US Patent 8199923 Active noise control system

An active control of an unwanted noise signal at a listening site radiated by a noise source uses a reference signal that has an amplitude and/or frequency such that it is masked for a human listener at the listening site by the unwanted noise signal and/or a wanted signal present at the listening site in order to adapt for the time-varying secondary path in a real time manner such that a user doesn't feel disturbed by an additional artificial noise source.