Mastering is audio adjustments performed to the final stereo mixes unifying the overall sound and sonic impact of the tunes in the album (CD, demo, DVD, LP, MP3, etc.) before going to manufacturing, replication or uploading songs for end listeners (the fans). In other words, it is the opportunity to make sure each song has the same tone, bass and highs, and the same overall volume so the end listener never has to touch their volume knob song to song and plays back evenly over radio and on the multitude of consumer playback systems and environments (from cars to headphones to supermarkets boom boxes, computer and so forth). Easier said than done, today’s mastering engineer works in a acoustically controlled environment they know the results of through listening experience and has specific mastering hardware and software geared toward getting these results in this final audio production stage.
The first mastering engineers, and many still today, also made final LP, 45 or 78 disc masters on a special lacquer disc with a record cutting lathe that physically cut the groove into the disc. Mastering engineers for records had (and have) to make the record disc loud as it could be without the groove being to deep and wide to fit all the songs on the record side and not make the bass note push the needle out of the groove. Too quiet and the listener or DJ broadcaster would have to turn up the volume potentially decreasing fidelity. Too much high end and it will sound shrill…and so on.
With CD’s, these physical constraints do not exist, however extreme care to make the mastered audio sound better, louder, bassier, brighter, punchier, without sacrificing clarity, or unknowingly adding subtle harmonic distortion which may sound better initially, but make the record harder to listen to over time (though brain fatigue sorting out the subtle audio distortion. Think about how palatable some Beatle’s records remain, yet many hit records less than a decade old are seldom revisited!). The creation of louder and louder recordings released in the past 15 years or so has created what the industry calls the ‘loudness wars’. No standardization for how much headroom should remain on a digital recording was established so popular recordings are jammed right against the maximum allowable level compromising peak integrity. The mastering engineer weighs these factors and more when mastering audio.