The Subwoofer DIY Page
Bandpass Systems: Notch Filter
last updated: 09 April 2005
The Subwoofer DIY Page Notch Filter
Your bandpass design may suffer from out of band noise, unless you take some steps to reduce it. One method of doing this is to use a passive Parallel Notch Circuit in series with the driver to get rid of the noise. This filter circuit looks like the following:

                     C
              +------||-------+
              |      L         |
    o---------+----~~~~~------+---------o
              |      R         |
              +---/\/\/\/\----+

To calculate the values of C, R, and L, you'll first need to find F, the midpoint of the noiseband. Also find F1 and F2, the frequencies at which the response drops by 3dB compared to the response at F. Alternatively, assume that the majority of the noise problem occurs at the pipe resonance frequency of the port (344/(2*L)), where L is the length of the port in metres, and also assume 0.95*F and 1.05*F as the -3dB frequencies F1 and F2.

Then:

    C = 0.03003/F (Farads)
    L = 0.02252/(F^2*C) (Henries)
    R = 1/(6.2832*C*(F1-F2)) (Ohms)

Note that capacitors are usually rated in microfarads (uF) and coils in milliHenries (mH). To convert:

    1 F = 1,000,000 uF
    1 H = 1000 mH

Example:

Center of noise band, F, is at 300 Hz -3dB points occur at 250 Hz and 360 Hz

    C = 0.03003/F
      = 0.03003/300
      = 0.0001001 Farads

    L = 0.02252/(F^2*C)
      = 0.02252/(90000*0.0001001)
      = 0.00248 Henries

    R = 1/(6.2832*C*(F1-F2)
      = 1/(6.2832*.0001001*(360-250))
      = 1/(6.2832*0.0001001*110)
      = 14.5 Ohms

Values required (closest match)

    C = 100 uF (bipolar)
    H = 2.50 mH (preferably air-core)
    R = 14.5 ohms

    power rating for R is
       approx = Pe*R/(R+Re)

    where,
    Pe = speaker power rating
    Re = speaker's min impedance