While
noiseIn common use, the word noise means unwanted sound or noise pollution. In both analog and digital electronics, noise or signal noise is an unwanted random addition to a wanted signal; it is called noise as a generalisation of the audible noise heard when listening to a weak radio transmission...
is by definition derived from a
randomRandomness is a concept with somewhat disparate meanings in several fields. It also has common meanings which may have loose connections with some of those more definite meanings...
signalIn the fields of communications, signal processing, and in electrical engineering more generally, a signal is any time-varying or spatial-varying quantity....
, it can have different characteristic statistical properties corresponding to different mappings from a source of randomness to the concrete noise.
Spectral densityIn statistical signal processing and physics, the spectral density, power spectral density , or energy spectral density , is a positive real function of a frequency variable associated with a stationary stochastic process, or a deterministic function of time, which has dimensions of power per Hz,...
(
powerIn physics, power is the rate at which work is performed or energy is converted. It is an energy per unit of time. As a rate of change of work done or the energy of a subsystem, power iswhere P is power, W is work and t is time....
distribution in the
frequency spectrumFamiliar concepts associated with a frequency are colors, musical notes, radio/TV channels, and even the regular rotation of the earth. A source of light can have many colors mixed together and in different amounts . A rainbow, or prism, sends the different frequencies in different directions,...
) is such a property, which can be used to distinguish different types of noise. This classification by spectral density is given "color" terminology, with different types named after different colors, and is common in different disciplines where noise is an important factor (like
acousticsAcoustics is the interdisciplinary science that deals with the study of sound, ultrasound and infrasound . A scientist who works in the field of acoustics is an acoustician. The application of acoustics in technology is called acoustical engineering...
,
electrical engineeringElectrical engineering, sometimes referred to as electrical and electronic engineering, is a field of engineering that deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after...
, and
physicsPhysics is a natural science; it is the study of matter and its motion through spacetime and all that derives from these, such as energy and force...
).
While
noiseIn common use, the word noise means unwanted sound or noise pollution. In both analog and digital electronics, noise or signal noise is an unwanted random addition to a wanted signal; it is called noise as a generalisation of the audible noise heard when listening to a weak radio transmission...
is by definition derived from a
randomRandomness is a concept with somewhat disparate meanings in several fields. It also has common meanings which may have loose connections with some of those more definite meanings...
signalIn the fields of communications, signal processing, and in electrical engineering more generally, a signal is any time-varying or spatial-varying quantity....
, it can have different characteristic statistical properties corresponding to different mappings from a source of randomness to the concrete noise.
Spectral densityIn statistical signal processing and physics, the spectral density, power spectral density , or energy spectral density , is a positive real function of a frequency variable associated with a stationary stochastic process, or a deterministic function of time, which has dimensions of power per Hz,...
(
powerIn physics, power is the rate at which work is performed or energy is converted. It is an energy per unit of time. As a rate of change of work done or the energy of a subsystem, power iswhere P is power, W is work and t is time....
distribution in the
frequency spectrumFamiliar concepts associated with a frequency are colors, musical notes, radio/TV channels, and even the regular rotation of the earth. A source of light can have many colors mixed together and in different amounts . A rainbow, or prism, sends the different frequencies in different directions,...
) is such a property, which can be used to distinguish different types of noise. This classification by spectral density is given "color" terminology, with different types named after different colors, and is common in different disciplines where noise is an important factor (like
acousticsAcoustics is the interdisciplinary science that deals with the study of sound, ultrasound and infrasound . A scientist who works in the field of acoustics is an acoustician. The application of acoustics in technology is called acoustical engineering...
,
electrical engineeringElectrical engineering, sometimes referred to as electrical and electronic engineering, is a field of engineering that deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after...
, and
physicsPhysics is a natural science; it is the study of matter and its motion through spacetime and all that derives from these, such as energy and force...
). However, different fields may use the terminology with different degrees of specificity.
