ITU-R BS.1170-4 and EBU R128 recommendations introduce a new paradigm for audio metering, which define a way to measure perceived loudness of audio material in a normalized and reproducible manner.
Please refer to the official documents freely available online at tech.ebu.ch/groups/ploud * or consult a reference book such as “Audio Metering. Measurements, standards and practice” by Eddy Brixen (Focal Press, ISBN 9780240814674) for detailed information on this subject.