The perceived loudness of a sound is measured in

The decibel is the relation between two values: the pressure produced by a sound wave and a pressure taken as a reference. Resulting in a dimensionless value.  

It should be noted that itself is not a unit of measure, since in reality the unit is bel (which is not part of the International System of Units) in honor of Alexander Graham Bell.  

However, given the amplitude of the measured elements in practice, its submultiple, the decibel, is used. That is, this quotient is a logarithmic expression, where