TheInfoList

Equalization in
sound recording and reproduction Sound recording and reproduction is an electrical, Mechanical system, mechanical, electronic, or digital inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of ...
is the process of adjusting the volume of different frequency bands within an
audio signal An audio signal is a representation of sound In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order ...
. The circuit or equipment used to achieve this is called an equalizer. Most
hi-fi High fidelity (often shortened to Hi-Fi or HiFi) is the high-quality reproduction of sound. It is important to audiophile An audiophile is a person who is enthusiastic about high-fidelity High fidelity (often shortened to Hi-Fi or HiFi) i ...
equipment uses relatively simple filters to make
bass Bass or Basses may refer to: Fish * Bass (fish) Bass () is a name shared by many species of fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in thi ...
and treble adjustments. Graphic and parametric equalizers have much more flexibility in tailoring the frequency content of 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. 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 studio A recording studio is a specialized facility for Sound recording and reproduction, sound recording, Audio mixing (recorded music), mixing, and audio production of instrumental or vocal musical performances, spoken words, and other sounds. T ...

s,
radio studio A recording studio is a specialized facility for sound recording Sound recording and reproduction is an electrical Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan S ...

s and
production control room The production control room (PCR) or studio control room (SCR) is the place in a television studio (MDR) A television studio, also called a television production studio, is an installation room in which video productions take place, either for ...
s, and live
sound reinforcement A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeaker A loudspeaker is an Acoustical engineering#Electroacoustics, electroacoustic transducer; a device which converts an electrical ...
and in
instrument amplifier An instrument amplifier is an electronic device The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons The electron is a subatomic particle, symbol or , w ...
s, such as
guitar amplifier A guitar amplifier (or amp) is an that strengthens the weak electrical signal from a on an , , or so that it can produce sound through one or more s, which are typically housed in a wooden . A guitar amplifier may be a standalone wood or met ...
s, to correct or adjust the response of
microphone A microphone, colloquially called a mic or mike (), is a device – a transducer A transducer is a device that energy from one form to another. Usually a transducer converts a in one form of energy to a signal in another. Transducers ar ...

s, instrument pick-ups,
loudspeakers A loudspeaker (or ''speaker driver'', or most frequently just ''speaker'') is an Acoustical engineering#Electroacoustics, electroacoustic transducer, that is, a device that converts an electrical audio signal into a corresponding sound. A ''spe ...
, and hall acoustics. Equalization may also be used to eliminate or reduce unwanted sounds (e.g., low-frequency hum coming from a guitar amplifier), make certain instruments or voices more (or less) prominent, enhance particular aspects of an instrument's tone, or combat
feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain A chain is a assembly of connected pieces, called links, typically made of metal, with an overall character similar to that of a in that it is flexib ...
(howling) in a
public address A public address system (or PA system) is an electronic system comprising microphone A microphone, colloquially called a mic or mike (), is a device – a transducer A transducer is a device that energy from one form to another. Us ...
system. Equalizers are also used in
music production A record producer is a recording project's creative and technical leader, commanding studio time and coaching artists, and in popular genres typically creates the song's very sound and structure. Virgil Moorefield"Introduction" ''The Producer as ...
to adjust the
timbre In music, timbre ( ), also known as tone color or tone quality (from psychoacoustics Psychoacoustics is the branch of psychophysics Psychophysics quantitatively investigates the relationship between physical stimulus (physiology), stimuli a ...

of individual instruments and voices by adjusting their frequency content and to fit individual instruments within the overall frequency spectrum of the
mix MIX is a hypothetical computer used in Donald Knuth's monograph, '' The Art of Computer Programming'' (''TAOCP''). MIX's model number is 1009, which was derived by combining the model numbers and names of several contemporaneous, commercial machi ...
.

# Terminology

The concept of equalization was first applied in correcting the
frequency response In electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons The electron is a subatomic particle In physical sciences, subatomic particles ...
of
telephone lines A telephone line or telephone circuit (or just line or circuit industrywide) is a single-user circuitCircuit may refer to: Science and technology Electrical engineering * Electrical circuit, a complete electrical network with a closed-l ...
using
passive Passive may refer to: * Passive voice, a grammatical voice common in many languages, see also Pseudopassive (disambiguation), Pseudopassive * Passive language, a language from which an interpreter works * Passivity (behavior), the condition of sub ...
networks; this was prior to the invention of electronic amplification. Initially, equalization was used to "compensate for" (i.e., correct) the uneven frequency response of an electric system by applying a filter having the opposite response, thus restoring the
fidelity Fidelity is the quality of faithfulness Eugene Santos or Faithfulness is the concept of unfailingly remaining loyal to someone or something, and putting that loyalty into consistent practice regardless of extenuating circumstances. It may be ex ...
of the
transmission Transmission may refer to: Science and technology * Power transmissionPower transmission is the movement of energy from its place of generation to a location where it is applied to perform useful Mechanical work, work. Power (physics), Power is d ...
. A plot of the system's net frequency response would be a flat line, as its response at any frequency would be equal to its response at any other frequency. Hence the term "equalization." Much later the concept was applied in
audio engineering An audio engineer (also known as a sound engineer or recording engineer) helps to produce a recording A record, recording or records may refer to: An item or collection of data Computing * Record (computer science), a data structure ** Record, ...
to adjust the frequency response in recording, reproduction, and live
sound reinforcement system A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers in Loudspeaker enclosure, enclosures all controlled by a mixing console that makes live or pre-recorded sounds louder and may also d ...

