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 characteristics (Butterworth, Bessel, etc). It is also possible to contrive crossovers that have notches (or to get mathematical, zeros) built into the roll-off, 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  gives the example of a horn loudspeaker being used near its cutoff frequency. He also cites the case ...
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