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Excerpt from Field Guide to Spectroscopy
Noise is unwanted fluctuation in a signal. Some level of noise can never be eliminated, because it results from the particulate nature of matter and light; this is fundamental noise. However, excess noise comes from imperfections in equipment, instrumentation, conditions, etc., and theoretically can be minimized or eliminated.
While too much noise is not good, it is irrelevant if the signal itself is strong enough. Thus, the concept of signal-to-noise ratio (SNR) is important. In the left spectrum below, a signal (in the position as indicated by the arrow) is obscured by the noise. In the right spectrum, the magnitude of the noise is lower, allowing for the signal to be detected without question. A SNR of at least 3 is necessary to demonstrate that a true signal exists.
There are different types of noise. Thermal noise comes from the temperature of electrical components. Shot noise arises from the existence of junctions in electrical components that charged particles (electrons) must cross. 1/ƒ noise, also called pink noise or frequency noise, is random noise of indeterminate origin but which has a larger magnitude at lower frequencies. White noise is random noise whose magnitude is constant no matter what the frequency.
The most controllable form of noise is environmental noise, which arises from the interactions of the system with the surroundings. Noise occurs at 60 Hz and its multiples (in the U.S.; 50 Hz and its multiples in other countries) because of AC power. Components picking up radio or TV signals (essentially acting as antennae) can convert those signals to voltage or current fluctuations. Flickering sources, power variations, shared electrical circuits, elevators, temperature and other weather fluctuations, and pedestrian traffic all cause noise.