What is Loudness Normalization?
Loudness normalization is when a streaming service or player automatically turns tracks up or down so they all play back at about the same perceived loudness, measured in LUFS.
Without normalization, a listener would have to keep reaching for the volume control as a playlist moved between quiet and loud masters. Services measure each track's integrated loudness and apply a gain change toward their reference level, for example about -14 LUFS on Spotify and YouTube and about -16 LUFS on Apple Music.
Louder tracks are turned down. Whether quieter tracks are turned up depends on the service and the listener's settings, and some services only turn up as far as the track's peaks allow. Some players also keep the level difference between songs on an album so that a quiet intro stays quiet.
Normalization is why the old race to make every master as loud as possible makes less sense today: a crushed master ends up at the same volume as a dynamic one, but sounds flatter.
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