An equalizer is a group of filter that allows the adjustment of volume levels of each range of frequencies within an audio signal range allowing to adjust the coloration of the listening media. Equalization is a highly sophisticated art. Good equalization is critical to a good mix.

When used correctly, an equalizer can provide the impression of nearness or distance, “fatten” or “thin” a sound, and help blend or provide separation between similar sounds in a mix, allowing both to be heard as intended. It can also be used to adjust a sound system to account for the acoustical response of a room or an outdoor venue.

In sound recording and reproduction, equalization is the process commonly used to alter the frequency response of an audio system using linear filters. Most hi-fiequipment uses relatively simple filters to make bass and treble adjustments. Graphic and parametric equalizers have much more flexibility in tailoring the frequency content of an audio signal. An equalizer is the circuit or equipment used to achieve equalization. Since equalizers, “adjust the amplitude of audio signals at particular frequencies,” they are, “in other words, frequency-specific volume knobs.”

Equalizers are used in recording studios, broadcast studios, and live sound reinforcement to correct the response of microphones, instrument pick-ups, loudspeakers, and hall acoustics. Equalization may also be used to eliminate unwanted sounds, make certain instruments or voices more prominent, enhance particular aspects of an instrument’s tone, or combat feedback (howling) in a public address system. Equalizers are also used in music production to adjust the timbre of individual instruments by adjusting their frequency content and to fit individual instruments within the overall frequency spectrum of the mix.

The most common equalizers in music production are graphic, semi-parametric, parametric and dynamic.

  • Graphic equalizers in consumer audio equipment can be provided with a single adjustment as a tone control that provides an adjustable frequency with a fixed Q or width. Other products provide a four to 10 band adjustment with a fixed Q or width.
  • Graphic equalizers in professional applications are provided with a 31-band or 1/3 octave adjustment of a 6 or 12 dB gain or boost. This provides 3 adjustments per octave.
  • Semi-parametric equalization in provided on each channel on a mixing board that provides the frequency adjustment with gain with a fixed Q or width.
  • Parametric equalizers provides the frequency adjustment with gain and an adjustable Q or frequency width. Depending on the application can provide from 6 to 20 bands of adjustments.
  • Dynamic equalization is used to focus on a frequency range of an input and when active or another input is active may cut or boost this setting to provide the sound required for that application.

In sound recording, equalization is used to improve an instrument’s sound or make certain instruments and sounds more prominent. For example, a recording engineermay use an equalizer to make some high-pitches in a vocal part louder while making low-pitches in a drum part quieter.

Equalization is commonly used to increase the ‘depth’ of a mix, creating the impression that some sounds in a mono or stereo mix are farther or closer than others, relatively. Equalization is also commonly used to give tracks with similar frequency components complementary spectral contours, known as mirrored equalization. Select components of parts which would otherwise compete, such as bass guitar and kick drum, are boosted in one part and cut in the other, and vice versa, so that they both stand out.

Equalizers can correct problems posed by a room’s acoustics, as an auditorium will generally have an uneven frequency response especially due to standing wavesand acoustic dampening. The frequency response of a room may be analyzed using a spectrum analyzer and a pink noise generator for instance. Then a graphic equalizer can be easily adjusted to compensate for the room’s acoustics. Such compensation can also be applied to tweak the sound quality of a recording studio in addition to its use in live sound reinforcement systems and even home hi-fi systems.

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. When the sound produced by the speakers is picked up by a microphone, it is further reamplified; this recirculation of sound can lead to “howling” requiring the sound technician to reduce the gain for that microphone, perhaps sacrificing the contribution of a singer’s voice for instance. Even at a slightly reduced gain, the feedback will still cause an unpleasant resonant sound around the frequency at which it would howl. But because the feedback is troublesome at a particular frequency, it is possible to cut the gain only around that frequency while preserving the gain at most other frequencies. This can best be done using a parametric equalizer tuned to that very frequency with its amplitude control sharply reduced. By adjusting the equalizer for a narrow bandwidth (high Q), most other frequency components will not be affected. The extreme case when the signal at the channel’s center frequency is completely eliminated is known as a notch filter.

