Page 6 - Spring 2009

Fig 2. Figure 7 from Alexander Graham Bell's 1876 patent of the telephone. 2
transducers drive membranes (a) and (i) at input (A) and output (I) horns. Thus, Bell’s work (and his competitors in the race to invent the telephone) can be considered as the beginning of microphone development. Although Bell’s patent described moving armature transmitters (telephone terminology for microphones) and receivers (telephone ter- minology for loudspeakers), the device that transmitted the now famous phrase “Mr. Watson, come here, I want to see you” was obtained using a microphone that consisted of a membrane containing a conducting needle in a slightly acidic water bath. The modulation of the membrane by Bell’s voice caused a time varying resistance in the circuit which drove a corresponding modulation of a moving armature receiver. That Bell used a liquid transmitter device that was the basis of Elisha Gray’s work, has led to claims that Bell stole Gray’s invention. However, Bell’s patent was submitted to the patent office in advance of when he could have known about the details of Gray’s work. Bell’s use of the liquid transmitter was part of the normal process of “reducing an invention to prac- tice.” Bell was also accused of fraud and misrepresentation by the government of the United States who moved to annul his patent in favor of Antonio Meucci, an inventor from Staten Island, New York who had filed a caveat (a one year renew- able notice of an impending patent) in 1871, about 5 years before Bell’s patent. The case was remanded to the Supreme Court for trial but due to Meucci’s death and Bell’s patent expiring, the true inventor of the telephone (who was entitled
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world and widely became known as the inventor of the tele- phone. It was not until the invention and refinement of the carbon microphone by Edison4 that adequate speech levels could be obtained over reasonable distances that telephony became practical. Microphone design continued at a rapid pace with the further development of the moving coil, or dynamic microphone invented by Siemens5 in 1874.
With the invention of the Audion vacuum tube by Lee DeForest and refinements by Western Electric in 1916, E.C. Wente of Bell Labs invented the first stable commercially viable condenser microphone based on a tensioned diaphragm.6 Figure 3 shows a drawing contained in Wente’s 1917 patent clearly showing the main features of a condenser microphone. Wente’s condenser microphone established the basic design rules that form the foundation of condenser microphones still used and built today. An interesting histor- ical fact is that Wente extensively used equivalent circuit
Fig 3. Image from Edward Wente's 1917 patent of the condenser microphone.6
analysis to model the basic frequency response on the con- denser microphone. He was clearly one of the first practi- tioners to use “lumped parameter models” to design and understand electroacoustic transducers.
Further progress was made with the commercialization of the ribbon microphone invented by Gerlach7 as a loud- speaker in 1924. The ribbon microphone inherently offered dipole directionality and improvements and variations of the ribbon element to obtain cardioid and hypercardioid patterns
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were developed by Olson at RCA. Microphone directivity
was desired in radio broadcasting for more spatial attenua- tion between the separate sources and for increased robust- ness against acoustic feedback in public address systems. Ben Bauer9 of Shure Brothers made a significant contribution in the late 1940’s by using a passive acoustic filter to control the sound propagation to the rear of a dynamic microphone. The acoustic filter he proposed formed an acoustic delay that allowed the formation of a directional microphone with only a single microphone diaphragm. Bauer’s design and variants of the principle is still widely used today to build directional microphones.
In 1956, in a consent decree with the U.S. Department of Justice, AT&T was forced to sell its audio transducer busi- ness. Most of the professional audio business was handed over to Altec. Coincidentally, this business divestiture occurred at the same time that Jim West, a summer student intern at Bell Labs, was experimenting on how to increase the sensitivity of high quality condenser headphones made from Western Electric 604AA condenser microphones. Jim had some quick success by following the work of Kuhl, Schodder, and Schroeder10 who had experimented with a dielectric made from a Mylar® membrane known at the time as the Sell11 transducer. The new large headphone transducer that Jim constructed produced much higher sound pressure levels than the earlier 640AA headphones. Unfortunately, this suc- cess was short lived when the transducer sensitivity fell off quite a bit within a few months. Kuhl, Schodder, and Schroeder had observed that the Sell transducer had to be reverse-biased periodically if one desired to use the trans- ducer over a long period of time.
This “problem” became an opportunity, as is so common in scientific breakthroughs. By 1959 Gerhard Sessler had joined Bell Labs and Jim had returned from the university to investigate the sensitivity problem with the headphone trans- ducer on which he had worked as an intern. In another of those strange coincidences that seems to play important roles
to the patent) was never determined.
Bell successfully demonstrated his invention all over the
Condenser Microphones 5