RMS (Root Mean Square)
Definition
A measurement of the average power of an audio signal over a window of time, giving a better indication of perceived loudness than instantaneous peak level alone.
In Simple Terms
A measurement of the average loudness of your audio over time, which better represents how your track actually sounds than a peak meter does. Think of peaks as the waves and RMS as the tide level.
In Practice
A mastering engineer monitors both peak and RMS levels simultaneously — peak ensures no clipping, while RMS indicates the overall density and energy of the mix.