Power-law noise
Many of these definitions assume a signal with components at all frequencies, with a power spectral density per unit of bandwidth proportional to 1/
f β and hence they are examples of
power-law noise. For instance, the spectral density of
white noiseWhite noise is a random signal with a flat power spectral density. In other words, the signal contains equal power within a fixed bandwidth at any center frequency...
is flat (
β = 0), while
flickerFlicker noise is a type of electronic noise with a 1/ƒ, or pink spectrum. It is therefore often referred to as 1/ƒ noise or pink noise, though these terms have wider definitions...
or
pink noisePink noise or 1/ƒ noise is a signal or process with a frequency spectrum such that the power spectral density is inversely proportional to the frequency. In pink noise, each octave carries an equal amount of noise power...
has
β = 1, and brown noise has
β = 2.
Technical definitions
Various noise models are employed in analysis, many of which fall under the above categories. AR noise or "autoregressive noise" is such a model, and generates simple examples of the above noise types, and more. The
Federal Standard 1037CFederal Standard 1037C, titled Telecommunications: Glossary of Telecommunication Terms is a United States Federal Standard, issued by the General Services Administration pursuant to the Federal Property and Administrative Services Act of 1949, as amended....
Telecommunications Glossary defines white, pink, blue, and black.
The color names for these different types of sounds are derived from a loose analogy between the spectrum of frequencies of sound wave present in the sound (as shown in the blue diagrams) and the equivalent spectrum of light wave frequencies. That is, if the sound wave pattern of "blue noise" were translated into light waves, the resulting light would be blue, and so on.
White noise
White noiseWhite noise is a random signal with a flat power spectral density. In other words, the signal contains equal power within a fixed bandwidth at any center frequency...
is a
signalIn the fields of communications, signal processing, and in electrical engineering more generally, a signal is any time-varying or spatial-varying quantity....
(or process), named by analogy to
white lightWhite Light may refer to:* Light with the color white*White Light/White Heat , The Velvet Underground's second album.*White Light , a 1971 album*White Light a 1980 novel by Rudy Rucker...
, with equal energy per cycle (hertz). This produces a flat
frequency spectrumFamiliar concepts associated with a frequency are colors, musical notes, radio/TV channels, and even the regular rotation of the earth. A source of light can have many colors mixed together and in different amounts . A rainbow, or prism, sends the different frequencies in different directions,...
in linear space. In other words, the signal has equal
powerIn physics, power is the rate at which work is performed or energy is converted. It is an energy per unit of time. As a rate of change of work done or the energy of a subsystem, power iswhere P is power, W is work and t is time....
in any band of a given bandwidth (power spectral density). For example, the range of frequencies between 40
HzThe hertz is a unit of frequency. It is defined as the number of complete cycles per second. It is the basic unit of frequency in the International System of Units , and is used worldwide in both general-purpose and scientific contexts...
and 60 Hz contains the same amount of sound power as the range between 4000 Hz and 4020 Hz has.
An infinite-bandwidth white noise signal is purely a theoretical construct. By having power at all frequencies, the total power of such a signal would be infinite. In practice, a signal is considered "white" if it has a flat spectrum over a defined frequency band (such as the range of human hearing, or the frequency response of audio/visual equipment).
Pink noise
The frequency spectrum of
pink noisePink noise or 1/ƒ noise is a signal or process with a frequency spectrum such that the power spectral density is inversely proportional to the frequency. In pink noise, each octave carries an equal amount of noise power...
is flat in
logarithmic spaceA logarithmic scale is a scale of measurement that uses the logarithm of a physical quantity instead of the quantity itself.A simple example is when equally-spaced divisions on the vertical axis of a chart are labelled 1, 10, 100, 1000, instead of 1, 2, 3, 4.Presentation of data on a logarithmic...
