The logarithm of the ratio of that quantity to a reference quantity of the same kind. The base of the logarithm, the reference quantity, and the kind of level must be specified.
Examples of kinds of levels in common use are electric power level, sound-pressure-squared level, voltage-squared level. Level as here defined is measured in units of the logarithm of a reference ratio that is equal to the base of logarithms. In symbols,
where L is level of kind determined by the kind of quantity under consideration, measured in units of log; r is the base of logarithms and the reference ratio; q is the quantity under consideration; and is the reference quantity of the same kind. Differences in the levels of two like quantities q and q2 are described by the same formula because, by the rules of logarithms, the reference quantity is automatically divided out:
logr (q1 /q0) - logr (q2 /q0) = logr (q1 /q2).