Mastering

When the mix is finished, it is still not quite ready for listeners. Mastering is the last check and polish before release. A mastering engineer listens to the stereo mix on accurate speakers, ideally in an acoustically treated room, and makes small, careful adjustments to the whole track at once rather than to individual instruments.

What mastering does

• Overall tone: gentle EQ to correct the balance of bass, middle and treble, so the song sounds good on many kinds of system.
• Dynamics: light compression and limiting to control peaks and set the final loudness.
• Stereo image: small adjustments to width and to the balance between left and right.
• Consistency: on an album or EP, every song should feel like part of one record, with matching loudness and tone.
• Sequencing: the running order, the gaps between songs, and fade-ins and fade-outs.
• Delivery: the final files in the required formats, sample rates and bit depths, with information such as track titles and identifying codes embedded.

Loudness

For years, releases competed to sound louder, pushed by heavy limiting at the expense of dynamics. Today many streaming services measure each track's loudness and turn louder tracks down to a common playback level, so making a master as loud as possible no longer makes it louder for most listeners, and it can make it sound flatter. Loudness for this purpose is usually measured in LUFS (loudness units relative to full scale). The right target depends on the music and on where it will be heard.

What mastering can't do

Mastering can't fix a bad mix. If the vocal is buried or the bass is boomy, the fix belongs in the mix, where each part can be adjusted separately. A mastering engineer may send a mix back with notes, or ask for stems so that problem groups can be adjusted, which is called stem mastering.

Preparing a mix for mastering

1. Leave headroom. Avoid squashing the mix with a limiter that pushes the peaks right up to the maximum level, unless the mastering engineer asks for it (gain).
2. Export at full resolution, for example 24-bit at the session's sample rate.
3. Leave a little silence before and after the music, and don't trim or fade unless the fade is part of the song.
4. Include notes: reference tracks you like, and anything you are unsure about.

Fresh ears matter here too: many artists have a different engineer master the music than mixed it, so someone new hears it with no attachment to earlier decisions.