The purpose of this paper is to present the methodology and results of an "in-situ" acoustic intensity measurement technique which was used to estimate the first seven natural frequencies and mode shapes of a surface plate of a gear housing. The acoustic intensity measurements were collected extremely close to the vibrating surface. Natural frequencies and mode shapes of this plate were also determined from conventional experimental modal analysis and from operating structural motion surveys; these compared well with the results estimated from radiating acoustic intensity data. The only exception seemed to be the case when plate natural frequencies were closely spaced. This use of microphones for "in-situ" modal analysis could be very attractive when conventional structural measurement techniques are neither feasible nor practical.
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