Feedforward Active Noise Control System Using Optical Laser Microphone to Overcome Causality Constraint

In this paper, a feedforward active noise control (ANC) system with an optical laser microphone is proposed and the effectiveness of the proposed system is introduced through real-world experiments. The main problem with the feedforward ANC system is the degradation of its noise reduction performance due to the causality constraint, that is, the ANC system should update the noise control filter and emit the anti-noise until unwanted noise reaches the error microphone. As a solution, the proposed ANC system utilizes the optical laser microphone as the reference microphone. The optical laser microphone picks up the vibration of the noise source by emitting an optical laser beam to the surface of the vibrating object. Since the reference signal obtained by the optical laser microphone is different from the unwanted noise obtained by the ordinary microphone as the error sensor, the proposed system utilizes the first-order differentiator to convert the vibration velocity into the acceleration for the improvement of the coherence between the unwanted noises obtained by the reference and error microphones. Simulation and experimental results show that the proposed system can reduce the unwanted noise by almost the same amount as the conventional system while the causality constraint is relaxed.