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Adaptive noise control algorithms have been successfully applied to reduce noise in acoustic, photonic, and mechanical systems for many years. Since the noise source and the environment are time-varying in general, it is often desired that an active noise control system be adaptive.
Adaptive filters are designed by minimizing an error function and can be realized as Finite Impulse Response (FIR), Infinite Impulse Response (IIR) or lattice and transform-domain filters. The most commonly used adaptive filter is the FIR filter using a least mean square (LMS) algorithm.
A serious issue with implementations of the LMS algorithm for noise cancellation is the requirement of a coherent reference signal, which must be well correlated with the disturbance or noise. A common practice is to measure the disturbance or noise directly and use it as the reference signal to the LMS algorithm. But, a direct measurement of disturbance may not be possible and even if it is possible, it will require that additional resources be used which eventually increase the cost of the operation or process.
Addressing the above, Navy scientists have developed a new method for generating the reference signal by utilizing the characteristics of the error signal, which is the difference between the responses of the system to disturbance and the control signals. Since the error signal has the frequencies of the disturbance, processing the error signal can generate a reference signal.
While jitter remains a serious issue in laser communications, vibration control in large machinery, noise control in acoustic systems and microphones, and jitter control in satellites and space systems are some of the other important applications for this technology.
- Under ideal conditions, the adaptive LMS algorithm has proven to drive the error to zero
- Filter does not require that a coherent reference signal be highly correlated with the noise
- The weights of the adaptive filter are adjusted by the LMS calculator to minimize the mean square of the error signal and therefore the weights are continuously updated so that the error is progressively minimized on a sample-to-sample basis
- Filter can also be used to attenuate measurement noise: since the reference signal is generated using the error signal, this method is designed to handle the primary disturbance as well as the measurement or process noise
- US patent 8,019,090 available for license