A filter is a process that removes frequencies from a signal.
Filter or equaliser?
An important thing to remember about filtering a signal is that once something is removed, it cannot be re-added. In recording, it is therefore often preferable to record without any filters cutting into the content of the audio source. Attenuating EQ is a milder choice that can be undone.
A common effect especially used for synthesisers and pornographic film soundtracks is a resonant low-pass filter of which the frequency follows the envelope (the volume) of the input signal, meaning transients open the filter making for a more percussive sound, as the sustain of a tone is restricted to low frequencies. An excellent example of an envelope filter in metal is Dragonaut by Sleep.
An auto-wah is a form of envelope filter that uses a band-pass filter instead of a low-pass filter. Other types of filters are less commonly used with envelopes.
Frequencies below the cutoff point pass, frequencies above the cutoff point are removed.
A low pass filter may be used to remove AC, oftentimes with the purpose of removing noise from DC power.
Frequencies above the cutoff point pass, frequencies below the cutoff point are removed.
A high pass filter may be used to remove DC, as it is effectively a 0Hz signal.
Frequencies within a certain band pass, frequencies outside the band are removed. A band pass filter can be constructed by combining a low-pass filter and a high-pass filter.
A common application for band-pass filters is the wah, which sweeps a resonant band-pass filter.
Frequencies outside a certain band pass, frequencies inside the band are removed.
No frequencies are removed. However, phase is altered around a target frequency.
Passive filters are most commonly constructed using capacitors and resistors. Inductors can be used too, though they are less commonly used due to cost and size.
An active filter combines filters with active electronics, which can boost a signal. This lets one create EQ, shelves, etc.
Filters can be created digitally and can achieve things not easily done with electronic processing. They are also perfectly consistent, which cannot be said for electronic filters.
Linear-phase filters, i.e. filters that do not alter signal phase, can be made using digital signal processing. Their drawback is that they introduce a slight signal delay. A common application for linear-phase filters is in the use of crossovers.