The term 'Loudness Unit' is used to describe the measurement of sound intensity. It can be measured in four different ways: Sone (4 letters), Phon (4 letters), Decibel (7 letters), and BEL (3 letters). A Sone is a unit of loudness perceived by the human ear, while a Phon is a unit of loudness relative to a reference level. A Decibel is a unit of sound pressure level, and a BEL is a unit of sound power level. All of these units are used to measure the loudness of sound.