rlpf in freq rq

   i - input signal
   f - frequency (hertz)
  rq - reciprocal of Q

A resonant low pass filter.

> import Sound.DF

> do { n <- white_noise_m
>    ; f <- fmap (\x -> x * 40.0 + 220.0) (sin_osc 0.5 0.0)
>    ; r <- rlpf n f 0.1
>    ; audition [] (out1 r) }