A new two-sensor active noise cancellation algorithm

A novel approach to active noise cancellation using two microphones is developed. The primary microphone is placed at the point where noise cancellation is desired and the secondary microphone is placed at some other point. The cancelling signal is generated as the output of a two-input/single-output FIR (finite impulse response) filter which is driven by the outputs of the secondary and the primary microphones. The output of the primary microphone is not only used to adjust the coefficients of this filter, but is also used as an input to the filter itself. The performance of the two-sensor algorithm is compared with that of an LMS (least mean square) algorithm and with that of a single-sensor algorithm. These three algorithms were applied to noise recordings made using a set of headphones worn inside the cabin of a propeller aircraft. The primary sensor was placed inside the headphones and the secondary sensor was attached to the outside of the headphones. Assuming that the transfer function between the cancelling speaker and the primary sensor was a pure delay of about 40 mu s, the new algorithm was able to attenuate the overall noise power 15 dB more than the LMS algorithm and 3 dB more than the single-sensor algorithm.<>