Mastering for digital formats is subtly different than mastering for vinyl or cassette reproduction. Cutting to vinyl, the mastering engineer has the physical considerations of the medium to ponder; how much signal can the medium cope with? Are there special EQing processes to be mindful of, such as Dolby noise reduction?
With digital formats, these issues are minor but with a couple of important exceptions: it’s crucial that you do not exceed 0dB with your audio as this will introduce digital distortion (see Headroom). And exactly how loud do you want the resulting file to be? It’s standard to leave headroom of between 0.5-1dB in your mastered audio file as some older CD players distort at consistently high amplitudes. Also, your metering system might not be 100% accurate – leave a small safety margin.
But there’s a more subtle issue to deal with when mastering for lossy formats. As most MP3 and AAC files are originally ripped from CDs and sound just fine for most listeners, there shouldn’t be a necessity to adjust your mastering to cope with the compressed format. But what about the reproduction medium? MP3s are often played on computer speakers: computer speakers are usually small and tinny, so should we enhance our bass frequencies to adjust for this?
Probably not: many computer systems already have a bottom end boost to cope with their physical limitations – adding yet more will leave mixes sounding unpleasantly boomy. The consensus in the mastering world is to master relatively ‘flat’, avoiding big EQ changes for MP3s.