s. Sound engineers correct the frequency response of a sound system so that the frequency balance of the music as heard through speakers better matches the original performance picked up by a
microphone A microphone, colloquially called a mic or mike (), is a device – a transducer A transducer is a device that energy from one form to another. Usually a transducer converts a in one form of energy to a signal in another. Transducers ar ...

.
Audio amplifiers Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to: Sound *Audio signal, an electrical representation of sound *Audio frequency, a frequency in the audio spectrum *Digital audio, representation of sound ...

have long had filters or controls to modify their frequency response. These are most often in the form of variable
bass Bass or Basses may refer to: Fish * Bass (fish) Bass () is a name shared by many species of fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in thi ...
and treble controls (shelving filters), 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 engineer An audio engineer (also known as a sound engineer or recording engineer) helps to produce a sound recording, recording or a live performance, balancing and adjusting sound sources using equalization (audio), equalization, Dynamic range compressi ...
s to modify frequency responses for aesthetic reasons. Hence in the field of audio electronics the term "equalization" is now broadly used to describe the application of such filters regardless of intent. This broad definition therefore includes all
linear filter Linearity is the property of a mathematical relationship (''function (mathematics), function'') that can be graph of a function, graphically represented as a straight Line (geometry), line. Linearity is closely related to Proportionality (mathema ...
s at the disposal of a listener or engineer. A British EQ or British style equalizer is one with similar properties to those on consoles made in the UK by companies such as Amek, Neve and
Soundcraft Soundcraft is a British designer and importer (formerly a manufacturer) of mixing console Sound Mixer Full In sound recording and reproduction In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epist ...
from the 1950s through to the 1970s. Later on, as other manufacturers started to market their products, these British companies began touting their equalizers as being a cut above the rest. Today, many non-British companies such as
Behringer Behringer is an audio equipment Audio equipment refers to devices that reproduce, record, or process sound In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''ph ...
and
Mackie Mackie is an American professional audio Professional audio, abbreviated as pro audio, refers to both an activity and a category of high quality, studio-grade audio equipment. Typically it encompasses sound recording, sound reinforcement sy ...
advertise British EQ on their equipment. A British style EQ seeks to replicate the qualities of the expensive British
mixing console In sound recording and reproduction Sound recording and reproduction is an electrical, Mechanical system, mechanical, electronic, or digital inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music ...

s.

# History

Filtering audio frequencies dates back at least to
acoustic telegraphyAcoustic telegraphy (also known as harmonic telegraphy) was a name for various methods of multiplexing In telecommunications and computer networks, multiplexing (sometimes contracted to muxing) is a method by which multiple analog or digital si ...
and
multiplexing In telecommunications Telecommunication is the transmission of information by various types of technologies over , radio, , or other systems. It has its origin in the desire of humans for communication over a distance greater than that ...
in general. Audio electronic equipment evolved to incorporate filtering elements as consoles in radio stations began to be used for recording as much as broadcast. Early filters included basic bass and treble controls featuring fixed frequency centers, and fixed levels of cut or boost. These filters worked over broad frequency ranges. Variable equalization in audio reproduction was first used by John Volkman working at
RCA The RCA Corporation was a major American electronics company, which was founded as the Radio Corporation of America in 1919. It was initially a patent trust owned by General Electric General Electric Company (GE) is an American multinatio ...
in the 1920s. That system was used to equalize a motion picture theater sound playback system. The Langevin Model EQ-251A was the first equalizer to use slide controls. It 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. The first true graphic equalizer was the type 7080 developed by
Art Davis Art Davis (December 5, 1934 – July 29, 2007) was a double-bassist, known for his work with Thelonious Monk, John Coltrane, Dizzy Gillespie, McCoy Tyner and Max Roach. Biography Davis was born in Harrisburg, Pennsylvania, where he began studying ...
's Cinema Engineering. It featured 6 bands with a boost or cut range of 8 . It used a slide switch to adjust each band in 1 dB steps. Davis's second graphic equalizer was the
Altec Lansing Altec Lansing, Inc. is a U.S. audio electronics Audio electronics is the implementation of electronic circuit 200px, A circuit built on a printed circuit board (PCB). An electronic circuit is composed of individual electronic component A ...

