Equalization (audio)

Equalization, or simply EQ, in sound recording and reproduction is the process of adjusting the volume of different frequency bands within an audio signal.

Broadcast and recording studios use sophisticated equalizers capable of much more detailed adjustments, such as eliminating unwanted sounds or making certain instruments or voices more prominent.

[3]: 73–74 The concept of equalization was first applied in correcting the frequency response of telephone lines using passive filters; this was prior to the invention of electronic amplification.

Later the concept was applied in audio engineering to adjust the frequency response in recording, reproduction, and live sound reinforcement systems.

These are most often in the form of variable bass and treble controls, and switches to apply low-cut or high-cut filters for elimination of low-frequency rumble and high-frequency hiss respectively.

Graphic equalizers and other equipment developed for improving fidelity have since been used by recording engineers to modify frequency responses for aesthetic reasons.

Later on, as other manufacturers started to market their products, these British companies began touting their equalizers as being a cut above the rest.

Audio electronic equipment evolved to incorporate filtering elements as consoles in radio stations began to be used for recording as much as broadcast.

[9] Born in Salt Lake City, Davis worked in Southern California most of his life for a series of companies including Cinema Engineering (from 1938), Langevin, Electrodyne, Cetec and Altec.

[10] The first true graphic equalizer was the type 7080, an active tube device developed in the 1950s by Davis's Cinema Engineering company.

Davis followed this in 1961 with the Langevin EQ-252-A having seven sliders, then reworked it for Altec Lansing to create the Model 9062A EQ which sold well into the 1970s.

In May 1972 Massenburg used the term parametric equalization in a paper presented at the 42nd convention of the Audio Engineering Society.

[15] Most channel equalization on mixing consoles made from 1971 to the present day rely upon the designs of Flickinger, Massenburg and McNeal in either semi or fully-parametric topology.

These are broad adjustments designed more to increase the listener's satisfaction than to provide actual equalization in the strict sense of the term.

The user can raise or lower each slider in order to visually approximate a graph of the intended frequency response.

Equalization in the context of audio reproduction is not used strictly to compensate for the deficiency of equipment and transmission channels.

An example of this is a low-cut or rumble filter, which is used to remove infrasonic energy from a program that may consume undue amplifier power and cause excessive diaphragm excursions in (or even damage to) loudspeakers.

An example of this is a high-cut or hiss filter, which is used to remove annoying white noise at the expense of the crispness of the program material.

High and low-pass filters are used in audio crossovers to direct energy to the speaker drivers capable of reproducing it.

However, the usual tone controls have a more limited range, since their purpose is not to eliminate any frequencies but only to achieve a greater balance when, for instance, the treble is lacking and the sound is not crisp.

In some cases, semi-parametric equalizers allow the user to select between a wide and a narrow preset bandwidth.

These can be described according to their so-called pole and zero frequencies, which are complex numbers in the case of second-order responses.

[a] The bass and treble controls in a hi-fi system are each a first-order filter in which the balance of frequencies above and below a point are varied using a single knob.

Thus a parametric equalizer section will have three controls for its center frequency F0, bandwidth or Q, and the amount of boost or cut usually expressed in dB.

During live events where signals from microphones are amplified and sent to speaker systems, equalization is not only used to "flatten" the frequency response but may also be useful in eliminating feedback.

For instance, the Bose 901 speaker system does not use separate larger and smaller drivers to cover the bass and treble frequencies.

[22] Tone controls (usually designated "bass" and "treble") are simple shelving filters included in most hi-fi equipment for gross adjustment of the frequency balance.

The treble control might be used to give the percussion a sharper or more "brilliant" sound, or can be used to cut such high frequencies when they have been overemphasized in the program material or simply to accommodate a listener's preference.

"Rumble" is a type of low-frequency noise produced in record players and turntables, particularly older or low quality models.

The preamplifier attached to the phono cartridge has a complementary filter boosting those low frequencies, following the standard RIAA equalization curve.