RMS (i.e., short for Root Mean Square) is a measurement of the average perceived loudness in music.

It doesn't take into account fast transients (or peaks) in the audio material in question, but rather the average level of sustained input audio signal for a given duration (e.g., 400 ms.) Now as an example, a record with an RMS level of -8 dB RMS is going to be louder than music with a -12 dB RMS.

N.B., RMS loudness meters are less used during composing music and the mixing stage but become even more essential, especially during the mastering stage of music and podcasts as well.
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