; it has equal power in bands that are proportionally wide. This means that pink noise would have equal power in the frequency range from 40 to 60 Hz as in the band from 4000 to 6000 Hz. Since humans hear in such a proportional space, where a doubling of frequency is perceived the same regardless of actual frequency (40–60 Hz is heard as the same interval and distance as 4000–6000 Hz), every octave contains the same amount of energy and thus pink noise is often used as a reference signal in
audio engineeringAudio engineering is a part of audio science dealing with the recording and reproduction of sound through mechanical and electronic means. The field draws on many disciplines, including electrical engineering, acoustics, psychoacoustics, and music. Unlike acoustical engineering, audio...
. That is, the human auditory system perceives approximately equal magnitude on all frequencies. The power density, compared with white noise, decreases by 3
dBThe decibel is a logarithmic unit of measurement that expresses the magnitude of a physical quantity relative to a specified or implied reference level. Since it expresses a ratio of two quantities with the same unit, it is a dimensionless unit...
per
octaveIn music, an octave , is the interval between one musical pitch and another with half or double its frequency. The octave relationship is a natural phenomenon which has been referred to as the "basic miracle of music," the use of which is "common in most musical systems." It may be derived from the...
(density proportional to 1/
f ). For this reason, pink noise is often called "1/
f noise".
Since there are an infinite number of logarithmic bands at both the low frequency (DC) and high frequency ends of the spectrum, any finite energy spectrum must have less energy than pink noise at both ends. Pink noise is the only power-law spectral density that has this property: all steeper power-law spectra are infinite if integrated to the DC, low frequency end, and all flatter power-law spectra are infinite if integrated to the high-frequency limit.
Brown(ian) noise
In fields that adopt precise definitions, the terminology "red noise", also called Brown noise or
Brownian noiseIn science, Brownian noise , also known as Brown noise or red noise, is the kind of signal noise produced by Brownian motion hence its alternative name of random walk noise...
, will usually refer to a power density which decreases 6 dB per octave with increasing frequency (density proportional to 1/
f 2) over a frequency range which does not include
DCDirect current is the undirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also be through...
(in a general sense, does not include a constant component, or value at zero frequency). In areas where terminology is used loosely, "red noise" may refer to any system where power density decreases with increasing frequency.
The first definition can be generated by an algorithm which simulates
Brownian motionBrownian motion is the seemingly random movement of particles suspended in a fluid or the mathematical model used to describe such random movements, often called a particle theory....
or by
integratingIntegration is an important concept in mathematics which, together with differentiation, forms one of the main operations in calculus. Given a function ƒ of a real variable x and an interval [a, b] of the real line, the definite integralis defined informally...
white noise. "Brown" noise is not named for a power spectrum that suggests the color brown; rather, the name is a corruption of Brownian motion. "Red noise" describes the shape of the power spectrum, with pink being between red and white. Also known as "random walk" or "drunkard's walk" noise.
Blue noise
Blue noise is also called azure noise. Blue noise's power density increases 3 dB per octave with increasing frequency (density proportional to
f ) over a finite frequency range. In computer graphics, the term "blue noise" is sometimes used more loosely as any noise with minimal low frequency components and no concentrated spikes in energy. This can be good noise for
ditherDither is an intentionally applied form of noise, used to randomize quantization error, thereby preventing large-scale patterns such as "banding" in images, or noise at discrete frequencies in an audio recording, that are more objectionable than uncorrelated noise...
ing;
retinaThe vertebrate retina is a light sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
l cells are arranged in a blue-noise-like pattern for this reason.
Violet noise
Violet noise is also called purple noise. Violet noise's power density increases 6 dB per octave with increasing frequency (density proportional to
f 2) over a finite frequency range. It is also known as
differentiatedIn calculus the derivative is a measure of how a function changes as its input changes. Loosely speaking, a derivative can be thought of as how much a quantity is changing at a given point; for example, the derivative of the position of a vehicle with respect to time is the instantaneous velocity...
white noise.
