Filter apparatus for actively reducing noise

A filter apparatus for actively reducing noise d from a primary noise source x, comprising:

• a secondary source signal connector for connecting to at least one secondary source, such as a loudspeaker, wherein said secondary source generates secondary noise y to reduce said primary noise d;
• a sensor connector for connecting to at least one sensor, such as a microphone, for measuring said primary and secondary noise as an error signal e;
• a first delay for delaying said error signal e and a time reversed secondary path filter G for providing a delayed filtered error signal e′;
• a reference signal connector for connecting to at least one reference signal x, said reference signal x being coherent with said primary noise d;
• a first control filter Wa arranged to receive said reference signal x and calculate a control signal for providing said secondary source signal u;
• a second delay arranged to receive said reference signal x and calculate a delayed reference signal x′;
• and
• an adaptation circuit arranged to adapt said first control filter Wa based on said delayed reference signal x′ and an error signal e″ characterized in that the filter apparatus further comprises:
• a third delay arranged to receive said control signal and calculate a delayed control signal y′;
• a second control filter Wb arranged to receive said delayed reference signal x′ and calculate an auxiliary control signal y″;
• wherein said adaptation circuit is arranged to adapt said second control filter Wb while receiving said error signal e″ as a sum of said auxiliary control signal y″ and an auxiliary noise signal d′, said auxiliary noise signal d′ constructed from a difference of said delayed filtered error signal e′ and said delayed control signal y′;
• wherein said adaptation circuit is arranged to adapt said first control filter Wa by a copy of said updated second control filter Wb;
• and
• a flatness improving preconditioning circuit for preconditioning the reference signals.