Reply To: I just need one HPF for my aux-fed subwoofer

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#54097
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Andreas
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The OP didn’t want to reduce level between 20 and 35Hz, he needs to cut everything below 35Hz (which totally makes sense since this region isn’t recognizable by our ears and simply eat power from the Amp and force sub speakers to extraordinary movement, except you have subsonic-able speakers, then you can feel this inside your body…).
If you set a notch filter at 20Hz which is on unity gain one octave higher (40Hz), that it will be unity also on one octave below, which is 10Hz. Next octave below would be 5Hz, then 2.5Hz, then 1.25Hz and so on. Such frequencies may be created by wind, for example.
A notch filter is a notch filter, not a high pass.
And, no, there should be no reasonable content below some 30Hz which is worth to be fed to your subs, that’s why the OP asks for a steep low cut (4th order would be 24dB/Oct). Technically there are many factors which normally would eliminate such low frequencies, like capacitors within the signal path. But such first order “highpasses” only cut with 6dB/Octave which isn’t much.