Grey noise
Grey noise is random pink noise subjected to a psychoacoustic equal loudness curve (such as an inverted A-weighting curve) over a given range of frequencies, giving the listener the perception that it is equally loud at all frequencies. This is in contrast to standard pink noise which has equal strength over a logarithmic scale of frequencies but is not perceived as being equally loud due to biases in the human
equal-loudness contourAn equal-loudness contour is a measure of sound pressure , over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tones. The unit of measurement for loudness levels is the phon, and is arrived at by reference to equal-loudness contours...
.
Red noise
- A synonym for Brownian noise
In science, Brownian noise , also known as Brown noise or red noise, is the kind of signal noise produced by Brownian motion hence its alternative name of random walk noise...
, as above
- A synonym for pink noise
Pink noise or 1/ƒ noise is a signal or process with a frequency spectrum such that the power spectral density is inversely proportional to the frequency. In pink noise, each octave carries an equal amount of noise power...
, as above
Orange noise
Orange noise is quasi-
stationaryIn the mathematical sciences, a stationary process is a stochastic process whose joint probability distribution does not change when shifted in time or space...
noise with a finite power spectrum with a finite number of small bands of zero energy dispersed throughout a continuous spectrum. These bands of zero energy are centered about the frequencies of
musicMusic is an art form whose medium is sound. Common elements of music are pitch , rhythm , dynamics, and the sonic qualities of timbre and texture...
al notes in whatever
scaleIn music, a scale is a group of musical notes collected in ascending and descending order, that provides material for or is used to conveniently represent part or all of a musical work including melody and/or harmony...
is of interest. Since all in-tune musical notes are eliminated, the remaining spectrum could be said to consist of sour, citrus, or "orange" notes.
Green noise
- "Green noise is supposedly the background noise of the world. A really long term power spectrum averaged over several outdoor sites. Rather like pink noise with a hump added around 500 Hz
The hertz is a unit of frequency. It is defined as the number of complete cycles per second. It is the basic unit of frequency in the International System of Units , and is used worldwide in both general-purpose and scientific contexts...
."
- The mid-frequency component of white noise, used in halftone
Halftone is the reprographic technique that simulates continuous tone imagery through the use of dots, varying either in size or in spacing. 'Halftone' can also be used to refer specifically to the image that is produced by this process....
dithering
- Bounded Brownian noise
Black noise
Black noise is also called silent noise.
- Silence
- Noise with a 1/fβ spectrum, where β > 2 (Manfred Schroeder, "Fractals, chaos
Chaos typically refers to a state lacking order or predictability. In ancient Greece, it referred to the initial state of the universe, and, by extension, space, darkness, or an abyss...
, power laws"). Used in modeling various environmental processes. Is said to be a characteristic of "natural and unnatural catastrophes like floods, droughts, bear markets, and various outrageous outages, such as those of electrical power." Further, "because of their black spectra, such disasters often come in clusters."
- Noise that has a frequency spectrum of predominantly zero power level over all frequencies except for a few narrow bands or spikes. Note: An example of black noise in a facsimile transmission system is the spectrum that might be obtained when scanning a black area in which there are a few random white spots. Thus, in the time domain, a few random pulses occur while scanning.
- "The output of an active noise control
Active noise control is a method for reducing unwanted sound.- Explanation :...
system which cancels an existing noise, leaving the local environment noise free. The comic bookA comic book is a magazine made up of narrative artwork, often accompanied by dialog and often including brief descriptive prose...
character Iron ManIron Man is a fictional character, a superhero that appears in comic books published by Marvel Comics. The character first appeared in Tales of Suspense #39 , and was created by writer-editor Stan Lee, scripter Larry Lieber, and artists Don Heck and Jack Kirby.Born Anthony Edward "Tony" Stark, he...
used to have a "black light beam" that could darken a room like this, and popular science fictionScience fiction is a genre of fiction. It differs from fantasy in that, within the context of the story, its imaginary elements are largely possible within scientifically-established or scientifically-postulated laws of nature...
has a tendency to portray active noise control in this light." The Batman BeyondBatman Beyond, also known as Batman of the Future, is an American animated television series created by The WB Television Network in collaboration with DC Comics and Warner Bros. Animation as a continuation of the Batman legacy. It began airing on January 10, 1999, and ended its run on December18,...
supervillian Shriek also had a weapon like this, which effectively blocked out all noise.
