Practical active systems for the cancellation of broadband noise in ducts have, to date, almost exclusively used a single secondary loudspeaker. With a single loudspeaker, a large change of impedance in the duct can be-realised so that the incident waves are reflected back in the upstream direction.In order to absorb the incident sound wave, rather than just reflect it, a secondary source pair has been used in this work based on a proposal by Swinbanks. The source pair is capable of generating a uni-directional downstream wave, and the effect of standing waves and extra resonances in the upstream direction can thus be avoided. The controller has been implemented on a digital signal processing device and has been made adaptive using the LMS algorithm. The control of broadband noise over a frequency range of 2-3 octaves has been achieved with attenuation levels of 20-40 dB and the adaptive control system has been shown to be robust. The performance of the Swinbanks' source has been compared to that of a single loudspeaker and that of a "maximally efficient" source that can be realised by two loudspeakers. It has been found that the Swinbanks' source is the preferable solution, giving good reductions in the downstream and upstream directions, stabilizing the adaptive algorithm by reducing feedback effects, and placing least demand on the loudspeakers.
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