Active cancellation of acoustic pressure and particle velocity in the near field of a source

In a local active noise control system the pressure signal from a single microphone is usually taken as the error signal and cancelled by the action of a secondary source to create a zone of quiet. In this paper it is shown that the strategy of cancelling the acoustic pressure and a component of the particle velocity at a point in the near field of a secondary source considerably improves the acoustic performance of a local active noise control system with respect to the case in which only the acoustic pressure is cancelled. It is also shown that in the near field of a secondary source the active cancellation of the acoustic pressure and the particle velocity component due to only the secondary source produces similar near field zones of quiet to those obtained when the acoustic pressure and the total particle velocity component are cancelled instead. This suggests that an array of two loudspeakers having a fixed gain and phase relationship could be used as the single secondary source. The acoustic performance of a secondary source array formed by two loudspeakers is theoretically studied when the acoustic pressure and the secondary particle velocity component is cancelled at a point in its near field with and without a diffracting head present. The results show that this sort of secondary source array can produce larger near field zones of quiet than a conventional loudspeaker cancelling the acoustic pressure only. Finally, the acoustic performance of such a secondary source array in a local active noise control system with a virtual microphone arrangement that projects the zone of quiet further away from the secondary source than the position of the physical microphone is also studied. For this sort of arrangement, it will be shown that the cancellation of the pressure and the secondary particle velocity at two different points in the near field of the secondary source gives better performance than the cancellation of both acoustic magnitudes at the same field point. (C) 1999 Academic Press.