Virtual microphone control for an active noise-cancelling casing

An active noise-canceling casing is very attractive for reduction of sound generated by devices. Such casing can provide good noise reduction for low frequencies, where a passive barrier would be too thick for practical use. The classical active noise control approach, where the goal is to minimize the sound pressure level around multiple microphones outside the casing can be used. However, it requires placing external microphones, what makes the overall technical solution not accepted for many applications. The active vibration control, where the goal is to minimize vibrations of all plates, requires only sensors on the plates. However, in this solution, in turn, noise reduction results are worse. This paper presents employment of the idea of the virtual microphone-based approach to improve results from the system based on vibration sensors only, which are used to estimate acoustic pressure at specific locations in the acoustic field. By using a two-stage structure, the system is tuned to reconstruct the same vibrations of the plates, which were present when the acoustic pressure were minimized directly in the square sense. A laboratory active noise-canceling casing used for experiments is made of 5 actively controlled aluminum plates mounted on a steel frame. It is passively isolated from the floor. On each plate, three electrodynamical actuators are installed. The control system is experimentally verified and obtained results are reported.