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New Thoughts on Active Acoustic Absorbers

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This paper continues an earlier exploration of using full-range loudspeakers as both acoustic sources and sinks, in an attempt to reduce the room decay time for bass frequencies. We develop the theory for a point active absorber immersed in the acoustic source field from a point source. This would apply to normal loudspeakers used as either sources or absorbers at frequencies below about 300 Hz, where they act as points. The result extends the theory of Nelson and Elliott for a point absorber interacting with a plane wave. An extra term occurs which has little net effect when averaged over frequency or distance. In rooms such cancellation occurs due to the varying distances from all the source images to the absorber. Impulse responses in several small rooms were measured from a source and an absorber loudspeaker to both a few listening microphones and a microphone mounted at the absorber. The efficacy of the active absorber is assessed and the results are enigmatic.

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