A Study on Optimal Filter of Feedforward Active Noise Control System Based on Analysis of Frequency Response

In this paper we introduce an experimental study on an optimal filter of a feedforward active noise control (ANC) system based on the analysis of a frequency response. The feedforward ANC system reduces the unwanted noise by using a noise control filter based on an adaptive digital filter. The optimal noise control filter is determined by the transfer functions of each acoustic path including microphones and loudspeaker. However this optimal filter is a noncausal filter and cannot be used in the ANC system. In this paper the analysis of the optimal filter of the feedforward ANC system is introduced. Analytical results show that the optimal filter has a frequency characteristic similar to that of the adaptive noise control filter and the noncausal components mainly exist at low frequencies below the resonance of the secondary loudspeaker. Also two adjustments of the optimal filter are introduced to use the optimal filter in the ANC system. Simulation results show that the adjusted filter without the noncausal component has a frequency response similar to that of the original optimal filter. Also the simulation results show that the adjusted optimal filter can reduce the unwanted noise when using it as fixed noise control filter: