An adjustable audio filter inserted in a circuit to divide and adjust its frequency response, altering or distorting the relative amplitude of certain frequency ranges of an audio signal. The effects processor used for equalization. Equalizers come in two varieties, graphic and parametric. A graphic equalizer typically has a number of fixed-frequency bands (5-10 in consumer equipment, 31 in professional equipment), each wired to its own front-panel slider. The control is over the amount of cut or boost (in dB) at each band. A parametric equalizer goes two steps further: the center frequency of each band can be selected by the user, as can the bandwidth. This affords more precise control over which frequencies will be affected by the boost or cut in amplitude. Because EQ circuitry with these controls is more expensive to build, a parametric equalizer will typically provide fewer bands than a graphic equalizer. A semi-parametric equalizer, sometimes found in multieffects devices, provides control over the center frequency of each band, but not over the bandwidth.
See also active equalizer, passive equalizer, shelving equalizer, Q.