement is nec-essary, and in large rooms rever-beration is a serious enemy if the talker-to-microphone distance is significant. Traditionally, this issue is met by passing around a close-talking microphone to those wishing to speak, or where possible, providing personal wireless microphones. Frequently, these are lost, forgotten after use, or inadver-tently taken home. The inexpensive, high-quality electret, used in large num-bers, offers the opportunity for steerable, spatially selective arrays.

An ambitious experimental system produced at Bell Labs employed 401 electrets in a planar rectangular array, approximately 3 m on a side (Figure 8). To maintain beam-width nearly inde-pendent of frequency, the microphones were arranged in three nested subar-rays, each spanning an octave in fre-quency. Beam width of about plus/minus 10° was realized after Hamming shading. For delay-sum beamforming, each electret was given a bucket-brigade chip, whose delay value could be set by a voltage computed for a given pointing direction. A per-sonal computer was dedicated to control the array [3], [13].

In Figure 8, Dr. Gary Elko stands beside the 3 m 3 3 m array, which brings to mind a story that I believe is originally attributed to him. Prior to instal-lation on the ceiling in Arnold Auditorium at Bell Labs, Gary had

the array set on the stage for testing. While behind the array, and with ear-phones on, he began to hear a man and a woman in quite intimate conversation. It being the noon hour, a girl and her boy-friend, seeking privacy, had apparently come into the empty auditorium, and they were seated in the very back row. Gary came out from behind the array and called out to the pair: “Hey, do you know what this is? It’s a very big micro-phone.” The private conversation promptly ended, and the young couple rapidly departed.

Additionally, the array was provided hardware sufficient for forming two beams simultaneously (Figure 9). One beam was programmed to constantly

scan the conference space and to detect the direction of the domi-nant sound energy (i.e., the active talker). When a sound source was located, the second beam was pointed to that direction and remained there until instructed by the scanning beam to move to a new direction.

This system was installed over the stage area of Bell Labs Arnold Auditorium (Figure 10), a space seating about 400 persons, and having a reverberation time of about 1 s [14]. It was utilized ef-fectively both for group tele-conferencing and for sound re inforcement. It well demon-strated the value of inexpensive, autodirective microphone arrays for good quality sound capture in

large, reverberant enclosures. (In the 1 min audio demonstration linked to Figure 10, there are two receivers to be compared: a single omni at the lectern on stage, and the 401 array above on the ceiling. Passage 1: a man talking ap-proximately three-fourths way to back of auditorium, and received only by the lectern omni. Note the reverberation. Passage 2: the same man received by the auto-directive array. Note the less-ened reverberant quality. Passage 3: a man talking along with a simultaneous random noise source at about 45°, and all received by the lectern omni. Note the interference. Passage 4: the same condition as Passage 3, but received by the overhead array. Note the lessened

noise interference.) These capa-bilities, along with advances in signal processing, have spawned other efforts [15], [16].

EPILOGUEThe electret has indeed come a long way since the research of the early 1960s. In truly great mea-sure, we owe these global advances in sound-to-electrical transduction to the initiative, diligence, and ingenuity of Gerhard Sessler and James West.

[FIG9] Beam patterns of the 401 array for three pointing directions. The sound source location scanning beam and the signal capture beam have similar characteristics.

20 10 0 0 10 20

0°

–0°

30°

–30°30°

Bruel and Kjaer330°

[FIG10] The large array installed overhead in the Bell Labs Arnold Auditorium, Murray Hill, New Jersey [14]. (continued on page 116)

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[social SCIENCES] continued

categorized by sociologist as micrody-namics, in contrast to the mesodynam-ics represents the organizational or institutional factors and the macrody-namics that drives our society and cul-ture. The computational study of microdynamics enables a bottom-up approach to sociological research, where microbehaviors are used to define large-scale behaviors (e.g., organizational behavior analysis based on audio micro-dynamics [7]). As important is the top-down influence of society and culture on individual and interpersonal dynam-ics. The joint analysis of micro-, meso-, and macrodynamics will enable a better understanding of cultural differences in human communicative behaviors.

ACKNOWLEDGMENTCode, data, and papers related to this work are available at http://projects.ict.usc.edu/multicomp/.

AUTHORLouis-Philippe Morency (lmorency@ict.usc.edu) is currently a research assis-tant professor at the University of Southern California (USC) and director of the Multimodal Communication and Computation Laboratory at USC In -stitute for Creative Technologies. His main research interests include multi-modal signal processing, machine learn-ing, computer vision, and social psychology. He received many awards for his work on human communication dynamics including three best paper awards in 2008 (at various IEEE and ACM conferences). He was recently selected by IEEE Intelligent Systems as one of the “Ten to Watch” for the future of AI research.

REFERENCES[1] J. B. Bavelas, L. Coates, and T. Johnson, “Listen-ers as co-narrators,” J. Personality Social Psychol., vol. 79, no. 6, pp. 941–952, 2000.

[2] J. DeVito, The Interpersonal Communication Book, 12th ed. Boston, MA: Allyn & Bacon, 2008.

[3] D. McNeill, Hand and Mind: What Gestures Reveal About Thought. Chicago, IL: Univ. of Chicago Press, 1996.

[4] L.-P. Morency, A. Quattoni, and T. Darrell, “Latent-dynamic discriminative models for con-tinuous gesture recognition,” in Proc. IEEE Conf. Computer Vision and Pattern Recognition, June 2007.

