Adaptive digital filters have been used in an experimental investigation of low-frequency equalization of a single-channel sound reproduction system in a car. The problems are discussed of adapting the digital filter so that a smooth transition is achieved between the equalized low-frequency (below 400 Hz) and the unequalized high-frequency response. Equalization of the response at only one point in the car is found to cause degradation in the response at others. Multiple-point equalization, in which the response at four positions is best equalized in a least-squares sense, was found to give only modest overall improvements in this case. The best strategy for a single filter appears to be weighted multiple-point equalization, in which the error at the most important listening position in the car is more heavily weighted in the adaptation algorithm. This gave worthwhile improvements in the response at the selected location, without significant degradations at other points. A very similar effect can also be achieved with the single-point equalization systems either by using a leak in the adaptive algorithm or by using an adaptive filter with a smaller number of coefficients.
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