Study on Feedforward Active Noise Control System with Optical Laser Microphone to Detect Reference Signal with Short Delay

In this paper, a feedforward active noise control (ANC) system with an optical laser microphone is proposed. The feedforward ANC system consists of a reference microphone, error microphone, and secondary loudspeaker. The main problem of the feedforward ANC system is the degradation of its noise reduction performance owing to the causality constraint, that is, there is a need for the ANC system to update the noise control filter and emit the anti-noise until the unwanted noise reaches the error microphone. The feedforward ANC system with the optical laser microphone is proposed to solve this problem. The proposed ANC system utilizes the optical laser microphone as the reference microphone that detects the vibration of the noise source. Since the optical laser microphone uses an optical laser, the unwanted noise is picked up more quickly than by an ordinary microphone. However, the coherence of the signals obtained by the reference and error microphones decreases and noise reduction performance may degrade since the proposed ANC system uses the ordinary microphone as the error microphone. To solve this problem, the proposed ANC system utilizes a differentiator for the reference signal because of the relationship between the sound pressure and the velocity. That is, the sound pressure is obtained by the differentiation of the velocity. Simulation results show that the proposed ANC system with the differentiator can reduce unwanted noise by almost the same amount as the conventional feedforward ANC system.