[5] L.-P. Morency, C. Sidner, C. Lee, and T. Darrell, “Head gestures for perceptual interfaces: The role of context in improving recognition,” Artif. Intell., vol. 171, no. 8-9, pp. 568–585, June 2007.

[6] L.-P. Morency, I. de Kok, and J. Gratch, “A proba-bilistic multimodal approach for predicting listener backchannels,” J. Autonom. Agents Multi-Agent Syst., vol. 20, no. 1, pp. 70–84, Jan. 2010.

[7] A. Pentland, “Social dynamics: Signals and behav-ior,” in Proc. IEEE Int. Conf. Developmental Learn-ing, San Diego, CA, Oct. 2004.

[8] N. Ward and W. Tsukahara, “Prosodic features which cue back-channel responses in English and Japanese,” J. Pragmat., vol. 23, pp. 1177–1207, 2000.

[9] P. Watzlawick, J. B. Bavelas, and D. D. Jackson, Pragmatics of Human Communication A Study of Interactional Patterns, Pathologies, and Para-doxes. Norton: New York, 1967.

[10] V. H. Yngve, “On getting a word in edgewise,” in Proc. 6th Regional Meeting Chicago Linguistic Society, 1970, pp. 567–577.

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[dsp HISTORY] continued from page 106

ACKNOWLEDGMENTSI am indebted to the following sources for their permission to reproduce figures drawn from my work and related work for this “History” column: Taylor and Francis Group, LLC for Figures 1, 2, 3, and 8, as referenced in [3]; Acoustical Society of America for Figure 4, as referenced in [4]; Alcatel-Lucent USA Inc. for Figures 6, 8, 9, and 10, as referenced in [3], [6], [13], and [14]; and Deutscher Apotheker Verlag for Figure 10, as referenced in [14]. I thank Robert A. Kubli for the display of commer-cial electret microphones (Figure 7). I also thank Ann Marie Flanagan for preparing all the figures.

AUTHORJames L. Flanagan (jlf@caip.rutgers.edu) is a Professor Emeritus at Rutgers University.

REFERENCES[1] M. D. Fagen, Ed., A History of Science and Engineering in the Bell System, vol. 1, The Early Years (1875–1925). Murray Hill, NJ: AT&T Bell Laboratories, 1984, p. 68.

[2] E. C. Wente, “A condenser transmitter as a uni-formly sensitive instrument for the absolute mea-surement of sound intensity,” Phys. Rev., vol. 10, pp. 39–63, 1917.

[3] J. L. Flanagan, “Acoustics in communications,” in Froel ich/Kent Encyclopedia of Tele-communications, vol. 1, New York: Marcel Dekker, 1991, pp. 67–96.

[4] L. L. Beranek, Acoustical Measurements Revised Edition. New York: Acoustical Society of America, 1988.

[5] G. M. Sessler and J. E. West, “Self-biased con-denser microphone with high capacitance,” J. Acoust. Soc. Amer., vol. 34, no. 11, pp. 1787–1788, 1962.

[6] J. L. Flanagan, “Communication acoustics,” in A History of Science and Engineering in the Bell System: Communication Sciences (1925–1980), S. Millman, Ed. Murray Hill, NJ: AT&T Bell Laboratories, 1984, ch. 2.

[7] G. M. Sessler and J. E. West, “The foil electret microphone,” Bell Labs Rec., vol. 47, no. 7, pp. 244-248, 1969.

[8] D. Hohm and G. M. Sessler, “An integrated sili-con-electret condenser microphone,” in Proc. 11th Int. Congr. Acoustics, 1983, vol. 6, pp. 29–32.

[9] G. M. Sessler, “Silicon microphones,” J. Audio Eng. Soc., vol. 44, no. 1–2, pp. 16–21,1996.

[10] G. W. Elko, F. Pardo, D. Lopez, D. Bishop, and P. Gammel, “Surface-micromachined MEMS mi-crophone,” in Proc. 115th Convention Audio Engi-neering Society, New York, 2003.

[11] G. W. Elko and K. P. Harney, “A history of consumer microphones: The electret condenser microphone meets micro-electro-mechanical-systems,” Acoust. Today, vol. 5, no. 2, pp. 4-13, Apr. 2009.

[12] J. L. Flanagan, J. D. Johnston, R. Zahn, and G. E. Elko, “Computer steered microphone arrays for sound transduction in large rooms,” J. Acoust. Soc. Amer., vol. 78, no. 5, pp. 1508-1518, 1985.

[13] G. W. Elko, J. L. Flanagan, and J. D. Johnston, “Computer-steered microphone arrays for large room teleconferencing,” in Proc. IEEE Workshop Applications of Signal Processing, New Paltz, NY, 1986, Paper 1.6.

[14] J. L. Flanagan, D. A. Berkley, G. W. Elko, J. E. West, and M. M. Sondhi, “Autodirective microphone systems,” Acustica, vol. 73, no. 2, pp. 58-71, 1991.

[15] J. L. Flanagan, D. A. Berkley, and K. L. Shipley, “HuMaNet: An experimental system for conferenc-ing,” J. Vis. Commun. Image Rep., vol. 1, no. 2, pp. 113-126, 1990.

[16] J. L. Flanagan and E. E. Jan, “Sound capture with three dimensional selectivity,” Acustica, vol. 83, no. 4, pp. 644-652, July/Aug. 1997. [SP]