Equalization
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In audio processing, equalization (EQ) is the process of modifying the frequency envelope of a sound. Etymologically, it means to correct, or make equal, the frequency response of another audio device. The term "equalizer" is sometimes applied to audio filters in general, though strictly speaking not all audio filters are equalizers.
Overview
There are a number of kinds of EQ. Each has a different pattern of attenuation or boost. A peaking equaliser raises or lowers a range of frequencies around a central point in a bell shape. A peaking equalizer with controls to adjust the level (Gain), bandwidth (Q) and center frequency is called a parametric equalizer. If there is no control for the bandwidth (it is fixed by the designer) then it is called a quasi-parametric or semi-parametric equalizer.
Shelving type equalizers increase or attenuate the level of a wide range of frequencies by a fixed amount. A "low shelf" will affect low frequencies up to certain point and then above that point will have little effect. A high shelf affects the level of high frequencies, while below a certain point, the low frequencies are unaffected.
One common type of equalizer is a graphic equalizer, which consists of a bank of band-pass filters, with an independent gain control for each band. Normally, these bands are tight enough to give at least 3dB or 6dB maximum effect for neighboring bands, and cover the range from around 20Hz to 20kHz (which is approximately the range of human hearing). A simple equalizer might have bands at 20Hz, 200Hz, 2kHz and 20kHz, and might be referred to as a 4-band equalizer. A typical equalizer for live microphone work might have as many as 24 or 32 bands.
Uses
Early telephone systems used equalization to correct for the reduced level of high frequencies in long cables. For example, a particular microphone might be more sensitive to low frequency sounds than to high frequency sounds, so an equalizer would be used to increase the volume of the higher frequencies (boost), and reduce the volume of the low frequency sounds (cut).
One of the most direct uses of equalization is at a live event, where there are microphones and speakers operating simultaneously. An equalizer is used to ensure that there are no frequency bands where there is a round trip gain of greater than 1, as these are heard as audible feedback. Those frequencies are cut at the equalizer to prevent this.
In practice, there is a degree of aesthetics involved, in addition to technical considerations.