Noise

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 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 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 aircraft, neighbours playing loud music, or road sounds spoiling the quiet of the countryside.

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)).

Audio 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

Noise is often generated deliberately and used as a test signal. The two most common types of such noise are:
 * white noise, which has a uniform spectral power density at all frequencies
 * pink noise which has a power spectral density that falls at 3dB/octave with rising frequency. The pink noise is often more useful in audio testing because it contains constant energy per octave (and hence per commonly used 1/3rd octave), rather than a preponderance of energy at high frequencies. In other words it contains energy that is distributed geometrically rather than linearly.

There are other less common kind of noise:


 * black noise, a term with numerous conflicting definitions, but commonly refers to silence with occasional spikes.


 * blue noise, contains more energy as the frequency increases.


 * brown noise, mimics the signal noise produced by brownian motion.


 * gray noise, similar to white noise, but has been filtered to make sound level appear constant at all frequency to the human ear.


 * green noise, an unofficial term which can mean the mid-frequencies of white noise, or the 'background noise of the world'.


 * orange noise, an unofficial term describing noise which has been stripped of harmonious frequencies.


 * purple noise, contains more energy as the frequencies increases.


 * red noise, an oceanograhic term which describes ambient underwater noise from distant sources. Also another name for brown noise.

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. The lower the temperature the lower is this 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, cooled in liquid nitrogen it gives a reduced response to low frequency sounds, and does not take account of the increased annoyance value of bass boom to man-made electronics, including the receiver itself. Transmitter power must be increased to overcome radio noise over long distances.

Video noise
In video and television, noise refers to the random dot pattern that is superimposed on the picture as a result of electronic noise, the 'snow' that is seen with poor (analog) television reception or on VHS tapes. Interference and static are other forms of noise, in the sense that they are unwanted, though not random, which can affect radio and television signals.