In the early 1930s, the movie industry was making strides in developing recording techniques for film scores. Yet, at that time only a small selection of microphones was available. So when MGM Studios in Culver City, CA, heard that the Siemens Co. of Germany had developed a cardioid microphone, a sample was immediately requested. The Siemens cardioid mic was first used on "Naughty Marietta" with Jeanette McDonald and Nelson Eddy.
The Siemens mic was much larger than the dynamic or condenser mics the studios had at the time. Since McDonald had a weaker voice than Eddy, the Siemens mic was first placed in front of McDonald in an attempt to achieve a better balance. Naturally, Eddy was in a quandary over why McDonald got the new mic and he insisted on one too -- refusing to record until he got a Siemens mic. The next morning when recording resumed, Eddy was provided with a Siemens mic replica built by the studio's prop shop and which contained a small omni mic inside -- thus marking the beginning of audio sugar-pills.
During this time, the equipment used was very heavy and bulky; every attempt was made to reduce size and weight. One area of such attempt was in microphone and camera connectors. James Cannon of Cannon Electric Co. in Los Angeles, CA, supplied connectors for the auditorium/projection room communications systems. Cannon, known as an ingenious person, was asked to develop a microphone connector. He came up with a six-pin connector which was the prototype of his famous P-type connector. It became an immediate success. Later, the small, three-conductor, camera motor cable connector was developed, and from 1929 on, the Cannon plug was history.
Even though there were several advancements in microphone technology the early models had not proved themselves reliable enough for broadcast work. But, in 1929 and 1930, at NBC's installation of new audio facilities for its Chicago Civic Opera House broadcasts, the 18 carbon microphones were replaced by three parabolic sound reflectors used in conjunction with a condenser microphone. This was the first application of highly-directional microphone techniques. NBC subsequently used these parabolic dishes in both the Philadelphia and New York Metropolitan Opera Houses and in its Time Square studios. In 1939, Mason and Marshall of Bell Labs reported on a design of a tubular microphone which used a single element and acoustical tubes of varying lengths to achieve a highly directional pickup pattern. This tubular design paved the way for the shotgun mics we are familiar with today.
The Olson/RCA Legacy
Perhaps the two most famous microphones to be commercialized were developed by Harry F. Olson at RCA. They were the 44A, B, BX velocity ribbon microphone series (1930-1940), and the 77A, B, C, D, and DX unidirectional ribbon microphone series (1931-1937). These vintage microphones are still in great demand; the 44BX could be found in many NBC studios, and the 77DX is currently used on NBC's "Tonight show" and on "Late Night with David Letterman."
When Olson developed the velocity mic it was a large step forward in microphone technology; it was the first high quality directional microphone. The effective solid angle of sound reproduction for the figure-eight velocity mic is one-third that of the omnidirectional mic. This means a reduction of 5 dB on the effective sound pickup of reverberation and other unwanted sounds. The directional properties of the velocity microphone were found to be useful in reducing effects of reverberation and increasing the intelligibility of reproduced speech.
The next logical step was the development of the unidirectional or cardioid pattern. Olson's 77A, which was introduced in 1933, consists of mini- and bidirectional capsules whose outputs are combined so that they yield the cardioid pattern. The cardioid pattern also affords the same effective angle of sound reception as the figure-eight, and hence the same advantages with the addition of a front-to-back rejection characteristic.
Sound Reinforcement
While RCA's microphone developments efforts were concentrated more towards broadcasting/recording applications, Western Electric, in 1939, introduced the 639A unidirectional microphone with sound reinforcement applications in mind. The 639A consists of a ribbon velocity element and a dynamic pressure element, whose outputs are combined so that they yield a cardioid pattern. Marshall and Harry of Bell Labs reported in 1941 that due to the reproducing characteristics of monaural sound systems in use, directivity needed to be supplied by the microphone in order to produce a more natural balance of direct-to-reverberant sound. They further stated, "This [feedback in a reinforcement system] is merely a special case of extraneous noise, and its effect can generally be reduced by directivity in the microphone."