Model 9062A EQ. In 1967 Davis developed the first 1/3 octave variable notch filter set, the Altec-Lansing "Acousta-Voice" system. In 1966, Burgess Macneal and
George Massenburg 280px, George Massenburg Labs's booth on the AES Convention George Y. Massenburg (born Baltimore, Maryland) is a recording engineer and inventor. Working principally in Baltimore, Los Angeles, Nashville, and Macon, Georgia, Massenburg is widely kno ...
began work on a new recording console. Macneal and Massenburg, who was still a teenager, conceptualized an idea for a sweep-tunable EQ that would avoid inductors and switches. Soon after, Bob Meushaw, a friend of Massenburg, built a three-band, frequency adjustable, fixed-Q equalizer. When asked who invented the parametric equalizer, Massenburg stated “four people could possibly lay claim to the modern concept: Bob Meushaw, Burgess Macneal, Daniel Flickinger, and myself… Our (Bob’s, Burgess’ and my) sweep-tunable EQ was borne, more or less, out of an idea that Burgess and I had around 1966 or 1967 for an EQ… three controls adjusting, independently, the parameters for each of three bands for a recording console… I wrote and delivered the AES paper on Parametrics at the Los Angeles show in 1972… It’s the first mention of `Parametric’ associated with sweep-tunable EQ.” Daniel N. Flickinger introduced the first parametric equalizer in early 1971. His design leveraged the high performance op-amp of his own design, the 535 series (USPTO #3727896) to achieve filtering circuits that were before impossible. Flickinger's patent (USPTO #3752928) from early in 1971 showed the circuit topology that would come to dominate audio equalization until the present day, as well as the theoretical underpinnings of the elegant circuit. Instead of slide potentiometers working on individual bands of frequency, or rotary switches, Flickinger's circuit allowed completely arbitrary selection of frequency and cut/boost level in three overlapping bands over the entire audio spectrum. Six knobs on his early EQ's would control these sweepable filters. Up to six switches were incorporated to select shelving on the high and low bands, and bypassing for any unused band for the purest signal path. His original model boasts specifications that are seldom met today. Other similar designs appeared soon thereafter from
George Massenburg 280px, George Massenburg Labs's booth on the AES Convention George Y. Massenburg (born Baltimore, Maryland) is a recording engineer and inventor. Working principally in Baltimore, Los Angeles, Nashville, and Macon, Georgia, Massenburg is widely kno ...
(in 1972) and Burgess McNeal from ITI corp. In May 1972 Massenburg introduced the term ''parametric equalization'' in a paper presented at the 42nd convention of the
Audio Engineering Society The Audio Engineering Society (AES) is a professional body for engineers, scientists, other individuals with an interest or involvement in the professional audio Professional audio, abbreviated as pro audio, refers to both an activity and a ...
. Most channel equalization on
mixing console In sound recording and reproduction Sound recording and reproduction is an electrical, Mechanical system, mechanical, electronic, or digital inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music ...

s made from 1971 to the present day rely upon the designs of Flickinger, Massenburg and McNeal in either semi or fully parametric topology. In the late 1990s and in the 2000s, parametric equalizers became increasingly available as
digital signal processing Digital signal processing (DSP) is the use of digital processing Digital data, in information theory and information systems, is information represented as a string of discrete symbols each of which can take on one of only a finite number of ...
(DSP) equipment, usually in the form of plug-ins for various digital audio workstations. Standalone outboard gear versions of DSP parametric equalizers were also quickly introduced after the software versions and are typically called Digital Parametric Equalizers.