An equalizer can be used to correct or “flatten” the frequency response of speakers rather than designing the speaker itself to be equalized. For instance, the Bose 901 speaker system doesn’t use separate woofers and tweeters to cover the bass and treble frequencies, but includes 9 full-range drivers more akin to what one would find in a table radio. However this speaker system is sold with an active equalizer designed to correct the poor frequency balance of those drivers. That equalizer must be inserted into the amplifier system so that the amplified signal that is finally sent to the speakers has its response increased at the frequencies where the response of these drivers falls off, producing a high fidelity reproduction regardless.

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 bass control may be used, for instance, to increase the drum and bass parts at a dance party, or to reduce annoying bass sounds when listening to a person speaking. 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.

A “rumble filter” is a high pass (low cut) filter with a cutoff typically in the 20 to 40 Hz range; this is the low frequency end of human hearing. “Rumble” is a type of low frequency noise produced in record players and turntables, particularly older or low quality models. The rumble filter prevents this noise from being amplified and sent to the loudspeakers. Some cassette decks have a switchable “Subsonic Filter” feature that does the same thing for recordings.


A graphic EQ typically consists of a bank of slider controls used to boost or cut fixed frequency bands. A well-designed graphic EQ creates an output frequency response that corresponds as closely as possible to the curve displayed graphically by the sliders. Designers of analog EQs must carefully choose the bandwidth of the filter and decide how the bandwidth should vary with gain and how the filters are summed or cascaded. In general, narrower bandwidth signifies a more precise EQ.

Some high-end digital graphic EQs offer greater precision.  This is the approach PreSonus took with the StudioLive mixer: The graphic EQ is a pool of shelving filters from which coefficients like cutoff frequency, bandwidth, and gain are extracted through a process of curve-fitting. The curve entered by the user is first oversampled. The system then works with an internal curve made up of 128 bands to find coefficients for the first shelving filter that, when subtracted from the user’s curve, will produce the flattest possible response: 0 dB. The resulting response is then used to find coefficients for the second shelving filter, using the same optimization process. Coefficients for all available shelving filters are found through a recursive process.

Unlike conventional designs, the frequency and bandwidth of the “bands” depends on the curve entered by the user. This allows for much tighter matching of that curve. Because of this innovative design, the curve fitting-process is capable of very steep transitions, and unlike conventional, analog graphic EQs, what you see is what you get. With a carefully drawn, smooth curve, the StudioLive EQ will have almost no frequency ripple.

Graphic EQs are generally used to fine-tune the overall mix for a particular room. For instance, if you are mixing in a “dead” room, you may want to boost high frequencies and roll off some of the lows. If you are mixing in a “live” room, you might need to lower the high-midrange and highest frequencies. In general, you should not make drastic amplitude adjustments to any particular frequency bands. Instead, make smaller, incremental adjustments over a wider spectrum to round out your final mix.
In general, most graphic EQs have between 7 and 31 bands. Professional sound-reinforcement graphic EQs generally have 31 bands, and the center frequency of each band is spaced 1/3 of an octave away from the center frequency of the adjacent bands, so that three bands (three sliders on the front panel) cover a combined bandwidth of one octave. Graphic EQs with half as many bands per octave are generally used when less precision is needed. You will often find this 2/3-octave design on monaural, 15-band (or fewer) graphic EQs in guitar amps, bass amps, and some stompboxes. In traditional graphic EQ designs, the center frequency of each band is fixed.

In the graphic equalizer, the input signal is sent to a bank of filters. Each filter passes the portion of the signal present in its own frequency range orband. The amplitude passed by each filter is adjusted using a slide control to boost or cut frequency components passed by that filter. The vertical position of each slider thus indicates the gain applied at that frequency band, so that the knobs resemble a graph of the equalizer’s response plotted versus frequency.

The number of frequency channels (and therefore each one’s bandwidth) affects the cost of production and may be matched to the requirements of the intended application. A car audio equalizer might have one set of controls applying the same gain to both stereo channels for convenience, with a total of five to ten frequency bands. On the other hand, an equalizer for professional live sound reinforcement typically has some 25 to 31 bands, for more precise control of feedback problems and equalization of room modes. Such an equalizer (as shown above) is called a 1/3-octave equalizer (spoken informally as “third-octave EQ”) because the center frequency of its filters are spaced one third of an octave apart, three filters to an octave. Equalizers with half as many filters per octave are common where less precise control is required—this design is called a 2/3-octave equalizer.

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