Active noise control with virtual sensors

Abstract

Low frequency noise can be a significant problem in small vehicular interiors and the required size and mass of any effective passive treatment depends upon the wavelength of the noise that is to be reduced. Consequently, a low frequency noise problem in such an environment requires large, space-intrusive control elements. Active noise control (ANC) is a potential solution for this low frequency noise problem and can be used in conjunction with passive noise control at higher frequencies, so that noise can be attenuated over a broad frequency range. Although ANC can significantly reduce low frequency noise at an error sensor location in an enclosed space, the zone of attenuation may be so small that a nearby observer might not experience any improvement at all. Using an energy density error sensor, rather than a simple microphone error sensor, can increase the size of the attenuation zone, so that the observer becomes enveloped in the region of reduced noise. However, the maximum attenuation is still likely to occur at the error sensor location, so it may be preferable to reduce the noise specifically at the observer location, by using either a "virtual microphone or a virtual energy density sensor". By extrapolating the signal from a number of remote microphones, either pressure or energy density can be estimated at the observer location and hence minimised. The extrapolation method also has the potential to adapt to environmental changes and observer movements. Here, the concept will be introduced and the performance of the virtual sensors demonstrated for a small aircraft cabin.