Study of causality constraint on feedforward active noise control systems

Feedforward active noise control (ANC) systems function as adaptive system identification, thus able to control both broadband and narrowband noises. When the acoustic/electric delays in the noise cancelling subsystem exceed the acoustic delay of the primary path, the causality constraint will be violated. We present a performance analysis of the feedforward ANC system for ducts (one-dimensional acoustic field) under the noncausal condition. The bandwidth of the noise controllable by the ANC system decreases as the degree of noncausality increases. Noise with narrower bandwidth can be more effectively cancelled out by the ANC system with a given degree of noncausality. Analysis also shows that the convergence speed for the adaptive weight vector is independent of the degree of noncausality.