Feedforward Active Noise Control with Coherence-Adjusting Filter for Improving Noise Reduction Performance under Low-Coherence Condition

We propose a feedforward active noise control (ANC) system with a coherence-adjusting filter. The feedforward ANC system can reduce the unwanted noise that is correlated with the reference signal picked up by the reference microphone. That is, the feedforward ANC system cannot reduce the unwanted noise when the unwanted noise and the reference signal are not correlated with each other. Hence, the coherence should be high to reduce the unwanted noise. However, only solution for the coherence problem is to place the reference microphone close to the error microphone. Under such a condition, the causality constraint is violated, and the noise reduction performance degrades. In this paper, we propose a feedforward ANC system with a coherence-adjusting filter that adjusts the reference signal to be similar to the unwanted noise. Then, the coherence between the reference signal and the unwanted noise increases, and the amount of noise reduction can be large even under the low-coherence condition. Simulation results show that the proposed ANC system can reduce the unwanted noise well compared with the conventional feedforward ANC system under different coherence conditions.