Instability detection and correction in sinusoidal active noise reduction systems

1. A method for operating an active noise reduction system that is designed to reduce sinusoidal noise, where the sinusoidal noise does not emanate from a rotating device, where there is an active noise reduction system input signal that is related to the frequency of the sinusoidal 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 sinusoidal noise, the method comprising:

detecting distortions of the noise reduction signal by comparing the zero crossing rate of the noise reduction signal to the zero crossing rate of the sinusoidal noise; and

altering the noise reduction signal based on the detected distortions.

2. The method of claim 1 wherein the zero crossing rates are compared in a window of time.

3. The method of claim 2 wherein the time period of the window is variable.

4. The method of claim 3 wherein a variation of the window period is based at least in part on the frequency to be cancelled.

5. The method of claim 1 wherein an adaptive filter uses coefficients that are based on one or more adaptive filter parameters to modify one or more of the amplitude and phase of the input signal, and wherein altering the noise reduction signal based on the detected distortions comprises altering the values of one or more adaptive filter parameters.

6. The method of claim 5 wherein the adaptive filter parameters comprise a leakage factor and an adaptation rate.

7. The method of claim 6 wherein the active noise reduction system outputs separate noise reduction signals for each of a plurality of transducers, and where the amount by which one or both of the leakage factor and the adaptation rate are altered is based on one or more of:

i) the scale of the difference between the zero crossing rate of the noise reduction signal and the zero crossing rate of the sinusoidal noise;

ii) a difference between the zero crossing rate of the noise reduction signal and the zero crossing rate of the sinusoidal noise coupled with a relatively large noise reduction signal amplitude; and

iii) detected distortions in more than one noise reduction signal.

8. The method of claim 5 wherein altering the values of the one or more adaptive filter parameters comprises automatically reducing the value of one or more of the adaptive filter parameters.

9. The method of claim 8 further comprising establishing minimum values of one or more of the adaptive filter parameters and maintaining the values at least at such minimums.

10. The method of claim 8 further comprising automatically increasing the values of one or more adaptive filter parameters after they have been reduced.

11. The method of claim 10 wherein the values of the one or more adaptive filter parameters are increased in steps.

12. The method of claim 1 wherein a source of the sinusoidal noise comprises resonance resulting from mechanical vibration.

13. A method for operating an active noise reduction system that is designed to reduce sinusoidal noise in a motor vehicle cabin, where the sinusoidal noise does not emanate from a rotating device, where there is an active noise reduction system input signal that is related to the frequency of the sinusoidal 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 sinusoidal 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 sinusoidal noise, and where distortions are detected by comparing the zero crossing rate of the noise reduction signal to the zero crossing rate of the sinusoidal 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.

14. The method of claim 13 wherein the zero crossing rates are compared in a window of time, where the time period of the window is variable and is based on the frequency to be cancelled.

15. The method of claim 13 wherein a source of the sinusoidal noise comprises resonance in a vehicle cabin resulting from vibration of cabin components.