The Best Audio Mastering Book, Video, and Course by John Rogers

In one sentence, it gives the mastering engineer more room to work with.

If I compared a hair stylist to a mastering engineer, hair length would be mix headroom.  If someone came in with 18" of hair, the skies the limit as to how she could style it.  But, if they came in with only 1" of hair, her style choices (options) are very limit.  In audio mastering, no headroom limits your options.

In the audio mastering process, a series of EQ boosts and cuts are performed.  Most of the time you're going to need to boost something, even if it's only a little +2db boost at 100hz.  Well, if the song is already at 0 volume level or higher, you might not be able to make a necessary boost without distorting.

And if a song has very low dynamic range (the meter barely moves) it's probably over-compressed. Which means it could lack punch, power, clarity, or could even limit EQ options.

I want a song mix with some headroom and decent dynamic range. "I" want to EQ it as necessary, "I" want to compress it as necessary, "I" want to be able to set the overall volume as necessary, and I don't want to work with a distorted mix.

This section actually ties in with a previous one where I talk about not giving the mastering engineer a song that's already 75% mastered by you.  Don't do it!  And if you do, just master the other 25% yourself and save your money.

In order to produce a good master, a mix needs proper headroom and dynamic range.

WHAT IS HEADROOM?
Headroom is the distance between an audio tracks peak level (when the meter is at its highest) and 0 level on the output meter.

As a song plays, the output meter on the stereo/main out bounces up and down with the music.  You can see how high (loud) an audio track peaks by looking at this meter.  Anything peaking over 0 level usually means distortion, so at all costs stay below 0 level.

How far is your peak loudness below 0 level? That's the simple definition of what headroom is.  So, if your meter is peaking -3dbs below 0 level, you have 3dbs of headroom.  If your meter is peaking right at 0 level, you have NO headroom.

Note - +3dbs to +6dbs of headroom is the standard recommended amount, but many times a song with more or a little less headroom can still be mastered with no problems.  Although it really makes no sense to give a mastering engineer a mix with 0dbs of headroom and risk the chance that it's slightly distorted.  There's no reason to do it because overall song volume is done in the mastering process, not in mixing.

WHAT IS DYNAMIC RANGE?
As a song plays, the output meter constantly goes up and down.

Dynamic range is the area between the peak level (when the meter is up) and the low level (when the meter is down). Roughly, 3dbs to 6dbs of movement between the high and low meter level is a typical amount of dynamic range, but this does depend on the genre of music. 

Note - One problem you want to watch out for (if you're mixing your own music) is dynamic range between song sections.  You don't want a +10db or more difference between the verses and choruses.  This creates a mastering problem because the verses will never be loud enough compared to the rest of the song.  Roughly a 6db difference is as high as you want to go.

Here's another popular question I see online, always with the wrong answer! "There's no such thing as radio ready mastering!"  REALLY?

Let me explain how this works.  If a record label, after paying them millions, submits a new release to the radio stations and it's very poorly mastered (distorted, over-level, super bright, heavily compressed) it will be REJECTED.  They will not play it on the radio.  They'll tell the label the quality of the song is terrible and will request a re-master (maybe even a remix).

Technically, it would never even go this far because an executive working for the record label would reject the poor master and get it corrected before the radio station even heard it.

Two Facts -

1. Some songs aren't ready for radio play and are rejected by management for poor quality reasons.  This happens very often in the music industry.

2. Songs that are playing on the radio, are radio ready!

If songs are rejected for radio play, then there has to be a radio ready mastering quality standard.  There is!  There's your answer.

Note - This section is basically a tip for those of you working with clients or if you plan on submitting your mixes to a mastering engineer.

DEFINATELY 100% NO!!

I've actually seen a few sound engineers online say the opposite.  They say to make your mix sound as close as possible to a finished master.  "All you want the mastering engineer to do is make the song louder." These comments are based on common sense and theory, NOT on real world experience working with thousands of clients (like I have)!

Note - If the only thing your songs really needed was for the mastering engineer to make them louder, why not just make them louder yourself and save the money?  Duh!  No, pay someone $500 to do it in 15 minutes.  I want that job!

Here's why you don't tell clients to partially master their own songs:

1. Because you're paying an experienced mastering engineer to properly do the entire job.  Here's a non-music example.  Before you get your car detailed, do you clean it spotless inside and out, but leave only the windshield dirty?  So that's all the car detailer has to do is wash your windshield and he's done?  NO!  That doesn't even make any sense.

You're paying for a car detail and you want every process that comes with it.  That's their specialty and you want their expertise in every area, not just the windshield washing process.  The same goes for audio mastering.

2. Mixing is not mastering.  For a song to sound like a commercially mastered song on the radio you MUST use effects on the stereo/main out bus.  When you do use effects on the stereo/main out bus you are mastering not mixing.  And you're not supposed to partially master your songs if they're going to a mastering engineer.  Read #1 again.

3. And the #1 reason is THEY CAN'T DO IT!  They don't have the knowledge, skills, replicating abilities, etc. That's why they're looking for a mastering engineer to begin with!

I explain to my clients to submit a mix that's clean, but slightly dull with lower overall volume levels, so that I can bring everything up to where it needs to be.  Nothing on the stereo/main out bus.

In the past, before I was giving this info, do you know what my clients sent me?  I would say half the submissions were partial masters where the clients were trying to make them sound radio ready.  The problem was they were TERRIBLE!  Way over-level, distorted, super bright and/or super bass, way too much compression, etc.  Many times they had everything wrong!

In a perfect world, yeah send me a song that's already mastered and I don't have to do anything to it.  But I know from years of experience working with actual people that this is a very unrealistic request.  Most can't do it, that's why they're looking for a mastering engineer to begin with!

I've also heard the comment that mastering is taking all the songs on a CD and making them all sound similar.  Really?  That's all mastering is?  So, I'm supposed to take the best mix on the CD and make all the other songs sound just like it?  That doesn't even make any sense!  What if the best mix on the CD sucks?

What if someone gives me only one song (which happens daily)?  It can't be mastered because there isn't an entire CD?

I make every song on the CD sound as close as possible to a commercial industry standard song. Each song is mastered to sound the very best it can, regardless of past or future songs on the CD.  Also, every song is mixed differently and requires different actions to achieve this.  After I do this, all the songs on the CD are comparable in every sonic area.

Audio mastering is taking a mix and bringing it up to commercial industry standards.