Abstract: An adaptive control system for reducing undesired signals comprises a processor (36) which provides secondary signals for sources (37) for interference with the undesired signals. Sensors (42) measure the residual Vibration which is indicative of the interference between the undesired and secondary signals. The processor (36) uses the residual signal to adjust the secondary signals to reduce the residual signals. Noise generation means (48) is provided to add a low level noise signal to the secondary signal and to provide a low level noise signal to the processor (36). The processor (36) is adapted to transform the low level noise signal and the residual signal from sensors (42) to provide the amplitude and phase of spectral components of the signals. The processor (36) modifies the secondary signals using these spectral components to obtain better reduction of the undesired signals.

Abstract: An adaptive control system for reducing undesired signals comprises sensors (31) to provide signals indicative of the undesired signals, and a processor (36) which processes the first signal to provide a secondary signal for output to sources (37) to interfere with the undesired signals. Sensors (42) are provided to detect the residual signals which are indicative of the interference between the undesired and secondary signals. Within the processor the signals indicative of the undesired signals and the residual signals are transformed into the frequency domain and collated. The outcome of the collation is inverse transformed and the processor adjusts the secondary signal using this inverse transform to reduce the residual signal from the sensors (42).