Just above the crossover frequency, the capacitor begins to add resistance to the circuit. Band Pass Filters Band Pass filters allow a range of frequencies to pass above a selected crossover frequency and below another selected crossover frequency, filtering out all frequencies outside this band. Narrow Band Pass Filters are to be used when the bandwidth is less than two decades. Narrow Band Pass filters are commonly used with mid-bass loudspeakers. These allow a narrow band of frequencies to pass.
Different formulas apply to selecting coils and capacitors for a narrow band pass filter.
These formulas should be used if the low pass frequency is less than 20 times the high pass frequency, if not, distortion will be evident. Most of the time the rise is well beyond the crossover frequency and is not necessary to limit the high frequencies to it. If a Zobel is not used, only use a high pass filter and not a band pass filter. Zobel A Zobel is a filter used to stabilized loudspeaker impedance in a crossover-speaker circuit. This consists of a capacitor with a value equal to one which gives a crossover frequency at the frequency where the impedance has doubled, in series with a resistor which has a value equal to 1.25 times the nominal loudspeaker impedance.