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Acoustical Research Employing Time Average Holography

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Dennis Gabor in 1947 conceived the concept of holography. Holography awaited a coherent monochromatic light source to become a practical concept. In 1963, Emett Leith introduced the laser to holography and began a rapid advancement of the art. The work of Emmett Leith, George Stroke and their co-workers at the University of Michigan refined the art to the point where its potential as a means of measuring the displacement the displacement of vibrating objects was successfully demonstrated. During the summer of 1967, a research study by the University of Michigan, under the direction of Norm Barnett, was instigated by Electro-Voice Inc. to explore the possibility of using time average holography in acoustical research and development. The University of Michigan study proved the great potential value of holography by yielding a series of pictures showing the mode of vibration of a microphone diaphragm reacting to sound fields ranging from 100 Hz to 10 kHz. Unfortunately, the complexity of the optics and the degree of training needed to operate equipment of this vintage precluded the use of holography as an everyday acoustical research tool. Refinements in equipment and techniques continued and by 1969 equipment became available that was stable enough to permit holographic analysis under conditions realizable in an acoustically oriented engineering laboratory. Electro-Voice Inc. installed January 1, 1970, a Jodon H.S.1A. holographic system and began a continuous program of basic research into the nature of mechanical vibrations occurring in acoustical devices. The intent of this paper is a discussion of holography as a measuring tool of acoustical research. Tests referred to in the text are meant to serve as examples of the type and scope of the information obtainable by this method.

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