Noise
In common use the word noise means unwanted sound, but 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 many of these areas, the special case of thermal noise arises, which sets a fundamental lower limit to what can be measured or signalled 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 noise spoiling the quiet of the countryside.
Environmental noise
Environmental noise is governed by laws, and standards which set maximum remommended levels of noise for specific situations, such as residential areas, areas of outstanding natural beauty, or factories. These standards often specify measurement using a weighting filter, most often A-weighting.
Industrial noise
Industrial noise is usually considered mainly from the point of view of environmental health, rather than nuisance, as sustained exposure can cause permanent hearing damage. A-weighted measurements are commonly use for this as well, and special exposure meters are available that integrate noise over a period of time to give an Leq value (equivalent sound pressure level), defined by standards.
Audio noise
In audio, recording, and broadcast sytems 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 weightingNoise is often generated deliberately and used as a test signal. Two types of deliberately generated noise in common use are referred to as 'white noise', which has a uniform spectral power density at all frequencies, or 'pink noise' which has a power spectral density that falls at 3dB/octave with rising frequency. The latter 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 prepondance of energy at high frequencies. In other words it contains energy that is destributed geometrically rather than linearly.
Radio noise
Radio noise can originate inside the receiver, or from external sources. If there was no noise picked up with radio signals, then tiny radio transmitters could be received at any distance just by making a radio receiver that was sensitive enough. In practice this doesn't work, and a point is reached where the only way to extend the range of a transmission is to increase the transmitter power. Two sources add electronic noise to the radio signal, thermal noise from the receiver input circuits, and radiated electromagnetic noise picked up by its aerial. Thermal noise can be made lower by cooling the circuits, but this is only usually worthwhile on radio telescopes. In other applications the limiting noise source depends on the frequency range in use. At low freqencies (LF)(long-wave, or medium wave) and at high frequencies, (HF) (Short-wave), interference caused by lightning, (electromagnetic storms) or by nearby electrical impulses in electrical switches, motors, vehicle ignition circuits, computers, and other man-man sources tends to swamp thermal noise. These are sometimes referred to as 'static'. At VHF and UHF these sources can still be important, but at a much lower level, such that thermal noise is usually the limiting factor. In the microwave region, cosmic background noise may be relevant. Electromagnetic noise can interfere with electronic equipment in general causing malfunction, and in recent years standards have been laid down for the levels of electromagnetic radiation that electronic equipment is permitted to radiate. These standards are aimed at ensuring what is referred to as electromagnetic compatibility or EMC.
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.
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 ciruits, usually mounted on the aerial dish, cooled in liquid nitrogen to a very low temperature.Electronic noise is often measured in uV/root Hz, a term that derives from the fact that doubling the bandwidth of the measurement doubles the power level measured, but voltage is proportional to the square root of power. Integrated circuit devices, such as op-amps commonly quote equivalent input noise level in these terms (at room temperature).