From an email-response I sent today:
I think there are three elements to successfully mastering an album:
1. overall EQ of the music
2. compression – we may not like it, but the audience is accustomed to hearing compression and therefore usually prefers a certain amount of it
3. sound level/volume of each song
The third element is the hardest to master – because a song will subjectively feel louder or softer than the previous song, but they may objectively be equals. That’s when the experience of a mastering engineer shines.
And THIS was the sponsored link Google served up in Gmail… Funny