CHAPTER 5 Notch Crossovers The crossovers examined in Chapter 4 are all-pole crossovers, which means that the filters used are relatively simple lowpass and highpass types, though of varying order and filter characteristic (Butterworth, Bessel, etc.). It is also possible to contrive crossovers that have notches (or to get mathematical, zeros) built into the rolloff, typically giving a much steeper filter slope, to begin with at least, than an all-pole crossover of a practical order. This can be very useful when drive units that are otherwise acceptable misbehave badly when taken just outside their intended operating range. Neville Thiele [1] gives the example of a horn loudspeaker being used near its cutoff frequency. He also cites the case of a mono subwoofer, where its contribution must be rolled-off as quickly as possible out of band to prevent it contaminating the stereo localisation cues from the main speakers. 5.1 Elliptical Filter Crossovers A crossover design based on the use of elliptical filters was published by Bill Hardman in Electronics World in 1999 [2] and this has been the basis for much discussion on the