Ethnomusicology

Noise
From Wikipedia, the free encyclopedia
This article is about noise as in sound. For other uses, see Noise (disambiguation).
In common use, the word noise means unwanted sound or noise pollution. In electronics noise can refer to the electronic signal corresponding to acoustic noise (in an audio system) or the electronic signal corresponding to the (visual) noise commonly seen as 'snow' on a degraded television or video image. In signal processing or computing it can be considered 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. In Information Theory, however, noise is still considered to be information. In a broader sense, film grain or even advertisements in web pages can be considered noise.

Noise can block, distort, or change/interfere with the meaning of a message in both human and electronic communication.

In many of these areas, 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 at the molecular level described by well-established thermodynamics considerations, some of which are expressible by relatively well known simple formulae.

Acoustic noise
When speaking of noise in relation to sound, what is commonly meant is meaningless sound of greater than usual volume. Thus, a loud activity may be referred to as noisy. However, conversations of other people may be called noise for people not involved in any of them, and noise can be any unwanted sound such as the noise of dogs barking, neighbours playing loud music, road traffic sounds, chainsaws, or aircraft, spoiling the quiet of the countryside.


[edit] Regulation
Main article: Noise regulation
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[1], 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.


[edit] Examples
Sounds that are generally regarded as acoustic noise include snoring.


[edit] 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" ('The Art of Sound' (1929)). Alberto Cavalcanti uses noise as a synonym for natural sound ('Sound in Films' (1939)) and as late as 1960, Siegfried Kracauer was referring to noise as non-speech sound ('Dialogue and Sound' (1960)).


[edit] Audio noise
Main article: Colors of noise
In audio, recording, and broadcast systems audio noise refers to the residual low level sound (usually hiss and hum) that is heard in quiet periods of programme.

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


[edit] Electronic noise
Main article: Electronic noise
Electronic noise exists in all circuits and devices as a result of thermal noise, also referred to as Johnson Noise. Semiconductor devices can also contribute flicker noise and generation-recombination noise. In any electronic circuit, there exist random variations in current or voltage caused by the random movement of the electrons carrying the current as they are jolted around by thermal energy. Lower temperature results in lower thermal noise. This same 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 stars, use front-end low-noise amplifier circuits, usually mounted on the aerial dish, and cooled with liquid nitrogen.


[edit] Visual noise
Main article: Visual noise
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 color, 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.