Noise

Definition

Noise is an unwanted and random perturbation on a signal of interest. Any measurement is affected by noise, which can be fundamental (e.g., shot noise), instrumental (e.g., vibrations), or due to a perturbing medium (e.g., atmospheric turbulence). When noise sources are independent, their signals add in a quadratic way. The signal-to-noise ratio (usually noted S/N) is the ratio of the magnitude of the useful signal to the noise amplitude: it is a convenient way to assess the relative importance of a disturbing effect. The result of an experiment established with a S/N < 5 is generally considered as not reliable.