A sound unit is a measure of the intensity of a sound. The most commonly used sound units are decibel (dB), sone, phon, BEL, and rel. Decibel is a logarithmic unit used to measure sound pressure level or loudness. Sone is a unit of perceived loudness, which is equal to the loudness of a 1 kHz tone at 40 dB. Phon is a unit of loudness level relative to a reference level of 40 dB. BEL is a unit of sound power level, which is equal to 10 times the logarithm of the ratio of two power levels. Finally, rel is a unit of sound pressure level relative to a reference level of 20 micropascals.