Feedback control and acoustical energy in active noise control

Active noise control using feedback is achieved by reducing noise at one or a few spatial locations in reverbernt enclosures. The natural question that follows from this is: what happens to the noise at other locations in the enclosure? This paper proves that for a 3D enclosure the feedback which is effective in reducing noise at one particular location, under certain choice of sensor locations, also reduces acoustical noise at all other location, i.e., feedback schemes provide global reduction in acoustical noise. This is a new result, assumed but not proved so far.