Decibel

From AudioLexic

A unit of measurement that compares two values. For any quantity given in decibel, it must be referenced to some other quantity.

If we say that a sound is 100 dB loud, we are really saying 100 dB SPL (sound pressure level), which is 100 dB greater than the average threshold of hearing. Audio signals are measured in dBm, where 0 dBm = 1 milliwatt into 600 Ω. Ratios, such as signal/noise, are given in dB; a tape recorder may have a signal/noise ratio of 60 dB.