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Journal of the Acoustical Society of America

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Jun 1979

Volume 65, Issue S1, pp. S2-S142

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back to top Session PP. Engineering Acoustics V: Historical Turning Points in Audio‐Frequency Transducer Development
Invited Papers
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The single element unidirectional microphone (A)

B. B. Bauer

J. Acoust. Soc. Am. Volume 65, Issue S1, pp. S106-S106 (1979); (1 page)

Online Publication Date: 11 Aug 2005

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The single element unidirectional microphone exhibiting the Limaçon directional pattern m + (1 − m) cos θ was an accidental discovery during an effort to develop a bidirectional piezoelectric microphone, affirming the research director's lament that the best inventions are those leading into unexpected directions. Refined through equivalent circuit analysis and embodied in many configurations this principle over the years has been employed in many important and interesting microphone applications in Public Address, Broadcasting, Recording, and others. The theory and some of the most important configurations and applications of this principle are described.
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Reciprocity and the calibration of microphones; its history and new problems (A)

Richard K. Cook

J. Acoust. Soc. Am. Volume 65, Issue S1, pp. S106-S106 (1979); (1 page)

Online Publication Date: 11 Aug 2005

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A reciprocal theorem is the basis of the reciprocity method for the absolute calibration of microphones and other electroacoustical instruments. Such theorems are commonly found in the mechanics and acoustics of gases, liquids. and solids, in electroacoustical systems, in electric circuits, and in the propagation of electromagnetic waves. The earlier invention of the condenser microphone facilitated the invention of the reciprocity method in 1940. It was then developed and eventually standardized as a very accurate and convenient method for measuring the response of a microphone in both amplitude and phase. The first applications were to the sound pressure calibration of microphones used at audio frequencies in air. Earlier calibration methods were based essentially on standard sources such as the thermophone, pistonphone, and electrostatic actuator, and on the use of a Rayleigh disk. Systematic differences between these led to a search for a more accurate method, and discovery of the reciprocity method. During the war years (1940–45) it was applied to the calibration of underwater hydrophones, and shortly thereafter to the calibration of accelerometers. The extension to freefield calibrations led to problems still present today, and for which we offer a solution.
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Principles and history of the acoustic‐suspension (air‐suspension) loudspeaker system (A)

E. Villchur

J. Acoust. Soc. Am. Volume 65, Issue S1, pp. S106-S106 (1979); (1 page)

Online Publication Date: 11 Aug 2005

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In the early 1950's loudspeakers were designed with relatively stiff mechanical suspensions. When these speakers were used as direct radiators the enclosure was made as large as possible, in order to prevent its acoustic stiffness from raising the speaker's fundamental resonance frequency. In the acoustic‐suspension speaker system the quantitative relationship between mechanical and acoustic stiffness is interchanged. The mechanical suspensions are made many times more compliant, and the cushion of air enclosed behind the cone correspondingly stiffer. This air is no longer treated as a necessary evil, but as an integral design element of the system: A linear acoustic stiffness is substituted for the nonlinear mechanical stiffness of the cone suspensions. The result is a substantial reduction in low‐frequency harmonic distortion, particularly in the stiffness‐controlled frequency region below resonance. The enclosure size, also reduced, is dictated by the acoustic stiffness required for a predetermined resonance frequency of the mounted speaker. A brief history of the acoustic‐suspension system will be presented, including published attacks on it and discussion of a patent suit in 1962.
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