As we have detailed, the main filter types are high- (HPF) and low-pass (LPF). Set your cutoff frequency with either type at, say, 500Hz and the HPF will allow everything above 500Hz to pass; the LPF allows everything below 500Hz to pass.
Band-pass filters
Band-pass filters allow a set range of frequencies to pass, so you can set the width and the slope of this range to very narrow and steep (using a high Q factor) to home in on very specific ranges to allow through. Do the same with a notch filter to surgically remove narrow frequency bands - great for removing glitches or noise from a mix.
Shelf filters boost or attenuate larger bandwidths of frequencies and look, you guessed it, like a shelf on the UI of a parametric EQ. These are used for wider but more subtle boosting or cutting of frequency ranges, not so much as creative effects (like a narrow notch or band-pass), but typically to brighten or darken a mix in a less dramatic fashion.
Dynamic and creative filters
More recent filter plugins have introduced modulation into the effects process, so parameters within the filter can be controlled by envelopes, step sequencers, LFOs and a lot more. This raises the bar from the typical filter sweeps we mention above, so it's not 'just' your typical filter frequency or resonance controls that are recorded or automated. Now filters have become modulation monsters with everything from those typical filter parameters to full-on analogue saturation or distortion being modulated by a range of sources. Filters have become characterful beasts, either creating the sound of classic hardware synths (see Arturia) or a sound all of their own with colour, distortion, delays and more pushing the humble filter into the world of the multi-effect.
It's fair to say, then, that while the best filter plugins use filtering as their base ingredient, most go a lot further, taking your creativity as far as your imagination allows.
Free filters
As with all plugin effects, you don't have to spend a fortune on filter plugins, and we have included filters across a wide range of prices. There are also free filters available; our favourites of these include TAL-Filter-2, the simple but effective AudiThing Filterjam, Ohmforce Fromage and The FilterBank plugin by Wolfgang Krumme.