I found this crossover circuit in a construction set and it puzzled me a while. I have absolutely no idea, why I should combine a 2nd order low pass with half a band stop. The low pass alone has a corner frequency of roughly 155Hz. A full scale notch filter would have a center frequency of roughly 73Hz (impedance as shown).
Finally the sub woofer sits in a very small enclosure (<10L) and is itself quite small. The vented cabinet seems to be tuned as expected (Hoge). I'm a bit uncertain about that since the parameters (volume, opening's surface and high channel length) don't differ that much in comparison to tunings over-emphasizing at the lower end (B4).
So I assume this is an additional filter trick to make the small thing sound better at lower frequencies. There is a peak at 34Hz before the band stop takes over, a drop around the mid frequency and the expected cut off now at roughly 210Hz.
Is this correct or did I miss something? There is no additional filter stage for satellite speakers. And my goal is to replace the passive filter network with an active one (before the amplifier).