The application of adaptive filters to active sound control

Active sound control involves the introduction of a number of secondary acoustic sources whose outputs are arranged so as to destructively interfere with the sound from some 'primary' source. The technique works best with low frequency periodic signals where the wavelength is long and the waveform predictable. The method used is to adapt the output from a number of loudspeakers so as to minimise the sum of the squares of the pressures at a number of microphones. This may be considered as an adaptive filter problem with a pure tone reference signal being fed to the loudspeakers via a bank of digital filters. The paper begins with a review of a multichannel LMS algorithm suitable for this task and goes on to explain some of its characteristics including results measured with a practical system feeding 16 loudspeakers to control 32 microphones.<
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