Using Headphones in Mastering
Headphones prevent crossfeed (left speaker to the right ear and vice versa). There are plugins and software that help to simulate crossfeed.
It is best to use speakers, but headphones are acceptable when you need to perform detailed listening.
Mixing only on headphones heightens the chance of making mistakes, regarding imaging and tone. The track may translate well to other people on headphones, but not necessarily to those listening on speakers.
Through repeated exposure to great sounding material, you will build up an internal reference of level and tone.
The start of this journey is building out a collection of reference material to help you understand how correct tonal balance and level should sound.
Listen to Mastering References -- a Spotify playlist by iZotope: https://izo.to/2NiDPTR
It would be best if you had .WAV files for reference material, ideally, 24-bit.
Setting Your Payback Level
Play everything through a single playback system that you can trust.
In mastering, you want to leave your playback level static, not changing it like in the mixing stage.
How do you do that?
Load 12 tracks into your D.A.W., and monitor the loudest section of each. You should notice that you can jump between tracks without changing the playback level. Then, put your mix in the same project, and you'll hear the differences immediately. It will become apparent what's different about your project vs the reference material. You begin to notice certain features, such as the level and tonal balance which you can use as a point of reference for adjusting your work.
Set the output on your interface or D.A.W. to the level you were monitoring the reference material at. Use this as baseline level for which well-mastered material sounds best at, in your environment (85 dB SPL for monitors recommended). That way, when you begin mastering, you can sit down and turn your output to this level, giving you an immediate sense of what needs addressing. Use the meter readouts from the reference material to provide you with a point of reference to inform your decisions when mastering.
MV: I think the idea behind this is to understand you're own listening environment, and at what level well-mastered music works best at. Say, well-mastered music hits -14 LUFS on your meter, and you like monitoring at 85 dB SPL (on speakers in a medium to large room), you can set your playback level to 85 dB SPL and use that -14 LUFS measurement as your goal level.
You can measure SPL (sound pressure level) on your phone with an app. Hold it in your listening position and go through your references tracks. What you want to end up with is a monitoring level in the neighbourhood of 85 dB SPL. 85 dB is a magic number as this is where our hearing is most even.
MV: This wasn't clarified but never monitor in headphones that loud. In headphones, around 60 - 70, dB is optimal to perverse hearing. But, find an optimal level for you that doesn't fatigue your ears over extended periods.
Be Aware Of Your Echo Chamber
One of the most significant challenges of trying to mastering your music is revisiting decisions you've made and doing something different.
Just by inviting someone to come and listen to your music, it makes you hear things differently. Ever had that experience? When showing your friend a track, suddenly you hear everything differently? It's the second listener effect.
Settings Up Your Listening Environment
Form an equilateral triangle between the speakers and your ears. The symmetry of your position in a room will cause you to experience different sonics. If you're off to one side, closer to a boundary on one side of the stereo image than the other, this will cause you to hear an inaccurate representation of the music. You may experience phasing issues such as comb filtering.
The worst place to listen in most environments is right in the middle of the room. All of the room modes will collide to the greatest extent in the geographical centre of a room.
You should position yourself 1/3 or 2/3's of the way back from the front wall if possible.
Don't place speakers near a boundary; boundaries will exaggerate the low-frequency response, a corner even more so.
Set up a playback system so the front plane is across the narrow wall and not a wide wall, that way the first reflections bouncing off the wide wall will take longer to get back to you. That first reflected sound has the highest chance of interacting with what you hear coming from your speakers.
You Can't Clean With a Dirty Mop
To work on a wide range of material, you need surgical tools that don't impart any colour onto the sound. You should be able to make the necessary adjustments without adding anything else.