There is quite a bit of mystery surrounding the mastering stage of the recording process. Different sources will have lengthy, and sometimes conflicting, ideas about what mastering really means. I’m going to boil it down to the basics in a single sentence.
Mastering is preparing your audio for publishing.
It used to mean preparing for duplication on an album, cassette, or compact disc. In our digital age of mp3 and iTunes, it now also means preparing for conversion to digitally distributed media. What exactly does preparing mean though? Preparing involves a few key concepts.
You want your final mixes to sound balanced across the frequency spectrum. The bass should not be so strong that it takes over the mix (unless maybe you are mixing a hip hop record). The highs should not be shrill and cutting. The mids should be smooth so they don’t cause listener fatigue. A high quality EQ can be used to take care those problems. A compressor is not typically though of as an EQ but you can use a multi-band compressor to even out the levels between frequency bands. A multi-band compressor could be used to compress the frequency range of the vocals differently than the frequency range below and above them. This can give you a more balanced frequency response across the spectrum without stepping on your vocal track.
We already talked a bit about compression in the section on equalization. Dynamic range is the difference between the softest and loudest sounds of your material. This range is typically reduced via compression during the mastering process. This can keep your listeners from needing to constantly adjust their volume control in order to hear every section of a song. There is a lot of discussion to be had on the topic of reduced dynamic range. There is a movement in the industry toward ever decreasing range while audiophiles feel this is sucking the soul out of music. Rather than choose a side, it is the mastering engineer’s responsibility to judge material on a case by case basis. If you are mixing a metal album you will typically go for a much more limited dynamic range than if you are mixing an intimate solo violin performance. Do what sounds right for the music.
Normalizing is the technique of getting all the songs for your album to be at roughly the same volume level. You don’t want each song on your album to be at a different volume level (unless it is an artistic decision). It can be very frustrating for your listener to have to turn the volume up because a song was too quiet, only to be blasted out of their headphones on the very next track. This can really make a record sound disjointed.
I hope this has taken some of the mystery out of mastering for you. Read some of my accompanying articles to get a little more concrete and practical advice on mastering your recordings.