An audio filter is a type of filter used for processing sound signals. Many types of filters exist for applications including graphic equalizers, synthesizers, sound effects, CD players and virtual reality systems.
In its simplest form, an audio filter is typically designed to pass some frequency regions through unattenuated while significantly attenuating others. In some applications, such as in the design of graphic equalizers or CD players, the filters are designed according to a set of objective criteria such as pass band, pass band attenuation, stop band, and stop band attenuation, where the pass bands are the frequency ranges for which audio is attenuated less than a specified maximum, and the stop bands are the frequency ranges for which the audio must be attenuated by a specified minimum.
In more complex cases, an audio filter can provide a feedback loop, which introduces resonance (ringing) alongside attenuation. Audio filters can also be designed to provide gain (boost) as well as attenuation.
In other applications, such as with synthesizers or sound effects, the aesthetic of the filter must be evaluated subjectively.
Audio filters can be implemented in analog circuitry as analog filters or in DSP code or computer software as digital filters.
Non-linear filters are also used in audio applications: for example, compressors, fuzz boxes, ring modulators.
Generically, the term 'audio filter' can be applied to mean anything which changes the timbre, harmonic content, pitch or waveform of an audio signal.