In common use, the word noise means any unwanted sound.
In physics and analog electronics, noise is a mostly unwanted random addition to a signal; it is called noise as a generalisation of the acoustic noise ("static") heard when listening to a weak radio transmission with significant electrical noise. Signal noise is heard as acoustic noise if the signal is converted into sound (e.g., played through a loudspeaker); it manifests as "snow" on a television or video image. High noise levels can block, distort, change or interfere with the meaning of a message in human, animal and electronic communication.
In signal processing or computing noise can be considered random unwanted data without meaning; that is, data that is not being used to transmit a signal, but is simply produced as an unwanted by-product of other activities. "Signal-to-noise ratio" is sometimes used to refer to the ratio of useful to irrelevant information in an exchange.
In biology, many different forms of cellular noise exist, where a measurement displays substantial variance around its mean: for example, transcriptional noise describes the variability in gene activity between cells in a population.
In many cases, the special case of thermal noise arises, which sets a fundamental lower limit to what can be measured or signaled and is related to basic physical processes described by thermodynamics, some of which are expressible by simple formulae.
In some fields, noise means unwanted information or data that is not relevant to the hypothesis or theory being investigated or tested.
In relation to sound, noise is not necessarily random. Sounds, particularly loud ones, that disturb people or make it difficult to hear wanted sounds, are noise. For example, conversations of other people may be called noise by people not involved in any of them; any unwanted sound such as domesticated dogs barking, neighbours playing loud music, portable mechanical saws, road traffic sounds, or a distant aircraft in quiet countryside, is called noise.
Acoustic noise can be anything from quiet but annoying to loud and harmful. At one extreme users of public transport sometimes complain about the faint and tinny sounds emanating from the headphones or earbuds of somebody listening to a portable audio player; at the other the sound of very loud music, a jet engine at close quarters, etc. can cause permanent irreversible hearing damage.
Regulation of acoustic noise
Noise regulation includes statutes or guidelines relating to sound transmission established by national, state or provincial and municipal levels of government. After a watershed passage of the U.S. Noise Control Act of 1972, the program was abandoned at the federal level, under President Ronald Reagan, in 1981 and the issue was left to local and state governments. Although the UK and Japan enacted national laws in 1960 and 1967 respectively, these laws were not at all comprehensive or fully enforceable as to address (a) generally rising ambient noise (b) enforceable numerical source limits on aircraft and motor vehicles or (c) comprehensive directives to local government.
Use in Music
Percussion instruments are inherently noisy by nature. Specifically, snare drums (vibrations of a circular membrane), create a very noisy wave shortly after they are struck. The double headed nature of this instrument means that the harmonics are extraordinarily complex, as between both of them, there are, theoretically, an infinite number of resonant modes, and thus harmonics. In practice, though, the sound produced is not infinitely harmonic, rather it is extremely dissonant, due to the large number of constructive and destructive interfaces.
In audio engineering it can also refer to the unwanted residual electronic noise signal that gives rise to acoustic noise heard as "hiss". This signal noise is commonly measured using A-weighting or ITU-R 468 weighting.
Acoustic noise in film sound
For film sound theorists and practitioners at the advent of talkies c.1928/1929, noise was non-speech sound or natural sound, and for many of them, noise (especially asynchronous use with image) was desired over the evils of dialogue synchronized to moving image. The director and critic René Clair writing in 1929 makes a clear distinction between film dialogue and film noise and very clearly suggests that noise can have meaning and be interpreted: "...it is possible that an interpretation of noises may have more of a future in it. Sound cartoons, using "real" noises, seem to point to interesting possibilities". Alberto Cavalcanti uses noise as a synonym for natural sound  and as late as 1960, Siegfried Kracauer was referring to noise as non-speech sound.
Electronic noise and communication channel noise
Electronic noise exists in all circuits and devices as a result of thermal noise, also referred to as Johnson–Nyquist noise. It is caused by random variations in current or voltage caused by the random movement of charge carriers (usually electrons) carrying the current as they are jolted around by thermal energy. Thermal noise can be reduced by reducing the temperature of the circuit. This phenomenon limits the minimum signal level that any radio receiver can usefully respond to, because there will always be a small but significant amount of thermal noise arising in its input circuits. This is why radio telescopes, which search for very low levels of signal from space, use front-end low-noise amplifier circuits cooled with liquid nitrogen.
There are several other sources of noise in electronic circuits such as shot noise, seen in very low-level signals where the finite number of energy-carrying particles becomes significant, or flicker noise (1/f noise) in semiconductor devices. A digitized and reconstructed analog signal is exposed to additive quantization noise.
In a communication channel, noise is an undesired random signal, often modelled as additive white Gaussian noise, that may be caused by thermal noise or electromagnetic interference from unknown sources. Noise should not be confused with crosstalk and other interference from other communication system transmitters. Phase or frequency modulated communication systems may suffer from phase noise due to synchronisation problems and time-invariant channel conditions, caused by mobility, fading and doppler shift. Deliberate generation of communication system noise and interference is called jamming.
Noise is also present in images. Electronic noise will be present in camera sensors, and the physical size of the grains of film emulsion creates visual noise. This kind of noise is referred to as "grain."
Noise is also used in the creation of 2D and 3D images by computer. Sometimes noise is added to images to hide the sudden transitions inherent in digital representation of colour, known as "banding". This adding of noise is referred to as "dithering." Sometimes noise is used to create the subject matter itself. Procedural noise (such as Perlin noise) is often used to create natural-looking variation in computer generated images.
The Earth ground and the built environment are subjected to permanent vibrations commonly referred to as seismic noise but nowadays preferably called ambient vibrations. These vibrations have natural (ocean waves, wind, etc.) and anthropogenic (traffic, machines, etc.) origins depending on the considered frequency range. These vibrations may be disturbing for people (housing close to railway tracks, etc.) and even cause solidian noise (acoustic noise created by particular vibrations of solids like rooms) but they are also used (recorded) to characterise the structures (civil engineering structure, underground) they are shaking in terms of dynamic properties and eventually physical properties.
The activity and regulation of our genes are also subject to noise. Transcriptional noise refers to the variability in gene activity between cells in genetically identical populations (even identical twins are non-identical). Noise in gene activity has tremendous consequences on cell behaviour, and must be mitigated or integrated. Noise impacts upon the effectiveness of clinical treatment, with resistance of bacteria to antibiotics demonstrably caused by non-genetic differences. Variability in gene expression may also contribute to resistance of sub-populations of cancer cells to chemotherapy. In certain contexts, such as the survival of microbes in rapidly changing stressful environments, or several types of scattered differentiation, noise may be essential.
||This article needs additional citations for verification. (September 2011)|
- Clair, René (1985). "The Art of Sound". In Weis, Elisabeth; Belton, John. Film Sound: Theory and Practice. New York: Columbia University Press. p. 92.
- Cavalcanti, Alberto (1939). "Sound in Films". Films: A Quarterly of Discussion and Analysis (Arno Press) 1: 25.
- Kracauer, Siegfried (1960). "Chapter 7: Dialogue and Sound". Theory of Film: The Redemption of Physical Reality. Oxford University Press. p. 102.
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- Article "Noise Control Techniques"
- Mountain & Plains ERC: A NIOSH Education and Research Center for Occupational & Environmental Health & Safety
- US National Institute for Occupational Safety and Health, - Noise
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