Master bus compression is one of the most-debated topics in mixing. Some engineers swear by it from the start of every mix; others avoid it entirely. The truth is in the middle: master bus compression done well makes a mix feel cohesive and finished; done badly it kills dynamics and creates the obvious "compressed" sound that defines bad amateur mastering. The difference is in the parameters.
What master bus compression does. When all the elements of a mix pass through a single compressor on the master bus, they move together — when one element (usually the kick or vocal) triggers the compressor, every element in the mix gets quieter for a fraction of a second, then comes back together. This shared movement is "glue" — the perception that all the elements belong to the same recording, in the same room, at the same time.
The standard master bus compressor settings. Ratio 1.5:1 to 2:1 (very gentle). Attack 30 to 100 ms (slow enough to let transients through). Release auto, or 0.3 to 1 second (slow enough to be musical, fast enough to recover between sections). Gain reduction 1 to 3 dB on average. Threshold set to achieve that gain reduction, not to a specific dB value.
Why these settings. The slow attack lets the snare crack and kick punch through unchanged — the master compressor only reacts to the body of the sound, not the transients. The slow release means the compression breathes with the song rather than pumping audibly. The gentle ratio means the compression is subtle even when triggered. The small gain reduction means you should be able to bypass the compressor and not hear an obvious difference — only feel that the mix is slightly less cohesive.
The classic master bus compressors. The SSL G-Series bus compressor (or any of its many emulations) is the industry standard. Its sound is described as "the sound of a record." The Fairchild 670 is the warmer, vintage alternative. Software emulations of both are widely available and inexpensive. Native DAW compressors can do the same job if you set them with the parameters above.
When to add master bus compression. Two schools of thought. School A: add the master bus compressor at the start of the mix and mix into it. The compressor influences your decisions throughout the mix process, leading to a more cohesive final result. School B: mix without master bus compression, then add it at the end. This gives you full dynamic range during mixing decisions and adds glue only as a final polish step. Both work; pick the one that fits your workflow.
The mix-into-the-compressor approach. With the compressor on from the start, every fader move you make affects the compressor differently. You hear the cumulative result of your mix decisions, not the isolated impact of each one. This approach forces you to think holistically about the mix from the start. The risk: if you mix into the compressor and then turn it off at the end, you may discover your mix doesn't hold up without the compressor crutch.
The polish-at-the-end approach. With no master bus compressor during mixing, every decision you make is purely based on the dry sound. At the end, adding the master bus compressor adds the glue without forcing earlier decisions. The risk: you may need to redo some balance decisions because the compressor changes the perceived loudness of different elements differently.
Reference levels matter for master bus compression. Aim for an average of -14 to -10 LUFS during mixing. Anything louder is fighting the loudness war and will sound over-compressed. Anything quieter and you will struggle to make confident decisions about subtle changes. -14 LUFS is the streaming standard for most platforms; mixing at this level means your mix translates accurately to streaming playback.
Common mistakes: too much gain reduction (audible pumping, the obvious compressed sound), too fast attack (kills the punch of drums and the dynamics of vocals), too aggressive ratio (over-compresses the loudest elements), forgetting to bypass and check (you may have a compressed mix that sounds good but doesn't need the compression), using a brick-wall limiter instead of a compressor (limiting is not the same as gluing).