decibel

n. (dB) A logarithmic unit in a scale of loudness or sound pressure level (SPL) equal to one-tenth of a bel, which is a 10-fold increase in sound pressure level. Because it is logarithmic, the dB scale is not additive in a linear fashion. The scale has been arbitrarily set at 0 for a 20-micropascal sound pressure level at 1-kHz tone, which is about the normal human threshold for hearing, so positive and negative values for dB are possible.