Series Notch Filters
By Roy Viggo Pedersen
31 Jan 1996
What is a series notch filter?
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. Using this filter on bass drivers calls for very large (and expensive) components. Many modern tweeters are using magnetic oil (Ferrofluid) in the voice coil gap to mechanically damp the resonance. These tweeters will probably not need such a curcuit, or does not depend so much on it.
When is it used?
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. Example: If you're using a tweeter without ferrofluid with a 1. order filter (damps 6 dB/octave) and the crossover frequency is two octaves above the driver resonance, you certainly need a notch filter. Lets say driver resonance is at 1000 Hz (typical) and the crossover frequency is at 4000 Hz (also typical, and two octaves above the resonance). Then the filter only damps 12 dB at driver resonance, and this will strongly affect the network, as this graph shows Fig. 1. Crossover response without notch filter.
We are using a capacitor of 5 microFarads for this filter at (approximately) 4000 Hz. You can see that the undamped driver resonance causes a big peak in the SPL responce curve at driver resonance.
The circuit
This circuit is also called a LCR circuit because it consist of a Inductor (L), a capacitor (C) and a resistance (R). They are wired in series Fig. 2. The circuit.
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