An equalizer, or EQ, is a tool used in sound recording and reproduction to adjust the volume of different frequency bands within an audio signal). It is a software or hardware filter that adjusts the loudness of specific frequencies. EQs are used in recording and radio studios, production control rooms, and live sound reinforcement, as well as in instrument amplifiers, such as guitar amplifiers, to correct or adjust the response of microphones, instrument pickups, loudspeakers, and hall acoustics).

There are different types of EQs, but the most common type of consumer EQ is known as a graphic EQ. Graphic EQs typically display sliders that correspond to different frequency bands, and each slider controls the gain of a specific frequency range. The bass and treble controls on car audio systems are basic EQ controls, and they allow the user to alter the relative level of each frequency range.

Equalizers are used to modify an instruments sound or make certain instruments and sounds more prominent). They can also be used to eliminate or reduce unwanted sounds, enhance particular aspects of an instruments tone, or combat feedback in a public address system). In music production, EQs are used to adjust the timbre of individual instruments and voices by adjusting their frequency content and to fit individual instruments within the overall frequency spectrum of the mix).

In summary, an equalizer is a tool used to adjust the gain of various frequency ranges in the complete frequency range of a given output device. It is used to adjust the frequency response of audio by boosting or cutting the energy of different frequency ranges or bands. EQs are used in various settings, including recording and radio studios, production control rooms, live sound reinforcement, and instrument amplifiers).