Decibels:

Audio signal amplitudes are measured in deciBells (dB). The range of loudness which can be perceived by the human ear varies over 6 orders of magnitude making linear measurements of loudness difficult to deal with. Therefore a logarithmic scale of loudness, the deciBel scale, has been adopted.

This logarithmic measurement of signal intensity describes the amplitude of a signal relative to a standard referrence amplitude. Therefore the dB scale is said to be a relative, not an absolute scale of measurement. The referrence used for 0dB varies according to the circumstances for which the signal is being measured. Here are some of the most common referrences used:

0dBm - one milliwatt

0 dBv - 1 Volt

0 dBa - similar to dBv except that the scale is frequency-weighted by a curve that describes the variation of sensativity with frequency for human hearing.

0 dBu - 0.775 Volts = 1 milliwatt in a circuit with resistance of 600 Ohms.

0 dBspl - 0.002 dines per CM squared.

One Further Note: A doubling of absolute signal amplitude represents an increase in dB of 6dB. To calculate dB for amplitude:

20 Log(Amp / RefAmp).

A doubling of absolute signal power represents an increase in dB of 3dB. To calculate dB for power:

10 Log(Power / RefPower).

(Handout on dB - curtasy of Jay Kadis).