This module is a kind of low pass filter. It works by calculating the
average sample from the number of samples contained in one cycle of a
particular frequency. The lp_filter module is very slow for low
frequency cutoff points and very inaccurate for high frequency cutoff
points. You're better off using the LADSPA plugin modules instead
(GLAME).

See also: hp_filter glame_filter glame_butterworth