# Filter types

Although the range of equalization functions is governed by the theory of
linear filter Linearity is the property of a mathematical relationship (''function (mathematics), function'') that can be graph of a function, graphically represented as a straight Line (geometry), line. Linearity is closely related to Proportionality (mathema ...
s, the adjustment of those functions and the flexibility with which they can be adjusted varies according to the topology of the circuitry and controls presented to the user. Shelving controls are usually simple first-order filter functions which alter the relative gains between frequencies much higher and much lower than the
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. A ''low shelf'', such as the bass control on most
hi-fi High fidelity (often shortened to Hi-Fi or HiFi) is the high-quality reproduction of sound. It is important to audiophile An audiophile is a person who is enthusiastic about high-fidelity High fidelity (often shortened to Hi-Fi or HiFi) i ...
equipment, is adjusted to affect the gain of lower frequencies while having no effect well above its cutoff frequency. A ''high shelf'', such as a treble control, adjusts the gain of higher frequencies only. These are broad adjustments designed more to increase the listener's satisfaction than to provide actual equalization in the strict sense of the term. A parametric equalizer, on the other hand, has one or more sections each of which implements a second-order filter function. This involves three adjustments: selection of the center frequency (in ), adjustment of the Q which determines the sharpness of the
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
, and the level or gain control which determines how much those frequencies are boosted or cut relative to frequencies much above or below the center frequency selected. In a ''semi-parametric'' equalizer there is no control for the bandwidth (it is preset by the designer) or is only selected between two presets using a switch. In a ''quasi-parametric'' equalizer, the bandwidth is depending on the gain level. With rising gain, the bandwidth gets wider. A graphic equalizer also implements second-order filter functions in a more user-friendly manner, but with somewhat less flexibility. This equipment is based on a bank of
filters Filter, filtering or filters may refer to: Science and technology Device * Filter (chemistry), a device which separates solids from fluids (liquids or gases) by adding a medium through which only the fluid can pass ** Filter (aquarium), critical ...
covering the audio spectrum in up to 30 frequency bands. Each second-order filter has a fixed center frequency and Q, but an adjustable level. The user can raise or lower each slider in order to visually approximate a "graph" of the intended frequency response. Since "equalization" in the context of audio reproduction is not used strictly to compensate for the deficiency of equipment and transmission channels, the use of high-pass and low-pass filters may be mentioned. A
high-pass filter A high-pass filter (HPF) is an electronic filter that passes signals with a frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast ...

modifies a signal by eliminating only lower frequencies. An example of this is a low-cut or
rumble filter A high-pass filter (HPF) is an Filter (signal processing), electronic filter that passes signal (electrical engineering), signals with a frequency higher than a certain cutoff frequency and attenuates signals with frequencies lower than the cutof ...
, which is used to remove
infrasonic Infrasound, sometimes referred to as low status sound , describes sound waves with a frequency below the lower limit of human audibility (generally 20 Hz). Hearing becomes gradually less sensitive as frequency decreases, so for humans to per ...
energy from a program that may consume undue amplifier power and cause excessive diaphragm excursions in (or even damage to) loudspeakers. A
low-pass filter A low-pass filter is a filter Filter, filtering or filters may refer to: Science and technology Device * Filter (chemistry), a device which separates solids from fluids (liquids or gases) by adding a medium through which only the fluid can pass ...

only modifies the audio signal by removing high frequencies. 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. A first-order low-pass or high-pass filter has a standard response curve that reduces the unwanted frequencies well above or below the cutoff frequency with a slope of 6 dB per octave. A second-order filter will reduce those frequencies with a slope of 12 dB per octave and moreover may be designed with a higher Q or finite zeros in order to effect an even steeper response around the
cutoff frequency In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force ...
. For instance, a second-order ''low-pass notch'' filter section only reduces (rather than eliminates) very high frequencies, but has a steep response falling to zero at a specific frequency (the so-called ''notch frequency''). Such a filter might be ideal, for instance, in completely removing the 19 kHz FM stereo subcarrier
pilot signal In telecommunication Telecommunication is the transmission of information Information can be thought of as the resolution of uncertainty; it answers the question of "What an entity is" and thus defines both its essence and the nature of i ...
while helping to cut even higher frequency subcarrier components remaining from the stereo
demultiplexer In electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow by amplifier, amplif ...
. In addition to adjusting the relative amplitude of frequency bands, an audio equalizer usually alters the relative
phase Phase or phases may refer to: Science * State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter) In the physical sciences, a phase is a region of space (a thermodynamic system A thermodynamic system is a ...
s of those frequencies. While the human ear is not as sensitive to the phase of audio frequencies (involving delays of less than 1/30 second), music professionals may favor certain equalizers because of how they affect the timbre of the musical content by way of audible phase artifacts.

## High-pass and low-pass filters

A
high-pass filter A high-pass filter (HPF) is an electronic filter that passes signals with a frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast ...

is a filter, an electronic circuit or device, that passes higher
frequencies Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time interval, used as a standard way of measuring or expressing duration. The SI base unit, base unit of time in the Internation ...
well but (cuts or decreases) lower-frequency components. A
low-pass filter A low-pass filter is a filter Filter, filtering or filters may refer to: Science and technology Device * Filter (chemistry), a device which separates solids from fluids (liquids or gases) by adding a medium through which only the fluid can pass ...

passes low-frequency components of signals while attenuating higher frequencies. Some audiophiles use a low-pass filter in the signal chain before their
subwoofer A subwoofer (or sub) is a loudspeaker A loudspeaker (or ''speaker driver'', or most frequently just ''speaker'') is an Acoustical engineering#Electroacoustics, electroacoustic transducer, that is, a device that converts an electrical audio s ...

