Musicians usually use decibels to determine the loudness of sound. Decibels are always ratios between quantities in the form of logarithms. Decibels are the only constant way of measuring sound quality. But even with the knowledge of decibels, the loudness quality has to be based on energy over time. Energy is difficult to measure because of the dissimilarities of meters that are used to measure sound.
Read the full article here:
Loudness and metering, just part 1. A must read