Mastering is the final creative step in the audio production process; it is the bridge between mixing and replication (the production of final media i.e. a Compact Disc) and is correctly called Pre-Mastering.
This is the last chance to enhance audio or repair problems in an acoustically designed room with excellent monitoring acting as an audio microscope. Mastering engineers offer a fresh experienced outside opinion, they know the technical difficulties that arise whilst mixing and have the knowledge and technical ability to fix problems and add finishing gloss to final mixes.
Having great equipment in a neutral acoustically treated room is one thing but having great hearing and the capability to use the right choice of equiptment to correct and implement change in a positive way is another.
The Mastering Engineer
Mastering engineers have to keep up to date with technology and be well versed with a broad spectrum of music trends with an open mind across the board. One day they could be mastering a classical orchestra the next a commercial punk rock track, so they have to be able to adapt to the different styles and be able to work in a relaxed, subjective, yet professional manor.
Mastering engineers usually have years and years of experience behind them; for instance some of the big name's are 50+ years old and still going strong! It does seem strange that even though the engineers are getting older their hearing is not? Well the truth to the matter is their hearing is probably weakening like everyone else's but they have the day in day out experience to know how to make something sound right (if that makes sense) say for example you play the trumpet five days a week for 40 years, your going to be better then someone who plays it twice a week for 10 years even if your ears aren't as sharp they are more experienced, this is the same with mastering engineers. We come across certain problems that you just wouldn't know how to or be able to rectify unless you had solved it before or had a certain technique which you knew worked.
A good mastering engineer will know their equiptment backwards. These are like a saw and hammer to a carpenter. If an engineer had a great class A compressor but didn't have the ear nor the experience/technique to use it, then it would sound no better then a £100 compressor. The key to being a good mastering engineer is, knowing which piece of equiptement to use for the job. One EQ/compressor sounds different from another so the right choice is what makes all the difference with certain styles of music.
This gives the older more experienced mastering engineers greater power, as they have earned the respect over the years and are able to work at a quick pace, problem solving and repairing audio. They are able to make excellent judgments after listening to the track just a few times over and then apply the changes that make all the difference.
Getting the final mix ready for mastering
Mastering as previously explained is the art of manipulating a stereo mix to sound its best! So as this could seem very limiting (which it can be) surely it would be best to get it right in the mix or pretty close to!
I sometimes get a job come in that is super hot (loud) and sounds like its had mix compression added on the master bus along with for example limiting, leaving no headroom whatsoever to play with and sounding overall squashed or lifeless. For these sorts of jobs I usually request a remix or a clean mix with at least -3dBFS (decibels before full scale) head room on the loudest sections and no mix processing.
The reason I ask for a clean mix with headroom is quite simply because these sort of things are best left to fresh ears. Not only that but the fact that a professional mastering room/engineer will have much better equiptment aimed at final stage mastering and the experience to make professional judgmental decisions.
Once you've completed a mix and say for example run it through a compressor to get a fatter sound (remember it might sound fatter but what's happening to your tops?), your blocking your mix into a dead end. A mastering engineer with a much more elegant mastering compressor and experience could apply compression in a different manner which would not only fatten up a mix but keep it as transparent and dynamic as possible.
Turn up your monitors to give yourself or others the loudness you require and don't be tempted just for loudness sake to compress or limit. I think one big mistake people make when they make their final mix down is compare it too other commercial releases which have already been professionally mastered. By all means check for levels and taste but don't try and match it loudness wise!
What you should be aiming for with your final mix is a well balanced and structured mix with ample head room and dynamics to allow a good mastering engineer to evaluate the mix in a natural flat room, using excellent responsive monitoring (no colouration) and allow him/her to add the final polish and loudness to the mix.