The term 'Unit of Loudness' refers to a measure of the intensity of sound. The most commonly used units are decibel (dB), phon, sone, and BEL. Decibel is a logarithmic unit that measures the ratio of two sound intensities. Phon is a unit of loudness level relative to a reference level of 40 dB. Sone is a unit of loudness that is equal to the loudness of a 1 kHz tone at 40 dB. Sones are a unit of loudness level relative to a reference level of 40 dB. Finally, BEL is a unit of loudness level relative to a reference level of 10 dB.