Audio filters work on the frequency of an audio signal to attenuate unwanted frequency and amplify wanted ones. They are used within anything related to sound, whether it is radio communication or a hi-fi system.

• https://www.masteringbox.com/filter-types/
• http://ethanwiner.com/filters.html
• https://en.wikipedia.org/wiki/Audio_filter
• https://en.wikipedia.org/wiki/Electronic_filter