Active sound control systems and sound reproduction systems

A method of creating a region in which the sound waves from a sound source are at least substantially reduced, comprising during a setting-up stage measuring the difference in the outputs of a test microphone at the position of the required region of sound reduction, p(ra), and the output of a control microphone, located in a second position, after it has been passed through a signal processing means, p(ra), and then using said measurements to determine the characteristics of signal processing means for use in a sound control system comprising a loudspeaker having an input and operable to generate sound waves for interference with unwanted sound to produce a region close to the user of the system in which the sound perceived by the user is substantially reduced, a microphone positioned at a position ro closer to the loudspeaker than to said region of sound reduction, loudspeaker control means for controlling said input to the loudspeaker and operable to energize the loudspeaker such that the sound waves emitted by the loudspeaker substantially cancel the unwanted sound waves in said region, the loudspeaker control means including signal processing means arranged to simulate a microphone output that would be obtainable if the microphone, instead of being positioned closer to the loudspeaker, as aforesaid, were to be positioned in a notional position ra relatively closer to the user, the simulated microphone output being used to control said loudspeaker input, the complex response of the or notional position microphone, p(ra), at the frequency of interest, being obtained from the responses of the microphone at said position ro having an output p(ro) using an implementation of the equation:

• p(ra)=p(ro)+[Z(ra)-Z(ro)]q.sub.s
• where Z(ro) is the electrical transfer response at the frequency of interest between the loudspeaker and the microphone at the position ro, Z(ra) is the electrical transfer response between the loudspeaker and the notional position microphone, at the position ra, and q.sub.s is the signal driving the loudspeaker.