In layman’s terms this is simply a measurement of a given noise level in a device as compared to the level of the signal. Higher numbers signify a greater difference, which is better. In technical terms it is the ratio of signal power at a reference point in a circuit to the noise power that would exist if the signal were removed (its noise floor). The maximum signal to noise ratio (which in many schools of thought is equivalent to dynamic range) of a given piece of equipment can be an important thing to know. This ratio is how much absolute noise it has compared to the highest signal voltage it can pass without distortion. While signal to noise ratio is often used as a specification to characterize relative quality differences in equipment, the way in which measurements must be done, and the degree to which they can differ, makes the true objectivity of such measurements highly suspect. Factors such as how much distortion can be allowed before you say the signal has reached “maximum” as well as other kinds of noise (like modulation noise) that may only show up when signals are present are just two examples of many variables that affect objective measurements. In digital equalizers the signal to noise ratio is a function of the maximum possible sine wave signal power compared to the quantization noise (a.k.a. quantization error) power. This is a very unambiguous value in linear PCM (Pulse Code Modulation) systems, but in other types of PCM systems the quantization noise (or quantization error) depends strongly on the level of the audio being recorded so it is very difficult to nail down the actual signal to noise ratio. It is sometimes useful to be able to compare S/N Ratio differences between equipment in certain applications, but it is more important to just understand the concept. Signal to noise ratio concerns us every time we pass audio (or video or data) though anything, and knowing what factors in our setup (such as gain structure) affect it is a fundamental part of building clean, quiet systems and mixes.