Fixed-parameter control of non-stationary acoustic noise

If random acoustic noise is stationary it can be modelled as a wide-sense stationary white noise filtered by a minimum phase filter. Such filter can be used to design an optimal control filter minimising variance of the signal being the effect of the acoustic noise and the secondary sound interference, or other cost function corresponding to acoustic noise reduction. However, in many environments the noise is subject to change. For instance, some of the noisy devices are switched on and off, speed of some rotors or fans changes, etc. As a result contribution of different frequency components may significantly vary in time. A control filter designed based on a disturbance model should be then updated. Solving the optimisation problem is time consuming and is rather avoided in on-line systems. Then, an adaptive approach is usually used. A drawback of adaptive systems is that they may suffer from convergence problems or some unpleasant transient acoustic effects may be generated. In this paper, the fixed-parameter approach to control is appreciated. To overcome the problems related to non-stationary noise, dominating frequency components / bands are distinguished for the acoustic environment and the optimisation is performed. Spectral and inner-outer factorisations are employed. Properties of such systems and systems designed if the noise model were known are compared.