- "As seen in the sales literature for an ultrasonic vermin repeller, black noise with a power density that is constant for a finite frequency range above 20 kHz. More accurately, ultrasonic white noise. This black noise is like the so-called black light
A black light or UV light is a lamp emitting electromagnetic radiation that is almost exclusively in the soft near ultraviolet range, and emits very little visible light...
with frequencies too high to be sensed, but still capable of affecting the environment."
Noisy white
In
telecommunicationTelecommunication is transmission over a distance for the purpose of communication. In earlier times, this may have involved the use of smoke signals, drums, semaphore, flags or heliograph. In modern times, telecommunication typically involves the use of electronic devices such as the telephone,...
, the term
noisy white has the following meanings:
- In facsimile
Fax is a telecommunications technology used to transfer copies of documents, especially using affordable devices operating over the telephone network. The word telefax, short for telefacsimile, for "make a copy at a distance", is also used as a synonym...
or display systems, such as televisionTelevision is a widely used telecommunication medium for transmitting and receiving moving images, either monochromatic or color, usually accompanied by sound. "Television" may also refer specifically to a television set, television programming or television transmission...
, a nonuniformity in the white area of the image, i.e., document or picture, caused by the presence of noiseIn common use, the word noise means unwanted sound or noise pollution. In both analog and digital electronics, noise or signal noise is an unwanted random addition to a wanted signal; it is called noise as a generalisation of the audible noise heard when listening to a weak radio transmission...
in the received signal.
- A signal or signal level
In a communications system, the signal level is the signal power or intensity at a specified point and with respect to a specified reference level, e.g., 1 mW.It is also the rms signal level of a voltage, e.g., 0.775 VRMS is 0 dBu....
that is supposed to represent a white area on the object, but has a noise content sufficient to cause the creation of noticeable black spots on the display surface or record mediumIn telecommunication, the term record medium has the following meanings:# The physical medium on which information is stored in recoverable form. For example, magnetic tape or disk....
.
Noisy black
In
telecommunicationTelecommunication is transmission over a distance for the purpose of communication. In earlier times, this may have involved the use of smoke signals, drums, semaphore, flags or heliograph. In modern times, telecommunication typically involves the use of electronic devices such as the telephone,...
, the term
noisy black has the following meanings:
- In facsimile
Fax is a telecommunications technology used to transfer copies of documents, especially using affordable devices operating over the telephone network. The word telefax, short for telefacsimile, for "make a copy at a distance", is also used as a synonym...
or display systems, such as televisionTelevision is a widely used telecommunication medium for transmitting and receiving moving images, either monochromatic or color, usually accompanied by sound. "Television" may also refer specifically to a television set, television programming or television transmission...
, a nonuniformity in the black area of the image, i.e., document or picture, caused by the presence of noiseIn common use, the word noise means unwanted sound or noise pollution. In both analog and digital electronics, noise or signal noise is an unwanted random addition to a wanted signal; it is called noise as a generalisation of the audible noise heard when listening to a weak radio transmission...
in the received signal.
- A signal or signal level
In a communications system, the signal level is the signal power or intensity at a specified point and with respect to a specified reference level, e.g., 1 mW.It is also the rms signal level of a voltage, e.g., 0.775 VRMS is 0 dBu....
that is supposed to represent a black area on the object, but has a noise content sufficient to cause the creation of noticeable white spots on the display surface or record mediumIn telecommunication, the term record medium has the following meanings:# The physical medium on which information is stored in recoverable form. For example, magnetic tape or disk....
.
External links