Active noise control with bias free pre-inverse adaptive system

Filtered-x algorithm has a possibility of making an active noise control (ANC) unstable due to the modeling error of a secondary path. A pre-inverse type ANC is proposed in order to solve the problem. The proposed ANC uses the filter which has the inverse transfer function of a secondary path before the secondary path. Whereas the filtered-x algorithm controls a primary path and a secondary path simultaneously by an adaptive filter, the pre-inverse type ANC can control a primary path and a secondary path independently. Therefore the proposed ANC is always stable. However, the adaptive filter estimating a secondary path converges on a solution with bias due to disturbance. Thus, the bias free adaptive algorithm is also proposed. The proposed adaptive algorithm takes advantage of the independence between the input signal and disturbance.