Quite a few people questioning this...some know a little about it, some are in the dark. I do this for a living and have been for many years. It doesn't make me any better than anyone else, but I know quite a bit about the subject and wanted to share what I know in hopes of helping you.
What is mastering? This question pops up from time to time. Most people think you just eq a few things, add a compressor and make the tune super loud. There's a bit more than that involved, honest.
The first thing a mastering engineer needs is good ears to make the right calls along with a good room and good monitors. While I'm on this subject, I'd like to share something I posted in another thread that I think is VERY important here.
The stereo master NEEDS to be polished in a way that only the mastering engineer can do. If you only have one set of monitors, you'd not be able to master correctly anyway. Seriously....think about it, when you render that audio and export that song, in your mind you're done unless you listen to it and want to make changes. How could you master it on the same speakers you mixed on and decide what to enhance? See my point?
See, when your tune is completely finished, the first thing the mastering engineer is going to do is listen for things you can't even hear because you are way too close to the material. Way too close! You figure, you have been working on this for how long? Maybe a month...maybe 6 months...who knows? You will struggle to make the right calls if you engineer, mix, perform on the material for long periods of time. The mastering guy has a fresh set of ears...he's not heard the mixes 20,000 times like you have when you did what needed to be done...and he's not biased. He'll listen to it and make mental notes about what he thinks may need to be changed on speakers that are perfect for the job.
As he makes his notes and gets to know the song/songs, he'll listen again to make sure what he has written down is what he thought he heard. On the 3rd listen, he'll have a scope up to see if his thoughts about what he would change eq wise are close. Now keep in mind, if you are thinking the master is supposed to come back to you sounding like something completely different, you hired the wrong mastering engineer. The object is to preserve the mix you have given him, he removes sub low rumbles, extreme high end, levels and compresses the mix and if it loses any spaciousness, he adds it back in. He checks for clicks, pops, artifacts, hums from guitars being left open where someone may not have slip edited a clip...the list goes on and on.
This is what I listen for and fix with every tune that comes my way:
Leveling of all audio for consistency, Fade ins, Fade outs, Stereo wideness and separation, Stereo narrowing, Noise, hiss, hum, crackle removal, Eqing, Compression, Over-all loudness maximization, Professional Dithering: higher audio projects from 24/96, 24/48 down to CD format at 16/44 and DC Offset removal.
Those are the things you need to be doing to totally improve your mastered tunes. There are times where it is also a good idea to provide what we calls "Stems" of stereo audio taken directly from your project mixed at the exact song levels with effects processed on the tracks. This allows the mastering engineer to fly in something that may need a little extra push. Most good mastering engineers will demand to have those things and will ask you to export your audio at a certain dB level so they have the headroom to work on your project. Each mastering engineer will have his own rules. It's best to follow them and hopefully, if the guy is anything like me, he'll work with you before you actually send the material to be mastered. This is one thing that I do that many do not. I have a listen review of the material and tell the client whether they should remix it or not before I even touch a thing. My name is at stake here too and none of us has time to polish what may be a turd or even take someones money for something that may not be worth it. This of course is at the engineers discretion.
To do this right, in my opinion, you need the right tools and you need to know what to listen for. I can't stress that enough. There are times when you shouldn't even waste the money going to an engineer to get things mastered. If you are just giving out a demo to people or are a hobbiest, there is no reason to spend the money on an engineer unless it's something you just want to sound great. If you will be selling the cd or have an indy label deal, by all means don't even procrastinate and waste your time trying to do this on your own. You're too close to the material to make the right calls. Trust me when I tell you!
Can I use Ozone or T-Raxx? Yeah you can and they do a decent job, but if you cannot achieve what I have mentioned above properly, you need to get the right software and tools to get the job done. Keep in mind, though Ozone and T-Raxx are good products that can help you achieve good results, they are consumer products that were not made to handle what you could be capable of doing in a real mastering environment with the right stuff. The great mastering guys we all know and love are not using programs like that because they are very limited. Waves plugs and the equivalent as well as a mastering program made for mastering is essential due to having graphs, numbers and other tools to assist you in making the right calls. No one needs to rely on a graph or a group of numbers when mastering. However, when those numbers are all correct and you can achieve a min/max sample value of 32767 or less with DC offsets of 0% at 16/44 you better believe you're mix will not only sound good, but it will literally look good on your graphs as well as your wave form presentation. They all walk hand in hand. Sure, sometimes you'll see a weird eq curve that just sounds good. If it sounds good, it is good first and foremost. Everything in moderation and graphs, charts and numbers only if you are having issues pinpointing something.