speaker enclosure A loudspeaker enclosure or loudspeaker cabinet is an enclosure (often rectangular box-shaped) in which speaker driver A speaker driver is an individual loudspeaker transducer A transducer is a device that energy from one form to another. ...
, to ensure that only deep bass frequencies reach the subwoofer. In audio applications these high-pass and low-pass filters are frequently termed "low cut" and "high cut", respectively, to emphasize their effect on the original signal. For instance, sometimes audio equipment will include a switch labeled "high cut" or described as a "hiss filter" (hiss being high-frequency noise). In the
phonograph A phonograph, in its later forms also called a gramophone (as a trademark since 1887, as a generic name in the UK since 1910) or since the 1940s called a record player, is a device for the mechanical and analogue recording and reproduction ...

era, many stereos would include a switch to introduce a high-pass (low cut) filter, often called a "rumble filter", to eliminate
infrasonic Infrasound, sometimes referred to as low status sound , describes sound waves with a frequency below the lower limit of human audibility (generally 20 Hz). Hearing becomes gradually less sensitive as frequency decreases, so for humans to per ...
frequencies.

## Shelving filter

While high-pass and low-pass filters are useful for removing unwanted signal above or below a set frequency, shelving filters can be used to reduce or increase signals above or below a set frequency. Shelving filters are used as common tone controls (bass and treble) found in consumer audio equipment such as home stereos, and on
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s and
bass amplifier A bass amplifier or "bass amp" is a Instrument amplifier, musical instrument electronic device that uses electrical power to make lower-pitched musical instrument, instruments such as the bass guitar or double bass loud enough to be heard by th ...
s. These implement a first-order response and provide an adjustable boost or cut to frequencies above or below a certain point. A ''high shelf'' or "treble control" will have a frequency response , ''H''(''f''), whose square is given by: : $, H\left(f\right), ^2 =$ where ''fp'' and ''fz'' are called the pole and zero frequencies, respectively. Turning down the treble control increases ''fz'' and decreases ''fp'' so that frequences higher than ''fp'' are attenuated. Turning up the treble control increases ''fp'' and decreases ''fz'' so that frequencies higher than ''fz'' are boosted. Setting the treble control at the center sets ''fz'' = ''fp'' so that , ''H''(''f''), 2 = 1 and the circuit has no effect. At most, the slope of the filter response in the transition region will be 6 dB per octave (thus a doubling of signal voltage and a consequent quadrupling of signal power for every doubling of frequency). Similarly the response of a ''low shelf'' (or "low shelving or "bass control") can be represented as : $, H\left(f\right), ^2 = \left(f_z/f_p\right)^2 \; .$ In this case, the inclusion of the leading factor simply indicates that the response at frequencies much higher than ''fz'' or ''fp'' is unity and that only bass frequencies are affected. A high shelving control in which ''fz'' is set to infinity, or a low shelving response in which ''fz'' is set to zero, implements a first-order low-pass or high-pass filter, respectively. 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. Since the range of possible responses from shelving filters is so limited, some audio engineers considered shelving controls inadequate for equalization tasks. On some bass amps and
DI box A DI unit (direct input or direct inject) is an electronic device typically used in recording studios and in sound reinforcement systems to connect a high-output impedance, line level, unbalanced output signal to a low-impedance, microphone level ...
es, the units provide both low and high shelving controls and additional equalization controls.

## Graphic equalizer

In the ''graphic equalizer'', the input signal is sent to a bank of
filters Filter, filtering or filters may refer to: Science and technology Device * Filter (chemistry), a device which separates solids from fluids (liquids or gases) by adding a medium through which only the fluid can pass ** Filter (aquarium), critical ...
. Each filter passes the portion of the signal present in its own frequency range or ''band''. 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 to 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 Vehicle audio is equipment installed in a car A car (or automobile) is a wheeled motor vehicle Electric bicycles parked in Yangzhou's main street, Wenchang Lu. They are a very common way of transport in this city, in some areas almost ou ...

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 frequencies of its filters are spaced one third of an
octave In music Music is the of arranging s in time through the of melody, harmony, rhythm, and timbre. It is one of the aspects of all human societies. General include common elements such as (which governs and ), (and its associated co ...

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.

## Parametric equalizer

Parametric equalizers are multi-band variable equalizers that allow users to control the three primary parameters:
amplitude The amplitude of a period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Period, a descriptor for ...

,
center frequency Center or centre may refer to: Mathematics *Center (geometry) In geometry, a centre (or center) (from Ancient Greek language, Greek ''κέντρον'') of an object is a point in some sense in the middle of the object. According to the spe ...
and
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
. The amplitude of each band can be controlled, and the center frequency can be shifted, and the bandwidth (which is inversely related to " Q") can be widened or narrowed. Parametric equalizers are capable of making much more precise adjustments to the sound than other equalizers, and are commonly used in sound recording and . Parametric equalizers are also sold as standalone outboard gear units. A variant of the parametric equalizer is the semi-parametric equalizer, which is also known as a sweepable filter. It allows users to control the amplitude and frequency, but uses a pre-set bandwidth of the center frequency. In some cases, semi-parametric equalizers allow the user to select between a wide and a narrow preset bandwidth.

