This paper details an approach to select measurement point locations for the inverse boundary element method. An accurate reconstruction of the vibration requires a well conditioned acoustic transfer matrix, which depends on measurement point selection. Matrix techniques can be used to regularize the solution though they often lead to poor reconstruction rank. A technique to determine the number of measurement points required, and their placement, prior to measurement has been developed using three criteria: uniqueness, completeness, and measurement point density. With this technique, the reconstruction error and the number of measurements can be minimized.
The application of the inverse BEM method as an advanced, non-contact noise source identification technique will be discussed. In the first and most conventional experiment, the inverse BEM is used to reconstruct the vibration pattern on the surface of a motorcycle engine oil pan cover. In the second experiment, the inverse BEM is used to determine the distribution of particle velocity on the open end of a rectangular duct. Measurements of the particle velocity using the two-microphone method are compared to those predicted using the inverse BEM. In the third experiment, the inverse BEM is used to identify the mechanism of an aeroacoustics noise problem (a vortex tone) created by flow over a circular rod. The spectrum of the tonal sound within the flow field is reconstructed using the inverse BEM, and a near field spatial reconstruction of the sound pressure shows clearly the nature of the vortex source. This paper will also show that only a relatively small number sound pressure measurements are required for the inverse BEM and that these measurements need not be in the near field of the source nor conformal to it to obtain good results.
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