Abstract: With reference to FIG. 1, an active sound control system comprises a loudspeaker LS having an input qs and operable to generate sound waves for interference with unwanted sound so as to produce a region close to the user of the system in which the perceived sound is substantially reduced. A monitoring microphone ro is positioned closer to the loudspeaker LS than to the region of sound reduction. Loudspeaker control means for controlling the input qs to the loudspeaker LS operate to energise the loudspeaker such that the sound waves emitted by the loudspeaker substantially cancel the unwanted sound waves in said region. The loudspeaker control means includes a signal processing means (FIG. 3) arranged to simulate a microphone output that would be obtained if that microphone, instead of being positioned closer to the loudspeaker LS than the user, were to be positioned in a notional position ra relatively close to the user.

Abstract: To reduce noise inside a motor car passenger compartment, two loudspeakers 37.sub.1, 37.sub.2 are driven by signals derived from a reference signal x(n) by adaptive filtering carried out by a programmed microprocessor and memory unit 36 which adapts the filtering in dependence on error signals e.sub.l (n) from four microphones 42.sub.1, 42.sub.2, 42.sub.3 and 42.sub.4 distributed in the passenger compartment. Reference filtering coefficients are initially determined by analysis of finite impulse responses when white noise is acoustically coupled from the loudspeakers 37 to the microphones 42, a white noise generator 48 being coupled to the unit 36. The reference signal x(n) is restricted to one or more selected harmonics or subharmonics of the fundamental noise frequency by a filter 34 which tracks the selected frequency. The selected frequency may be obtained from a coil 31 in the ignition circuit of the vehicle.