Numerical studies of actively generated quiet zones

Abstract

A local active control system has been investigated, in which the pressure at a point in front of a piston secondary source is driven to zero. The spatial extent of the zone of quiet created around the point of cancellation has been determined numerically, for both a uniform and diffuse primary sound field. The 10 dB zone of quiet generally becomes larger the further the cancellation point is from the secondary source, up to a limit of about one tenth of an acoustic wavelength. At low frequencies, the zone of quiet forms a shell round the secondary source, although this is less pronounced for diffuse primary fields. This finding may be used to reduce the pressure in front of the piston, at low frequencies, by cancelling the pressure in the plane of the piston.