RMS (Root Mean Square) is a time-averaged measure of a signal's amplitude — square the samples, average them over a window, take the square root — which tracks the energy in the sound rather than its instantaneous spikes. Because perceived loudness depends on sustained energy, RMS correlates far better with how loud something sounds than peak does, and it underlies the level detection inside compressors and limiters. For loudness compliance it has been superseded by LUFS, which adds the frequency weighting and gating that RMS lacks, but RMS remains an everyday tool in metering, dynamics processing, and quick level comparisons.
