Parallel Notch Filter Circuit
Explanation of Terms
C is the capacitor value required for your filter.
L is the required inductor value for your filter.
R is the desired resistor value for this filter.
F is the frequency (in Hertz) halfway between F 1 and F 2.
F is the frequency point 1 below F that is -3dB SPL relative to F.
F is the frequency point 2 above F that is -3dB SPL relative to F.
For the example driver below, the center point of the response peak (F) would be at about 3583 (Hertz). F 1 and F 2 would be located at approximately 1015 and 6150 respectively.
Frequency Response Example
At first glance, the response of the speaker above does not look like a very good candidate for a driver in a quality loudspeaker system. A broad peak in the frequency response of a speaker like the example above can be easily corrected through the use of a notch filter, however.
Note : This filter can be used on any type of speaker, it is not only for midranges - this was just an easy example to model.
If you need some logarithmic paper to aid in your SPL measurements, I have made two graphics to help. The first one can be used for low frequency plotting, and the second is a full-range scale. Just click the link for the one you need, and press the "print button" on your browser after the image is done loading. I will add some notes about the methods used to properly measure frequency response as soon as I can.
Calculate Your Notch Filter
To use this form, enter F, F1, and F2. Then, click the "Calculate Values" button, and your results will appear in the empty text boxes.
Enter F Hertz Enter F 1 Hertz Enter F 2 Hertz C = Microfarads L = Millihenries R = Ohms