Abstract: An adaptive control system for reducing undesired signals comprises a processor (36) to generate secondary signals which are provided to secondary sources (37). Sensors (42) provide at least one residual signal to said processor (36) which is indicative of the interference between the undesired and secondary signals. The processor (36) is operative to adjust the secondary signals using the residual signals to reduce the residual signals. If the adaptive control system operates erroneously or there is a fault this is indicated. Such faults can be detected by increasing and decreasing the secondary signals and detecting whether there is a corresponding increase and decrease in the residual signals. Also, the rate of change of the amplitude of the secondary signals and the rate of change of the frequency of the reference signal can be monitored to determine whether a fault condition exists. Also the impulse response or transfer function of the system can be monitored to determine a fault condition.