31 Jan 1996 Get an explanation of what a notch filter is and what its use is. The function of the series notch filter is to dampen the effects the driver resonance has on filter networks.
Most drivers has a large impedance peak at it's resonance.
For crossovers to works as expected, it's important to have a nearly resistive impedance, at least in the crossover frequency region. Most drivers benefits from using notch filters, specially tweeters and midranges where the component values can be kept small. These tweeters will probably not need such a curcuit, or does not depend so much on it. The series notch filter is most useful when the crossover frequency in a highpass filter is near the driver resonance. But this depends on the filter damping.