The best way to remove low to low-mid frequencies is actually quite simple for recordings.Import a mp3 or wave file into Audacity (Freeware audio editor)Split the stereo track into 2 mono tracks.Invert the phase 180 degrees on one of the mono tracks. (both mono tracks must be pan center)Play both tracks at the same time. The low frequencies will cancel each other out. (sometimes even guitar and vocal range depending on the song.)I am not yet sure how (or even tried) to isolate the frequencies canceled out by the phasing invert. Sometimes a little EQ tweaking can also help. This also works well to create backing tracks for bass players.
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Make or buy an Audio crossover circuit. Depending on the parts used you can filter out all of the higher frequencies so only the low end reaches the woofer.http://en.wikipedia.org/wiki/Audio_crossover Unless your talking about trying to separate a single instrument from a sound file.