BUTTERHP, BUTTERLP, BUTTERBP, BUTTERBR

These generators are implementations of second-order hipass, lopass, bandpass, and bandreject butterworth filters. They offer an almost flat passband and very good precision and stopband attenuation. I find the formnt generator produces more musically pleasing results than butterbp, but your mileage may vary.
  make-butterhp frequency
  make-butterlp frequency

  make-butterbp frequency bandwidth
  make-butterbr frequency bandwidth

  butterhp b signal
  butterlp b signal
  butterbp b signal
  butterbr b signal

frequency is the cutoff or center frequency in hertz, and bandwidth is the bandwidth in hertz. Do not set bandwidth > (/ *srate* 2).

The values for frequency and bandwidth can be set within a run loop:

  set-butterhp b frequency
  set-butterlp b frequency

  set-butterbp b frequency bandwidth
  set-butterbp-frequency b frequency
  set-butterbp-bandwidth b bandwidth

  set-butterbr b frequency bandwidth
  set-butterbr-frequency b frequency
  set-butterbr-bandwidth b bandwidth