IEC 60027-3 Ed. 3.0 b:2002, Letter symbols to be used in electrical technology - Part 3: Logarithmic and related quantities, and their units

Applies to logarithmic quantities and units. Quantities that can be expressed as the logarithm of a dimensionless quantity, such as the ratio of two physical quantities of the same kind, can be regarded and treated in different ways. In many cases, differences do not affect practical treatment.