I always knew that based on the recording and production process, some tracks play louder than others. But I never knew that this was done intentionaly to make a track "stand out" or more noticeable when played back to back with other tracks, such as from a jukebox. And these wars for sound level happened mostly around the time when a particular media format is most popular.
http://en.m.wikipedia.org/wiki/Loudness_war#section_5
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