SubFilter is a freeware subsonic filter plug-in with a selectable cutoff slope of -12, -24 or -36dB/Octave.
The filter is extremely useful for removing thumps, bumps and low frequency rumble noise from your audio.
SubFilter has 2 controls: cutoff slope and crossover frequency. The cutoff frequency is variable from 10Hz to 200Hz. The passband is flat (Butterworth filter) and has unity gain.