Although this de facto standard is now formally known as ReplayGain,[1] it was originally known as Replay Gain and is sometimes abbreviated RG.
ReplayGain works by first performing a psychoacoustic analysis of an entire audio track or album to measure peak level and perceived loudness.
ReplayGain-capable audio players use the replay gain metadata to automatically attenuate or amplify the signal on a per-track or per-album basis such that tracks or albums play at a similar loudness level.
Alternatively, a tool can amplify or attenuate the data itself and save the result to another, gain-adjusted audio file; this is not perfectly reversible in most cases.
Some lossy audio formats, such as MP3, are structured in a way that they encode the volume of each compressed frame in a stream, and tools such as MP3Gain take advantage of this for directly applying the gain adjustment to MP3 files, adding undo information so that the process is reversible.