Signal to Noise   

This is the difference, measured in decibels, between the noise floor of an audio system and the largest signal it can record without distortion. See dynamic range.
Any piece of electronic audio equipment has a certain amount of operating noise caused by random movement of electrons (thermal noise) in its circuitry. 

The difference between the level of this noise floor and the level of the maximum signal the device can pass gives the signal to noise specification. The higher the figure the quieter the operation of the device. A signal to noise spec in excess of 90 db is very good. 

Care should be taken to operate audio equipment so as to take advantage of the available signal to noise specifications. This means ensuring that the signal fed to the device is of a sufficiently high level. To low a level will result in the noise floor of the device becoming audible.