Abstract: An active noise control system for controlling noise propagated through a duct comprises a first microphone located to receive the propagated noise at a point in a plane perpendicular to the length of the duct. A loudspeaker is located substantially at that plane for introducing sound into the duct. An analog first control circuit is operative in response to the noise received by the first microphone to feed a signal to the loudspeaker such that the sound introduced thereby destructively interferes with the noise at that point. A second microphone is located further downstream to receive residual noise propagated along the duct despite the destructive interference. A second control circuit, responsive to the residual noise received by the second microphone, controls parameters of the analog first control circuit so as to minimize the residual noise. In particular, the first control circuit includes an integrator which has a transfer function which is electrically controlled by the second control circuit.

Abstract: An active noise control system for reducing the amount of noise propagated along a duct comprises a microphone mounted in the wall of the duct for detecting the noise, and an active antiphase noise source, such as a loudspeaker, which is mounted substantially in the center of the cross-section of the duct. A control circuit coupled between the microphone and the antiphase noise source includes an integrator having a specific transfer function, which improves the loop gain of the microphone/source loop at low frequencies, and secures stability by altering the phase.