Microphones part 2 - The Electrical Era

  • Microphones part 1 - The Carbon Era 1915-1925
  • Microphones part 2 - The Electrical Era 1925-1945
  • Microphones part 3 - The Modern Era 1945-present
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    • orchestra before 1925
    Victor orchestra recording in Camden before 1925, from AT&T Archives
    orchestra after 1925
    Victor orchestra recording in Camden after 1925, from AT&T Archives
    Before the electrical revolution (Fig. 1a), musicians recorded in studios crowded together playing into a large acoustical horn that focused the soundwaves into the vibrating diaphragm of a acoustic phonograph that cut grooves in a wax-coated master disc. After the development of amplifiers and microphones (Fig. 1b), musicians could record in the same seating arrangement used in live concerts, with a single microphone to send electrical signals to an amplifier connected to an electro-magnetic disc-cutter.
    1920 condenser mic
    diagram from Olson 1976
    Wente with condenser 1931/05/09
    #48936 from AT&T Archives
    The condenser microphone was developed in 1916 by E. C. Wente at Bell Labs. It was improved over the next 10 years and became in 1926 the Western Electric 394-W microphone used to produce the first generation of sound motion pictures.
    1920 condenser mics enclosures
    diagram from Olson 1977
    RCA model AA ca. 1930
    from NMAH
    WE "cannon" condenser mic ca. 1929
    from "Dawn of Sound" exhibit
    The condenser microphone required an amplifier enclosure, containing a vacuum tube amplifier very close to the microphone element in order to amplifiy the faint signal to a more distant phonograph pickup or radio transmitter.
    Neumann "bottle" condenser mic 1928
    from Neumann History
    microphone top of Neumann "bottle"
    from Neumann History
    amplifier bottom of Neumann "bottle"
    from Neumann History
    Georg Neumann manufactured condenser microphones that became famous world-wide for quality. The CMV3 was the first model sold by his company that started in Berlin in 1928. The "bottle" microphone was used by the German radio and recording industry in the 1930's, and was used to broadcast the speeches of Nazi leaders and the Olympic games from Berlin in 1936.
    1931 Western Electric 618A
    diagram from Olson 1977
    Western Electric 618A
    from Fagen 1975
    1938 Western Electric 630A
    diagram from Olson 1977
    The moving-coil, or "dynamic" microphone was developed by W. C. Wente and A. C. Thuras at Bell Labs in the late 1920's, and was patented in 1931. Unlike the ealier condenser design with a fixed plate behind the vibrating diaphragm, this microphone used a wire coil behind the diaphragm that moved with a "velocity" independent of the sound frequency. The sensitivity of the voltage output depended on the resistance, or "impedance" of the moving coil system. A low impedance of 30 ohms allowed transmission over a long cables without loss of quality. The model 618A was unidirectional and the later model 630A was omnidirectional with a frequency response of 30-15,000 Hz.
    1931 RCA 44A (a) & 44B (b)
    diagram from Olson 1977
    Crosby 1939
    Crosby recording 1939 film
    from Judy McDonald
    The ribbon, or "velocity" microphone was introduced by RCA in 1931 as the model 44A and became one of the most widely used microphones in vocal recording. It used a small ribbon 2 inches (50 mm) long and 2.4 mm wide that moved inside a magnetic field according to the difference in sound pressure on each side of the ribbon. The velocity of the moving ribbon was independent of the sound frequency, producing a high-impedance signal. The mic was bidirectional with a pickup pattern like a figure-8, toward the front and back, eliminating unwanted noise from the sides. The advantages of high sensitivity and directionality made it ideal for "crooners" like Bing Crosby. The disadvantage of large size with a weight of over 8 lbs made it suitable only for fixed locations such as the recording studio or movie sound stage.
    1933 RCA 77A
    diagram from Olson 1977
    1937 RCA 77B
    diagram from Olson 1976
    The unidirectional, or "cardioid" RCA 77A ribbon microphone had a front-only pickup pattern rather than the figure-8 pattern of the earlier model 44. Because this front-only pattern resembled an inverted heart, it became known as the "cardioid" pattern. This mic had a velocity ribbon clamped in the center with the top half enclosed as a pressure-type ribbon. The RCA 77B introduced in 1937 was smaller and new magnetic materials produced greater sensitivity.
    1939 Western Electric 639A
    diagram from Olson 1976
    1939 Western Electric 639A
    from NMAH
    The Western Electric 639A unidirectional ribbon velocity microphone was sold by Altec starting in 1939; it was used by Voice of America ca. 1950.
    1941 Shure Unidyne
    diagram from Olson 1977
    1948 RCA KU-3A
    diagram from Olson 1976
    The single-element unidirectional microphone such as the Shure Unidyne was popular during World War II.

    Sources:

  • Microphones part 1 - The Carbon Era 1915-1925
  • Microphones part 2 - The Electrical Era 1925-1945
  • Microphones part 3 - The Modern Era 1945-present

    • © 1999 by Steven E. Schoenherr. All rights reserved.
    Return to Recording Technology History Notes | this page revised 8/16/1999