An audio equalizer, or EQ, alters sound by adding to or reducing frequencies within a signal. Treble and bass controls on audio equipment are two types of equalizers that adjust high and low frequencies, respectively.
An audio equalizer contains a series of filters. Depending on the equalizer and its set of filters, it applies positive and negative gain to audio signals within a certain range of frequencies. Applying negative gain, or attenuating, is also known as cut, whereas applying positive gain is boosting.
Audio software with built-in audio equalizers generally possesses enough filters to encompass a wide frequency spectrum. By helping to adjust inadequacies or inconsistencies in the tonal structure of recorded sound, the end result is that audio equalizers impart more natural-sounding warmth, resonance and color to recorded tones that sound unnatural, dry, sharp, flat or cold. Consequently, there are different methods of audio equalization that uniquely achieve this goal.
Shelving equalization is the equal boosting or attenuating of all frequencies that fall below or above a particular point. Bell equalization entails the attenuating or boosting of frequencies around a certain point, with frequencies farthest away from the point being affected to lesser degrees.
Graphic and parametric equalization are similar, except that the latter corresponds more closely to bell equalization. However, both forms allow a listener to manipulate frequencies with individual corresponding bars, sliders, knobs or controls, with each bar for a frequency spectrum. Virtual graphic and parametric equalization are most common in audio software and media players.