Every electrical system produces a certain amount of low-level electrical activity called
noise. The noise floor is the level of noise inherent in a system. It is nearly impossible to
eliminate all the noise in an electrical system, but you don’t have to worry about the
noise if you record your signals significantly higher than the noise floor. If you record
audio too low, you raise the volume to hear it, which also raises the volume of the
noise floor, causing a noticeable hiss.
The more a signal is amplified, the louder the noise becomes. Therefore, it is important
to record most audio around the nominal (ideal) level of the device, which is labeled
0 dB on an analog audio meter.
The signal-to-noise ratio, typically measured in dB, is the difference between the
nominal recording level and the noise floor of the device. For example, the
signal-to-noise ratio of an analog tape deck may be 60 dB, which means the inherent
noise in the system is 60 dB lower than the ideal recording level.