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Active Acoustic Absorbers Revisited

This paper continues an earlier exploration of using low-frequency loudspeakers as acoustic absorbers, with absorption cross-sections that far exceed their geometric size. Theory for a point active absorber immersed in the acoustic field of a point source is reviewed as it would apply to normal loudspeakers used as either sources or absorbers at low frequencies for which they act as compact sources. This theory contains an extra problematic term, which is suppressed if averaged over frequency or distance. In rooms, such suppression is expected due to the varying distances from all the source images to the absorber. Impulse responses in several small rooms were measured from a few sources and absorber loudspeakers to both a few listening microphones and near-field microphones mounted at the absorbers. These data were used to implement active absorption. The efficacy of the active absorber is assessed and the results are somewhat enigmatic. A room simulation was done to check on the actual measurements, and results are similar to theoretical expectation. The discrepancy should perhaps be attributed to the rather complex implementation issues.

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