L. A. Devries
J. S. Bolton
1077 Ray W. Herrick Labs., School of Mech. Eng., Purdue Univ., West Lafayette, IN 47907-1077
Acoustical holography has been applied most often to visualize the sound field of planar sources driven by known forces. In this paper the application of acoustical holography to machinery noise source identification, when it is possible to enclose the source entirely by a spherical measurement surface, is considered. That procedure amounts to representing the sound field on the measurement surface as a summation of spherical harmonics, each of which is associated with a particular spherical Hankel function. Once the spherical harmonics have been identified, each may be propagated inward towards the source for the purpose of source identification, or outward to visualize the sound field. The combination of those procedures with the use of multiple reference microphones to visualize the sound field radiated by multiple incoherent sub-sources will also be described. The procedures will first be illustrated by visualizing the sound field radiated by several simple sources (i.e., monopoles and dipoles) when operated either individually or in combination. Next, the vibration of a refrigeration compressor shell is visualized, both when mechanically driven at a single interior point and by internal pressure pulsations. Finally, results for a compressor in normal operation will be shown.