Virtual microphone control for a light-weight active noise-reducing casing

An important disadvantage of classic active noise control is that in general case the destructive interference is only at local areas. Such systems are thus inefficient in reducing noise generated by machines, which is a common problem in industrial environment, but also for home/office appliances. Then, the noise source is usually passively isolated by applying a casing with a sound absorbing material. Such passive isolation is, however, inconvenient for low frequencies, for which the walls should be too thick and heavy to be effective. Such walls may also reduce thermal dissipation factor of the casing. The authors have been developing the idea of improving noise isolation of device casings by applying active control of vibrations of the casing walls. For the purpose of control the emission of noise to the environment must be measured, for instance by microphones located around the casing, or some signal highly correlated to the emitted noise must be acquired. Performed research demonstrates that structural sensors measuring vibrations can be used, although, unfortunately, reduction of walls vibrations does not result in sufficient reduction of the noise emission. However, in this paper it is shown that those signals can be used in a Virtual-Microphone Control scheme to estimate acoustic signals at desired locations around the casing, which in turn are considered as the error signals for the active control algorithm. The control system is experimentally verified for a light-weight casing, and obtained results proving its efficiency are reported.