Signal-to-noise ratio (S/N)  --  Proportion of amplitudes (intensity) of the wanted signal to the unwanted noise in an electrical transmission system. It sets an upper limit to the dynamic range of the system. Customarily expressed in dB units. This ratio is expressed in many different ways, for example, in terms of peak values in the case of impulsive noise and in terms of root-mean-square values in the case of random noise, the signal being assumed sinusoidal (see also Weighted noise).