Active noise reduction

A system (100) for controlling noise comprises an array (2) of concelling tranducers (loudspeakers) (2a). Located some distance away from cancelling array (2) is a detection system (3) comprising a series of microphones (3a), the system casting an acoustic "shadow" or quiet region (4). Located on or adjacent the primary source (1) emitting the noise to be controlled is a synchronising sensor (5) which may be a microphone, vibration transducer or electrical transducer, the output of sensor (5) being fed, along with the output from detection array (3), into an adaptive control system (102) the output of which is fed back to the cancelling units (2). The adaptive control system (102) comprises a low pass filter (15) producing a dc component V from a mathematical convolver (multiplier) (14), and a digital oscillator (16) which generates the cancelling frequency which is controlled by V. Also included arc frequency multipliers (17, 18 and 19) which multiply (2, 3) and n times respectively.