Global noise reduction in free-field conditions by using active noise-controlling casings

Active noise control (ANC) methods have been extensively used for reducing the noise level over the past decade. In particular, in the range of low frequencies they can effectively reduce the noise level higher than passive control methods. However, except in some special cases, in ANC only local zones of quiet are obtained. In this paper one of such exceptions is investigated—an active noise-controlling casing. More specifically, a noise source is placed inside the casing. Casing vibrations are actively controlled to achieve global noise reduction in free-field conditions. The global noise reduction is obtained using the Switched Error FXLMS algorithm and using error signals from error microphones located at appropriate points around the casing. It is shown that selected microphone array configurations can provide global noise reduction in free-field conditions. Performance of this approach is experimentally verified in a laboratory room with an exemplary acoustic treatment.