Virtual Microphone Control for an Active Noise-Cancelling Casing

An active noise-canceling casing is very attractive for reduction of sound generated bydevices. Such casing can provide good noise reduction for low frequencies, where a passive barrierwould be too thick for practical use. The classical active noise control approach, where the goal is tominimize 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 acceptedfor many applications. The active vibration control, where the goal is to minimize vibrations of allplates, requires only sensors on the plates. However, in this solution, in turn, noise reduction resultsare worse. This paper presents employment of the idea of the virtual microphone-based approach toimprove results from the system based on vibration sensors only, which are used to estimate acousticpressure at specific locations in the acoustic field. By using a two-stage structure, the system is tunedto reconstruct the same vibrations of the plates, which were present when the acoustic pressure wereminimized directly in the square sense. A laboratory active noise-canceling casing used for experimentsis made of 5 actively controlled aluminum plates mounted on a steel frame. It is passivelyisolated from the floor. On each plate, three electrodynamical actuators are installed. The controlsystem is experimentally verified and obtained results are reported.