Marshall and Harry implemented two field tests on the 639A mic, one of a broadcast of a 30-piece orchestra and another of a sound reinforcement system. In the studio, the 639A allowed for an ideal acoustical location of the mic and it was commented that the bass reproduction was much clearer than with other mics. Marshall and Harry attributed the clarity of bass to the suppression of reverberant bass energy pickup, where the studio's acoustical treatment was deficient. The 639A was installed at the House of Representatives, in Washington, D.C., where feedback conditions were so severe that other types of microphones had proven inadequate in providing sufficient reinforcement. The 639B has a six-position switch that yields omni, cardioid, several types of hypercardioid, and figure-eight patterns. In the House of Representatives, the hypercardioid afforded an increase of 5 dB in the system gain -- wherein was the difference between success and failure of the entire installation.
In the late 1930s, Benjamin Bauer of Shure Brothers developed a new cardioid dynamic microphone that used a single element and acoustic means to achieve its directional pattern. The Shure Unidyne made it debut in 1941 and was a turning point for microphone design, manufacture, and reinforcement applications.
Crystal Microphones
In 1880, Jacques and Pierre Curie discovered the piezoelectric effect. Piezoelectric crystals were first used by Langevin in 1917, in connection with his research efforts in underwater acoustics using ultrasonic transducers. In 1919, using Rochelle Salt, Alexander Nicolson first demonstrated a variety of piezoelectric devices, including loudspeakers, phonograph pickups, and microphones. Problems of manufacturing crystals with uniformity and the necessary shapes prevented the commercial production of any of these devices. Almost 10 years later, C.B. Sawyer and C.H. Tower developed processes to manufacturer uniform complex-shaped piezo crystals. This led the way for many piezoelectric or crystal transducers, as they were first called.
Electrets
Work on the electret condenser microphones dates back to as early as 1928. These microphones used permanently polarized wax plates. Eventually, microphones with wax electrets were offered commercially by Bogen (1938 to 1940) under the name No-Voltage Velotron. The first large-scale application of electric transducers was during WWII, when wax-electret microphones were used in Japanese field equipment. The wax-electrets, however, did not catch on due to their instability and very small capacitance which complicates the mic-preamp design. From 1948 through the early 1960s, work continued in electret microphone technology, turning up materials such as acrylics, ethyl cellulose, polystyrene, vinyl polymers, and ceramic electrets.
In 1962 and 1965 electret microphones in which the diaphragm was composed of a metalized thin foil of Mylar or Teflon, respectively, which has been converted into an electret were proposed. Finally, in 1968, Sony brought out the finest electret condenser microphone. Later, around 1971, Primo Company Ltd. introduced an electret mic with a monolithic IC preamp. Foil-electrets are manufactured in countless numbers; the Japanese production of electrets alone is estimated to exceed 20 million units per year.
The Future
Once the past has been clearly laid out before us, the future is easy to imagine. Many inventions of the future will be stolen from early predecessors. Those who worked in the labs in the 1800s and early 1900s left us with a long list of inventions to be implemented with modern materials, and new electronic and manufacturing technologies. These new devices can be categorized into mechanical and non-mechanical transducer systems. Diaphragms made of materials yet to be patented will use various modulation and sampling techniques to convert their motion into data. Optical A/D microphones are currently being developed. On the horizon we see the technologies of fiber optics, lasers, and interferometers applied to the electrical and digital transduction of acoustical phenomena.
"There is room for improvement in even the best microphones....Possibly an understanding of the limitations can be had by considering that, for perfection, a microphone should possess no inertia, and should produce an output directly proportional to the air pressure applied." Although this quote is from the book, Public Address Systems, by James R. Cameron published in 1935, it still holds true today.
BIBLIOGRAPHY
Microphones for Recording, Harry F. Olson, JAES, p. 676, 25:10/11, 1977.
Microphone Techniques in Radio Broadcasting, O. B. Hanson, JASA, p. 8-93, July 1931.
Bogen Catalog (1939), p. 16 & W.A. Bruno, U.S. Patent #2,284,039 (26 May, 1942; filed 16 July, 1940).
High Polymer Piezo Film in Electroacoustal Transducer Applications, Jesse Klapholz, presented at the 79th AES Convention, October, 1985. preprint #2308 (C-20).
FOR FURTHER READING
The Microphone Handbook, John Eargle, Elar, 1981.
Microphones, Jon R. Sank, JAES, p. 514, 33:7/8, 1985.
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