Equalization (or equalisation, EQ) is the process of changing the frequency envelope of a sound in audio processing. In passing through any channel, an audio signal will "spread" from its original qualities. The goal of equalization is to correct, or make equal, the frequency response of a signal.
The term "equalizer" is often incorrectly applied to audio filters, such as those included on DJ mixing equipment and hi-fi audio components. However, these "equalizers" are typically general all-purpose audio filters, which can be arranged to produce the effect of low pass, high pass, band pass and band stop filters. Such filters are true equalizers only when arranged to reverse the effects of internal circuitry on sound output.
A pass filter attenuates either high or low frequencies while allowing other frequencies to pass unfiltered. A high-pass filter modifies a signal only by taking out low frequencies; a low-pass filter only modifies the audio signal by taking out high frequencies. A pass filter is described by its cut-off point and slope. The cut-off point is the frequency where high or low-frequencies will be removed. The slope, given in decibels per octave, describes a ratio of how the filter attenuates frequencies past the cut-off point (eg. 12 dB per octave). A band-pass filter is simply a combination of one high-pass filter and one low-pass filter which together allow only a band of frequencies to pass, attenuating both high and low frequencies past certain cut-off points.
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 a 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 the graphic equalizer, which consists of a bank of sliders for boosting and cutting different bands (or frequencies ranges) of sound. Normally, these bands are tight enough to give at least 3 dB or 6 dB maximum effect for neighboring bands, and cover the range from around 20 Hz to 20 kHz (which is approximately the range of human hearing). A simple equalizer might have bands at 20 Hz, 200 Hz, 2 kHz and 20 kHz, and might be referred to as a 4-band equalizer. A typical equalizer for live sound reinforcement might have as many as 24 or 31 bands. A typical 31-band equalizer is also called a 1/3-octave equalizer because the center frequencies of sliders are spaced one third of an octave apart.
In Multitrack recording and sound reinforcement systems, individual channels have equalization for aesthetic reasons, while the combined mix of sound is processed through equalization for practical reasons. Any acoustic space will cause some sound frequencies to be louder than others. This is due to standing waves produced by the size of the room and the materials in it. Equalization is used to compensate for the discrepancies of a room's acoustics. Ideally, a sound system would produce a flat frequency response. The frequency response of a room is examined with a Spectrum analyzer and usually a graphic equalizer, with matching frequency bands, is used to compensate for the room acoustics. This is standard practice for sound recording studios, live sound reinforcement systems and some High fidelity sound systems.
One of the most direct uses of equalization is at a live event, where microphones and speakers operate 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.
All audio records, or vinyls, have had equalization applied to the sound waveform before the consumers' record was made because of the limitations of equipment for recording and manufacturing the record. One scheme was used prior to 1940. Some 100 formulae were used until 1955, when the RIAA standard formula was implemented. As an example of the use of equalization in record production, low frequencies are reduced before the sound is imprinted onto the vinyl, making the groove take up less physical space so that more music can fit on the record. For this reason, record players boost the low frequencies back up to their original level before playback, to compensate for the reduction during printing.
Early telephone systems used equalization to correct for the reduced level of high frequencies in long cables, typically using Zobel networks. These kinds of equalizers can also be used to produce a circuit with a wider bandwidth than the standard telephone band of 300 Hz to 3.4 kHz. This was particulary useful for broadcasters who needed "music" quality, not "telephone" quality on landlines carrying program material. It is necessary to remove or cancel any loading coils in the line before equalization can be successful. Equalization was also applied to correct the response of the transducers, 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).
Modern digital telephone systems have less trouble in the voice frequency range as only the local line to the subscriber now remains in analog format, but DSL circuits operating in the MHz range on those same wires may suffer severe attenuation distortion which is dealt with by automatic equalization or by abandoning the worst frequencies. Picturephone circuits also had equalizers.
The individual channels of a mixing board and the sound of electric instruments are equalized for aesthetic reasons. Some guitar effects units, in particular, the wah-wah pedal is based on equalization. Equalization is used to manipulate the timbre of musical instruments and sounds.