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Mix bus or simply bus compression is the practice of using a compressor on a group of tracks sub-mixed together, as opposed to only compressing each track individually. For example, I may use a compressor on my kick, snare, toms, and overhead individually, but I might also use a compressor on the entire drum sub mix to achieve an effect that's not possible through adjusting the individual compressors in isolation. Bus compressors are generally associated with the phrase "gluing the mix together" and the pedigree has a fairly interesting history, dating back to the first bus compressors installed in large analog consoles.
The A&R executives used to call the bus compressor insert button the record button because it instantly glued the entire mix together and made the mix sound like a record. Take a listen to this mix before and after bus compression through a plug-in version of the famous SSL bus compressor. Here is the mix before adding the SSL bus compression. (music playing) And here is the mix with the SSL bus compression active.
(music playing) The change is very subtle but significant nonetheless. The overall goal with bus compression is very similar to any other type of dynamics control: by taming the peaks or the or the transients, I can reduce my overall dynamic range and pull up some of the lower-level material to inflate the body of the track and give it a little extra push in average loudness.
The bus compressor also grabs hold of any straggling transients that might jump out of the mix too far, gluing the elements of the mix together better. Many engineers like to mix through their bus compression, placing it on the master bus before starting their mix. This can help them mix quicker by influencing their processing decisions as everything passes through the compressor. Because everything is getting compressed at the master bus, you tend to use less compression on individual tracks. This technique can work well if you're an experienced mixer and know what to listen for and how to tweak the mixed-bus compression to changes in your individual track processing.
This can also work against you if the volume levels of your mix change radically over the course of the mixing process. If you don't update your bus compressor's threshold, you may end up hitting it too hard, which in turn may cause you to make less-than-ideal decisions about the processing of individual tracks. Personally, I like to use a slow attack and a fast release setting, and never higher than a two-to-one or four-to-one ratio. Most times the idea is not to compress the entire mix very hard, just to control the dynamics gently in a transparent way.
So my advice is, if you're going use a master bus compressor over your entire mix, use a light touch.
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