Instability detection and correction in sinusoidal active noise reduction system

A method for operating an active noise reduction system that is designed to reduce in a motor vehicle cabin harmonic noise emanating from the engine or propeller shaft of the motor vehicle, where there is an active noise reduction system input signal that is related to the frequency of the harmonic noise to be reduced, and where the active noise reduction system comprises one or more adaptive filters that output a generally sinusoidal noise reduction signal that is used to drive one or more transducers with their outputs directed to reduce the harmonic noise, wherein an adaptive filter uses coefficients that are based on one or more of the leakage factor and the adaptation rate of the adaptive filter to modify one or more of the amplitude and phase of the input signal, the method comprising:

• detecting distortions of the noise reduction signal, where a distortion is based at least in part on differences between the frequency of the noise reduction signal and the frequency of the harmonic noise, and where distortions are detected by comparing the zero crossing rate of the noise reduction signal to the zero crossing rate of the harmonic noise;
• and
• altering the values of one or more, of the leakage factor and the adaptation rate of the adaptive filter based on the detected distortions, to alter the noise reduction signal.