Loudness normalization¶

Musium normalizes the perceptual playback loudness to make tracks sound equally loud. It does this by estimating the loudness from audio data, and compensating playback volume for that.

Playback volume¶

The volume slider in Musium treats 0 dB volume as 0 dB gain for the output device, but it does compensate for track or album loudness relative to a target loudness of -23 LUFS. When the Musium volume slider is set to 0 dB, this means that a track with a loudness of -23 LUFS will play back without volume adjustment (at maximum volume of the device), and a track with a loudness of -10 LUFS will play back with device volume at -13 dB. When the Musium volume slider is set to -5 dB, a track with a loudness of -10 LUFS will play back with device volume at -18 dB.

It is possible to set volumes beyond 0 dB in Musium, but this is only effective for inherently loud tracks: a track with a loudness of -23 LUFS is already playing at the maximum device volume, but a track with a loudness of -13 LUFS has enough headroom to allow the volume to be set to 10 dB.

Track and album loudness¶

When two tracks from the same album play consecutively, Musium adjusts for the album’s loudness rather than the track loudness. In other words, Musium does not change the relative volume of tracks on the same album. You can listen to the album the way it was mastered.

When tracks from different albums play consecutively, Musium adjusts for track loudness, because this allows slightly better matching than album loudness.

Computing loudness¶

Musium automatically computes the loudness for any new tracks when they are added to the library. This can take some time, especially on devices that do not have a fast CPU, like a Raspberry Pi. The upside is that this also instantly verifies that Musium can decode the file. Loudness is saved to the database.

Loudness measurement¶

Musium computes the integrated loudness as defined in ITU-R BS.1770-4. For album loudness, it computes the integrated loudness over the concatenation of all tracks on the album.

To reproduce the measurement externally, the BS1770 flacgain utility can be used to analyze a collection of flac files, and to write loudness to BS17704_* tags. In the past Musium relied on these tags for loudness information, but since Musium 0.11.0 they are no longer used.

ReplayGain¶

ReplayGain has been very influential for loudness normalization, and it is widely supported by both taggers and players, but unfortunately it has become ambiguous over time. ReplayGain does not store the loudness of the track directly, instead it stores the gain that is needed to bring the track to target loudness. The target loudness was initially well-defined, but tools started offering different options to accomodate to the reference loudness of different standards such as EBU R-128, ATSC A/85, and ReplayGain 2.0. This means that ReplayGain tags from different sources do not necessarily have the same meaning, which defeats the purpose of normalization. In practice this means that ReplayGain is only really useful if you can be certain that all tags were produced by the same program with the same settings. Musium sidesteps this problem by computing the loudness itself rather than depending on ambiguous tags.