Spatial active noise reduction with multiple error microphones

Recently, research in the field of active noise control (ANC) has increased considerably. Currently there are three different approaches to extend the upper limiting frequency and attenuation zone of ANC systems. The first approach is to locate a single error microphone in close proximity to the noise source. This approach extends the upper limiting frequency but achieves local noise reduction. The second approach is to place a single error microphone at a remote point relative to the noise source. The problem with this approach is that due to coherence problems poor results are achieved. The third approach is .to place several error microphones around the noise source in various deployments. This approach, named as multiple error sensor technology (MEST), appears to be the most promising one. In this paper, three major phenomena were empirically investigated: the first is the contribution of MEST compared with a single error sensor, the second is the optimal deployment of the error microphones, and the third is the influence of the number of error microphones on the ANC performance. A significant improvement to ANC compared with classical single error microphone methods was achieved by applying MEST. Spatial broadband noise reduction of 6.4 dB(A) was demonstrated.