The direct design of fixed-pole parallel second-order filters is a very effective way of obtaining equalizers having the desirable logarithmic frequency resolution for audio applications. The frequency resolution of the parallel filter design is controlled directly by the choice of pole frequencies, similarly to Kautz filters. This paper reviews and compares different pole positioning strategies, such as using a predetermined pole set, warped IIR filter based pole positioning, multi-band warping, and custom warping. In addition, a new technique based on the ripple density of the transfer function is proposed. The methods are tested on loudspeaker and room response equalization applications. It is shown that a predetermined logarithmic pole set gives acceptable equalization despite its simplicity, while the most accurate results for a given filter order are obtained by multi-band and custom warping. The ripple-density based pole positioning presents a useful compromise between accuracy and design complexity.
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