Some songs are louder than others, and it's annoying to have to
constantly tweak the volume knob to keep things at a reasonable level.
Enter "replay gain".  You use a tool to go through your music and add
tags marking how loud each song/album is, then you play them with
something that understands the tags and adjusts the volume for you.
Very convenient.  Robert Downes (Bobulous) has a [nice post with
scripts for tagging FLAC files][post] (the heart of his scripts is
`metaflac --add-replay-gain *.flac`).  Then just add

    replaygain "album"

to `/etc/mpd.conf` and restart [[mpd]].  Voilà!

If you're not using `mpd`, modern versions of [flac][] come with an
undocumented `--apply-replaygain-which-is-not-lossless` option.  It's
annoying that they haven't gotten around to documenting this option
(since 2004!), but the syntax itself is pretty simple (description
from [this post][syntax]):

    --apply-replaygain-which-is-not-lossless[=<specification>]

where `<specification>` has the format:

    [<preamp>][a|t][l|L][n{0|1|2|3}]

    <preamp>    Floating point dB to add to the existing gain.
    a|t         Album (a) or track (t) gain.
    l|L         Peak-limit (l) or 6dB hard limiter (L)
    n{0|1|2|3}  Noise shaping from none (0) to strong (3) when
                dithering back to integer amplitudes.

The default is `0aLn1`.  If you look in the `flac` source, this
documentation is in `doc/html/documentation_tools_flac.html` (where it
is commented out).

[post]: http://www.bobulous.org.uk/misc/Replay-Gain-in-Linux.html
[flac]: http://flac.sourceforge.net/
[syntax]: http://www.hydrogenaudio.org/forums/index.php?showtopic=17293&pid=171075&mode=threaded&start=#entry171075
[[!tag tags/linux]]
[[!tag tags/fun]]