Audio equalization, more commonly referred to as EQ, is the process of altering the frequency response of an audio signal. The purpose of equalization is to increase or decrease the amplitude of chosen frequency components in the signal. This is achieved by applying an audio filter.
EQ can be applied in a variety of situations and for a variety of reasons. Sometimes, the frequencies of the original audio signal may have been affected by the physical response of the microphones or loudspeakers, and the audio engineer wishes to adjust for these factors. Other times, the listener or audio engineer might want to boost the low end for a certain effect, “even out” the frequencies of the instruments, or adjust frequencies of a particular instrument to change its timbre, to name just a few of the many possible reasons for applying EQ.
Equalization can be achieved by either hardware or software. Two commonly-used types of equalization tools are graphic and parametric EQs. Within these EQ devices, low-pass, high-pass, bandpass, bandstop, low shelf, high shelf, and peak-notch filters can be applied.