An active control system for attenuating tonal noise in a defined region is described. In its most basic form the system includes sensors for generating signals indicative of the residual noise in the region after attenuation and the uncontrolled sound affecting the region, signal processing circuit for processing the generated signals differently depending on the tonal content thereof, an adaptive filter supplied with at least one of the generated signals whose characteristic is controlled by the processing circuitry, a transducer for producing tonal-noise-attenuating disturbance in the region and delay means for delaying signals relating to the uncontrolled noise before or after or during the adaptive filtering. The system finds direct application in a personal headset or ear defender.