# Filter functions

The responses of
linear filter Linearity is the property of a mathematical relationship (''function (mathematics), function'') that can be graph of a function, graphically represented as a straight Line (geometry), line. Linearity is closely related to Proportionality (mathema ...
s are mathematically described in terms of their
transfer function In engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad rang ...

or, in layman's terms,
frequency response In electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons The electron is a subatomic particle In physical sciences, subatomic particles ...
. A transfer function can be decomposed as a combination of ''first-order'' responses and ''second-order'' responses (implemented as so-called biquad sections). These can be described according to their so-called
pole Pole may refer to: Astronomy *Celestial pole, the projection of the planet Earth's axis of rotation onto the celestial sphere; also applies to the axis of rotation of other planets *Pole star, a visible star that is approximately aligned with the ...
and
zero 0 (zero) is a number A number is a mathematical object A mathematical object is an abstract concept arising in mathematics. In the usual language of mathematics, an ''object'' is anything that has been (or could be) formally defined, and ...
frequencies, which are
complex numbers In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). I ...
in the case of second-order responses.

## First-order filters

A first-order filter can alter the response of frequencies above and below a point. In the transition region the filter response will have a slope of up to 6  per
octave In music Music is the of arranging s in time through the of melody, harmony, rhythm, and timbre. It is one of the aspects of all human societies. General include common elements such as (which governs and ), (and its associated co ...

. 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. A special case of first-order filters is a first-order high-pass or low-pass filter in which the 6 dB per octave cut of low or high frequencies extends indefinitely. These are the simplest of all filters to implement individually, requiring only a capacitor and resistor.

## Second-order filters

Second-order filters are capable of
resonance Resonance describes the phenomenon of increased amplitude The amplitude of a ic is a measure of its change in a single (such as or ). There are various definitions of amplitude (see below), which are all s of the magnitude of the differ ...

(or anti-resonance) around a particular frequency. The response of a second-order filter is specified not only by its frequency but also its Q; a higher Q corresponds to a sharper response (smaller bandwidth) around a particular center frequency. For instance, the red response in the accompanying image cuts frequencies around 100 Hz with a higher Q than the blue response which boosts frequencies around 1000 Hz. Higher Q's correspond to
resonant Resonance describes the phenomenon of increased amplitude The amplitude of a Periodic function, periodic Variable (mathematics), variable is a measure of its change in a single Period (mathematics), period (such as frequency, time or Wavelen ...
behaviour in which the half-power or −3 dB bandwidth, ''BW'', is given by: :$BW \ = \ F_0 / Q$ where ''F''0 is the
resonant Resonance describes the phenomenon of increased amplitude The amplitude of a Periodic function, periodic Variable (mathematics), variable is a measure of its change in a single Period (mathematics), period (such as frequency, time or Wavelen ...
frequency of the second-order filter. ''BW'' is the bandwidth expressed in the same frequency unit that ''F''0 is. Low Q filter responses (where ''Q'' < ) are not said to be resonant and the above formula for bandwidth does not apply. It is also possible to define the Q of a band-pass function as: :$Q \ = \ \frac \ = \ \frac,$ where ''N'' is the bandwidth in octaves. The reverse mapping is: :$N \ = \ 2 \log_2\left\left( \frac + \sqrt \right\right) \ = \ \frac \operatorname\left\left( \frac \right\right).$ A second-order filter response with ''Q'' of less than 1/2 can be decomposed into two first-order filter functions, a low-cut and a high-cut (or boost). Of more interest are
resonant Resonance describes the phenomenon of increased amplitude The amplitude of a Periodic function, periodic Variable (mathematics), variable is a measure of its change in a single Period (mathematics), period (such as frequency, time or Wavelen ...
filter functions which can boost (or cut) a narrow range of frequencies. In addition to specifying the center frequency ''F''0 and the Q, the specification of the filter's zeros determines how much that frequency band will be boosted (or cut). Thus a
parametric equalizer Equalization is the process of adjusting the balance between frequency components within an electronic Signal (electronics), signal. The most well known use of equalization is in sound recording and reproduction but there are many other appli ...
section will have three controls for its center frequency ''F''0, bandwidth or Q, and the amount of boost or cut usually expressed in . The range of second-order filter functions is important because any analog filter function can be decomposed into a (usually small) number of these (plus, perhaps, simpler first-order responses). These are implemented directly by each section of a parametric equalizer where they are explicitly adjusted. And each element of a graphic equalizer based on a
filter bankIn signal processing, a filter bank (or filterbank) is an array of bandpass filters that separates the input signal into multiple components, each one carrying a single frequency Sub-band coding, sub-band of the original signal. One application of a ...
includes one such element whose Q is not adjustable by the user.

