340 Chapter 11 11.16 Equalisation by Adjusting All Filter Parameters Much greater flexibility is possible if other filter parameters apart from the cutoff frequencies are manipulated. If we assume we have a fourth-order Linkwitz­Riley crossover, then the lowpass path has two cascaded Butterworth second-order filters. Both the cutoff frequency and the Q of each Butterworth filter could be altered to meet specific requirements, giving us four variables. The highpass path of the crossover similarly has four variables, so there are many more degrees of freedom. The problem is that all these variables have an interactive effect on the final response, and tweaking them to get a desired response is going to be a deeply tiresome business; some sort of automatic means of optimisation is highly desirable. The Linear-X Systems Filtershop CAD software [12] contains optimisation routines that can take measured drive unit amplitude response as input, and optimise the parameters of a chosen filter configuration to get the desired final response. It is a most impressive software package. This approach has the advantage that it has much more ability to correct response irregularities than simple frequency-offsetting. It also requires no extra circuitry to perform the equalisation; on the other hand adding dedicated equalisation stages will only have a small impact on the price of a typical active crossover. It does have the serious drawback that the resulting circuitry is likely to be wholly opaque, with no indication of what parameters have been tweaked to compensate for what response irregularities. Good documentation is essential, and if specialised software is used to perform the optimisation, it will need to be kept available so that design changes can be made in necessary. Passive crossovers must stringently minimise component count and power losses, so equalisation often has to be performed by manipulating filter cutoff frequencies and Q's. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] D. Keele, What's so sacred about exponential horns? AES Preprint No. 1038, 1975. H.F. Olson, Direct radiator loudspeaker enclosures, JAES 17 (1) (1969) 22­29. S. Linkwitz, Loudspeaker System Design, Electronics World, London, 1978. D. Self, Small Signal Audio Design, Chap. 10, Focal Press, Boston, MA, 2010, p. 271, ISBN 978-0-240-52177-0. D. Self, Small Signal Audio Design, Chap. 10, Focal Press, Boston, MA, 2010, p. 276, ISBN 978-0-240-52177-0. D. Self, Small Signal Audio Design, Chap. 10, Focal Press, Boston, MA, 2010, p. 286, ISBN 978-0-240-52177-0. N. Thiele, An active biquadratic filter for equalising overdamped loudspeakers, AES Convention Paper 6153, 116th Convention, 2004. S. Linkwitz, Loudspeaker System Design: Part 2, Wireless World, London, 1978, p. 71. R.A. Greiner, M. Schoessow, Electronic equalisation of closed-box loudspeakers, JAES 31 (3) (1983) 125­134. Heinzerling, Christhof, Adaptable Active Speaker System (subtractive), Electronics World, London, 2000, p. 105. D. Self, Small Signal Audio Design, Chap. 19, Focal Press, Boston, MA, 2010, p. 523, ISBN 978-0-240-52177-0. C. Strahm, Linear-X Systems Filtershop Application Manual, Application Note 5, p. 223.