Sound control systems which employ sound cancellation may deteriorate due to ageing or change of conditions. The system described is contructed to change according to the sound level at a point where cancellation is to be achieved by sound from a loudspeaker driven by modified signals from a microphone which picks up sounds from a noise source. The signals from the microphone pass through a filter circuit having a transfer function which is controlled by a controller. Signals from the point where cancellation is required, and after modification, from the microphone and the filter circuit are employed by the controller to derive a control signal for the filter circuit.