Given the benefits of using acoustic energy density for active noise control in enclosures, it was hypothesized that active structural acoustic control (ASAC) might also benefit by incorporating an energy-based structural error quantity. Power flow, or structural intensity, and structural energy density were studied for use in an ASAC system. Power flow was found to be unsuitable for general-purpose use in this application. Its minimization properties are such that for a general case, it may be impossible to predict structural or acoustic response based on a minimization of power flow amplitude in a two-dimensional setting. Additionally, sensor placement is complicated by the large changes in power flow field orientation caused by a small mass loading for a lightweight structure. Structural energy density was found to be a suitable error metric, and provides a slight improvement over velocity-based ASAC in enclosed spaces. A genetic algorithm was used to study structural energy density sensor placement on a simply supported plate. At modal frequencies, optimum control was achieved by placing the sensor at antinodes. The placement of the control force was found to be less critical, but showed a slight tendency towards locations remote from the disturbance force with low velocity cross-derivative.
Conventional near-field acoustical holography (NAH) requires a coherent field. For a coherent source, a scan-based approach can be used with one reference microphone to stitch the phase. For a noncoherent field, scan-based NAH can be performed if the virtual coherence technique is used. This technique uses multiple reference microphones to decompose the partial field into mutually uncorrelated partial fields, which are then processed by some NAH method. In particular, the statistically optimized near-field acoustical holography (SONAH) method is used with modified Tikhonov regularization. Numerical experiments are performed on a series of point sources with source strengths chosen to mimic the source characteristics of high-powered jets. The experiments are designed to aid in determining the proper number and location of reference microphones for doing NAH work on high-powered jet noise. [Work supported by Blue Ridge Research and Consulting and the Air Force Research Laboratory.]
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