MP3Gain adds metadata which your player can use to adjust the volume of a track or even track section using the PRO version.
MP3Gain also uses the ReplayGain standard which also specifies album gain to keep intended inter-track dynamics using fields "Peak track amplitude", "Peak album amplitude", "Track replay gain", and "Album replay gain". The target loudness is specified as the loudness of a stereo pink noise signal played back at 89 dB sound pressure level or −14 dB relative to full scale. This is based on SMPTE recommendation RP 200:2002, which specifies a similar method for calibrating playback levels in movie theaters using a reference level 6 dB lower (83 dB SPL, −20 dBFS).
Audacity's loudness normalization (which unlike normalization considers more than peaks) modifies your audio data in the source file. It offers two options, "perceived loudness" (default) and RMS:
- perceived loudness: the default -23 LUFS (the EBU standard) will produce audio that is approximately 25% of full scale.
- RMS: This will change the amplitude such that the result has the desired RMS level The default setting is -20dB which will also produce low level audio.
Both LUFS and RMS normalization ensures that different audio projects come out at a relatively uniform volume.
Not all players support MP3Gain or even ReplayGain metadata, so normalizing your audio files to a proper standard which doesn't clip or compress it too much is best, after which you should update the loudness metadata tags using your gain tools.
Note that unintended noise like pops and essing affect the maximum, so you may want to remove those first.