"If you listen to music on CD much, you'll notice that some CDs sound much louder than others, and I'm not talking about Ministry's The Mind is a Terrible Thing to Taste versus Sarah McLachlan's Touch. Two CDs of the same genre, when played on the same CD player, at the same volume, can have drastically different playback volume. This difference carries over when you rip the CD to MP3, and can be really annoying when you're going from song to song on your MP3 playlists on your computer or portable music player. One way to iron out the differences is to use MP3Gain to adjust your MP3s to have the the same volume."
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