Signal to Noise Ratio

The ratio between a recorded signal and spurious background noise. Background noise is most commonly present due to wires and instrumentation reacting to electro-magnetic emissions from surrounding electrical equipment e.g., are welders, vehicle ignition circuits, machine tools. The signal-to-noise ratio is the ratio between the coherent and the noncoherent power at the output.

The larger the number the better.

The signal to noise ratio is defined as:

where

γ² = coherence between input and output signals

In audio systems the signal to noise ratio is often defined as:

where

SNR = signal to noise ratio [dB]

The following table gives some typical devices and their associated signal to noise ratio.

Audio device	SNR (dB)
Plain Old Telephone System (POTS)	40
AM Radio	50
LP Records	50
Cassettes	70
FM Radio	70
Instantaneous Human Hearing Range	85
CD Player	90
Studio Recording Equipment	120
Total Human Hearing Range	120