# Uses

In
sound recording Sound recording and reproduction is an electrical Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion ...

, equalization may be used to adjust frequency responses for practical or aesthetic reasons, where the end result typically is ''unequal'' volume levels for the different frequencies. For example, equalization is used to modify an instrument's sound or make certain instruments and sounds more prominent. A
recording engineer An audio engineer (also known as a sound engineer or recording engineer) helps to produce a sound recording, recording or a live performance, balancing and adjusting sound sources using equalization (audio), equalization, Dynamic range compressi ...
may 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 away or closer than others. Equalization is also commonly used to give tracks with similar frequency components complementary spectral contours, known as . Selected components of parts that 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 Acoustics is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other wo ...
, as an auditorium will generally have an uneven frequency response especially due to
standing waves In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through Sp ...

and acoustic dampening. For instance, the
frequency response In electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons The electron is a subatomic particle In physical sciences, subatomic particles ...
of a room may be analyzed using a
spectrum analyzer A spectrum analyzer measures the magnitude of an input signal versus frequency within the full frequency range of the instrument. The primary use is to measure the power of the spectrum of known and unknown signals. The input signal that most comm ...
and a
pink noise Pink noise or noise is a signal (information theory), signal or process with a frequency spectrum such that the power spectral density (power per frequency interval) is inversely proportional to the frequency of the signal. In pink noise, each ...
generator. 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 A recording studio is a specialized facility for Sound recording and reproduction, sound recording, Audio mixing (recorded music), mixing, and audio production of instrumental or vocal musical performances, spoken words, and other sounds. T ...

in addition to its use in live
sound reinforcement system A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers in Loudspeaker enclosure, enclosures all controlled by a mixing console that makes live or pre-recorded sounds louder and may also d ...

s and even home
hi-fi High fidelity (often shortened to Hi-Fi or HiFi) is the high-quality reproduction of sound. It is important to audiophile An audiophile is a person who is enthusiastic about high-fidelity High fidelity (often shortened to Hi-Fi or HiFi) i ...
systems. During live events where signals from microphones are amplified and sent to
speaker Speaker may refer to: Roles * Speaker (politics), the presiding officer in a legislative assembly * Public speaker, one who gives a speech or lecture * A person producing speech, sometimes also called a speaker-hearer Electronics * Loudspeaker, a ...

systems, equalization is not only used to "flatten" the frequency response but may also be useful in eliminating
feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain A chain is a assembly of connected pieces, called links, typically made of metal, with an overall character similar to that of a in that it is flexib ...
. 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 Gain or GAIN may refer to: Science and technology * Gain (electronics) In , gain is a measure of the ability of a (often an ) to increase the or of a from the input to the output port by adding energy converted from some to the signal. ...
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 filter's center frequency is completely eliminated is known as a
notch filter In signal processing Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and synthesizing signals such as audio signal processing, sound, image processing, images, and scientific measurements. Signa ...
. An equalizer can be used to correct or modify the frequency response of a loudspeaker system rather than designing the speaker itself to have the desired response. For instance, the Bose 901 speaker system does not use separate larger and smaller drivers to cover the bass and treble frequencies. Instead it uses nine drivers all of the same four-inch diameter, more akin to what one would find in a table radio. However, this speaker system is sold with an active equalizer. 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, and vice versa, producing the response intended by the manufacturer.
Stereophile ''Stereophile'' is a monthly magazine that focuses on high-end audio equipment, such as loudspeakers and Audio power amplifier, amplifiers, and audio-related news, such as online audio streaming. History and profile Founded in 1962 by J. Gordon H ...
magazine
Bose 901 Loudspeaker Review
1995.
Tone control An audio filter is a frequency dependent amplifier circuit, working in the audio frequency range, 0 Hz to beyond 20 kHz. Audio filters can amplify (boost), pass or attenuate (cut) some frequency ranges. Many types of filters exist for di ...
s (usually designated "bass" and "treble") are simple shelving filters included in most
hi-fi High fidelity (often shortened to Hi-Fi or HiFi) is the high-quality reproduction of sound. It is important to audiophile An audiophile is a person who is enthusiastic about high-fidelity High fidelity (often shortened to Hi-Fi or HiFi) i ...
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 crossover network is a system of filters designed to direct electrical energy separately to the
woofer A woofer or bass speaker is a technical term for a loudspeaker A loudspeaker (or ''speaker driver'', or most frequently just ''speaker'') is an Acoustical engineering#Electroacoustics, electroacoustic transducer, that is, a device that conve ...

