The ambiguity of acoustic sources—A possibility for active control?

An Acoustic-Based Modal Filtering Approach to Sensing System Design for Active Control of Structural Acoustic Radiation: Theoretical Development

“…3. Are the terms in C + in equation (19) frequency independent? This is necessary for practical implementation of the modal filtering procedure, where sensor signals are multiplied by a weighting value and added to produce the measurement quantities of interest.…”

“…The expression in equation (19) has the form of a standard modal filtering problem [1,12], where a number of sensor measurements, in this case acoustic pressure, are decomposed into a number of modal quantities, in this case acoustic multipole amplitudes. In practice, it is often better to use many more measurement points (say, 3-10 times as many) than resolved acoustic modes.…”

“…With this, a relatively large number of sensor signals are first decomposed into a much smaller group of signals that are used to evaluate the global performance measure. This is expressed mathematically by substituting equation (19) into equation (11):…”

“…However, previous use has focused on problems of optimising the number and placement of control actuators to minimise an acoustic radiation performance measure [19][20][21][22]. The aim here is somewhat different: to use multipole sources as a basis for development of a sensing system design strategy.…”

Verification of a near-field error sensor placement method in active control of compact noise sources

“…Some authors 11,12 place the error sensors in the acoustic far field of the primary source. Hansen and Snyder 13 advocated far-field error sensing techniques by arguing that far-field noise reductions are often associated with increases in near-field levels.…”