Fixing it in the master, or fixing it in the mix?

Mastering works on one file in which every element is already summed together. There is no vocal channel, no kick, no guitar. There is one signal.

That single fact decides almost every “mix or master” question, and it gives you a test that works nearly every time.

The test

Can you name the element causing the problem?

• “The vocal is buried.” — a name. Mix.
• “The kick disappears under the bass.” — two names. Mix.
• “The hi-hats are harsh.” — a name. Mix.
• “The whole track is slightly dark.” — no name. Master.
• “It does not feel as solid as my reference.” — no name. Master.
• “It is quieter than everything else.” — no name. Master.

If a problem has a name, the fix needs access to that element, and mastering does not have it.

Why master-bus EQ cannot separate anything

Say the low mids are muddy because a synth pad is too thick there.

At the mastering stage you cut 250 Hz. The mud reduces — and so does the body of the kick, the weight of the vocal, the warmth of the bass and the fullness of the guitars. You fixed one instrument by thinning five.

Sometimes that trade is acceptable. Usually it is not, and it is why masters of unbalanced mixes sound thin, hollow or oddly scooped.

In the mix, the same fix is a 2 dB cut on one channel and nothing else changes. That is the entire argument.

What mastering genuinely fixes

Problem	Mastering can help?
Whole track slightly dark or bright	Yes — broad EQ
Track feels loose across sections	Yes — gentle compression
Not loud enough for release	Yes — limiting, within reason
Peaks exceed the ceiling	Yes — that is the limiter’s job
Tracks on an EP do not match	Yes — that is a mastering job
Slightly narrow or slightly wide	Sometimes, carefully
One instrument too loud	No
Two instruments colliding	No
Vocal unintelligible	No
Arrangement too dense	No
Phase problems between mics or layers	No

The pattern: mastering adjusts the whole, and cannot address a relationship between parts.

The grey areas

“The low end is too heavy.” Could be either. If it is heavy across the whole track, a broad master-bus shelf is legitimate. If it is heavy only when the bass plays a particular note, that is a mix fix — probably a dynamic EQ on the bass.

“It sounds harsh.” If everything is slightly bright, master. If it is the cymbals, mix.

“It is not punchy.” Almost always the mix. Punch comes from transients and arrangement, and mastering can only preserve those, not create them. If your limiter is removing punch, see what is a clipper for the usual remedy.

“It collapses in mono.” Always the mix, and specifically a phase problem.

The cost comparison

Going back to the mix feels like a defeat and a delay. It is usually neither.

• In the master: twenty minutes of careful compromise, a result that is better but not right, and side effects on four other elements.
• In the mix: two minutes, one channel, no side effects.

Professional mastering engineers ask for revised mixes constantly, for exactly this reason — see how the pros master.

How to avoid the situation entirely

Most “can mastering fix this” questions come from mixing without checking.

• Mix at low volume, where balance problems are obvious.
• Check in mono early, not at the mastering stage.
• Check on a small speaker while you still have channels to adjust.
• Reference against finished tracks during the mix, not after it.
• Mix with everything playing — see mix with everything playing.

A mix that has passed those checks arrives at mastering needing very little, which is exactly the state that makes a master sound effortless.

The rule, one line

Mastering makes a finished thing better. It does not make an unfinished thing finished.

If you are asking a master to solve something, write the problem down in a sentence first. The presence or absence of a specific instrument’s name in that sentence is your answer.