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Equalization, equalisation or EQ is the process of using passive or active electronic elements or digital algorithms for the purpose of altering (originally flattening) the frequency response characteristics of a system.[1]

Amplitude equalization is usually meant when it is stated without qualification but any frequency dependent response characteristic is capable of having equalization applied. Most notably there is phase and time-delay equalizations. There is also spatial directivity equalization.[2]



There are many kinds of EQ. Each has a different pattern of attenuation or boost. A peaking equalizer 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 (Hz) 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.

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 a combination (in series) 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.

Variable equalization was first used by John Volkman working at RCA in the 1920s. They were used to equalize a motion picture theater playback systems.[3]

Graphic equalizer

1/3-octave graphic equalizer with LEDs that show signal presence per frequency band]]

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. The number and width of filters depends on application. A simple car audio equalizer might have one bank of filters controlling two channels for easy adjustment of stereo sound, and contain five to ten filter bands. A typical equalizer for professional live sound reinforcement has some 25 to 31 bands, necessary for quick control of feedback tones and room modes. Such an equalizer is called a 1/3-octave equalizer (spoken informally as "third-octave EQ") because the center frequency of each filter is spaced one third of an octave away from its neighbors, three filters to an octave. Equalizers with half as many filters per octave are common where less precise general tone-shaping is desired—this design is called a 2/3-octave equalizer.

Historically, the first use of slide controls in an equalizer was in the Langevin Model EQ-251A which featured two passive equalization sections, a bass shelving filter and a pass band filter. Each filter had switchable frequencies and used a 15 position slide switch to adjust cut or boost.[4] The first true graphic equalizer was the type 7080 developed by Art Davis's Cinema Engineering. It featured 6 bands with a boost or cut range of 8 dB. It used a slide switch to adjust each band in 1 dB steps. Davis's second graphic equalizer was the Altec Lansing Model 9062A EQ. In 1967 Davis developed the first 1/3 octave variable notch filter set, the Altec-Lansing "Acousta-Voice" system.[5]


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.

Most audio records, 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 particularly 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.

See also


  1. ^ Ballou, p571.
  2. ^ Ballou, p426.
  3. ^ H. Tremaine, Audio Cyclopedia, 2nd. Ed., (H.W. Sams, lndianapolis, 1973)
  4. ^ Langevin EQ-251A Schematic
  5. ^ Operator Adjustable Equalizers: An Overview

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Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

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Alternative spellings


equal + -ization




equalization (plural equalizations)

  1. The act of equalizing, or state of being equalized.

Related terms



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