and
tweeter A tweeter or treble speaker is a special type of loudspeaker A loudspeaker (or ''speaker driver'', or most frequently just ''speaker'') is an Acoustical engineering#Electroacoustics, electroacoustic transducer, that is, a device that conver ...

of a 2-way speaker system (and also to the
mid-range speaker A mid-range speaker is a loudspeaker A loudspeaker (or ''speaker driver'', or most frequently just ''speaker'') is an Acoustical engineering#Electroacoustics, electroacoustic transducer, that is, a device that converts an electrical audio sign ...
of a 3-way system). This is most often built into the speaker enclosure and hidden from the user. However, in
bi-amplification Bi-amping and tri-amping is the practice of using two or three audio amplifier An audio power amplifier (or power amp) is an electronic amplifier that amplifies low-power electronic audio signals such as the signal from radio receiver ...
, these filters operate on the low level audio signals, sending the low-frequency and high-frequency signal components to separate amplifiers, which connect to the woofers and tweeters, respectively. Equalization is used in a reciprocal manner in certain communication channels and recording technologies. The original music is passed through a particular filter to alter its frequency balance, followed by the channel or recording process. At the end of the channel or when the recording is played, a complementary filter is inserted which precisely compensates for the original filter and recovers the original waveform. For instance, FM broadcasting uses a
pre-emphasis Typically, prior to some process, such as transmission over cable, or recording to phonograph record or tape, the input frequency range most susceptible to noise is boosted. This is referred to as "pre-emphasis"before the process the signal will und ...
filter to boost the high frequencies before transmission, and every receiver includes a matching
de-emphasis Typically, prior to some process, such as transmission over cable, or recording to phonograph record or tape, the input frequency range most susceptible to noise is boosted. This is referred to as "pre-emphasis"before the process the signal will und ...
filter to restore it. The
white noise In signal processing Signal processing is an electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, elect ...

that is introduced by the radio is then also de-emphasized at the higher frequencies (where it is most noticeable) along with the pre-emphasized program, making the noise less audible.
Tape recorder An audio tape recorder, also known as a tape deck, tape player or tape machine or simply a tape recorder, is a sound recording and reproduction Sound recording and reproduction is an electrical Electricity is the set of physical ...
s used the same approach to reduce "
tape hiss Tape hiss is the high frequency noise present on analog signal, analogue magnetic tape recordings caused by the size of the magnetic particles used to make the tape. Effectively it is the noise floor of the recording medium. It can be reduced by th ...
" while maintaining fidelity. On the other hand, in the production of
vinyl records A phonograph disc record (also known as a gramophone disc record, especially in British English British English (BrE) is the standard dialect A standard language (also standard variety, standard dialect, and standard) is a language va ...

, a filter is used to reduce the amplitude of low frequencies which otherwise produce large amplitudes on the tracks of a record. Then the groove can take up less physical space, fitting more music on the record. The preamplifier attached to the phono cartridge has a complementary filter boosting those low frequencies, following the standard
RIAA equalization RIAA equalization is a specification for the recording and playback of phonograph record A phonograph disc record (also known as a gramophone disc record, especially in ), or simply a phonograph record, gramophone record, disc record, lon ...
curve.

# See also

*
Electronic filter Electronic filters are a type of signal processing filter in the form of electrical circuits. This article covers those filters consisting of lumped electronic components, as opposed to distributed-element filter A distributed-element filte ...
*
Equalization (communications) In telecommunication, equalization is the reversal of distortion incurred by a signal transmitted through a Channel (communications), channel. Equalizers are used to render the frequency response—for instance of a telephone line—''flat'' from en ...
*
Loudness compensation Loudness compensation is a setting found on some High fidelity, hi-fi equipment that increases the level of the high and low frequencies. This is intended to be used at low listening levels, to compensate for the fact that as the loudness of audio ...
*
Weighting filter A weighting filter is used to emphasize or suppress some aspects of a phenomenon compared to others, for measurement or other purposes. Audio applications In each field of audio measurement, special units are used to indicate a weighted measure ...

# General sources

* Glen Ballou, "Filters and equalizers", ''Handbook for Sound Engineers'', Fourth edition, Focal Press, 2008 .

# External links

* Playback equalization for 78rpm shellacs and early LPs (EQ curves, index of record labels)
Audacity Wiki

Discriminating EQ frequencies by ear

* ttps://web.archive.org/web/20110721132848/http://www.idc.ul.ie/idcwiki/index.php/Equalisation EQ Condensed Overview
Audio EQ Cookbook

PreSonus Equalizer Terms and Tips

WikiRecording's Guide to Equalization
{{DEFAULTSORT:Equalization (audio) Audio effects Effects units Linear filters Sound recording Tone, EQ and filter