A feedback perspective on the LMS disturbance feedforward algorithm

Much of the recent active noise control research uses a least means squares (LMS) adaptive disturbance feedforward algorithm evolved from signal processing. In contrast, many other similar vibration control problems rely on linear feedback control techniques. The distinction between the two approaches is discussed. It is well known that for harmonic disturbances, the LMS algorithm is precisely LTI feedback of the error, and thus in this case, both approaches are identical. The primary differences are the implicit sensor placement in the LMS approach, differing modelling requirements, and the handling of changing disturbance characteristics. Understanding the similarities and differences between the approaches is important, as some results are more clearly obtained using one approach or the other. Similarities, differences, and common misconceptions are illustrated by means of two noise control examples.