Unfortunately, there's no one-size-fits-all approach to mix bus processing, so it's important to know why, what and how you're doing it!
There’s a few different cases to be made for adding processing to your mix bus. YouTube audio influencers will use words like ‘glue’, ‘grit’ and ‘weight’, while experienced, busy mix engineers might discuss ‘top down’ mixing to speed up their workflow as they battle against revisions from major labels.
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While mix bus processing shouldn’t literally be the glue that holds your mix together, it can add a cohesive sheen across everything, tying it all together and serving as the sugar on top—even if you add the sugar first in a top down approach! To help demystify this processing, that can have a profound, mix-ruining effect on your carefully crafted balance, we’ve listed a few common tips to get you started!
Compression
This one is used by a lot of your favourite mix engineers, but a little know-how can help you to understand when, why and how to compress your mix. Faster compressors can work well to tame your mix, if you just want things to be tied together a little more. Fast compressors can help to shape and hone the transient response of different elements, all at once, so you’ll be helping to squeeze everything together a touch. If using a plugin, there’ll often be a mix knob available that allows you to blend in the compressed signal in parallel, and this can be great to experiment with compressing harder than you usually would!
A slower compressor can have a more audible effect, not reacting to the faster transients, but swooping in to clamp down just after the transient, so they can help sources like snare and kick to poke through while the rest of the mix gets squashed. Tube compressors and more vintage-style compression is generally slower, the tube saturation imparting its own colour and heft.
A little rebalance can sometimes be necessary if you’re happy with the affect of the compression, despite it shifting the balance a little. Another common compression trick is to sum all your instruments together separately to vocals and compress them, giving the impression that the vocals are popping out more than the ‘band’, which are glued together and cohesive.
Limiting and not limiting
Limiting differs from compression in that it doesn’t allow audio beyond a threshold, like a brickwall, whereas compression will allow it through according to the preset ratio. Again, the Mix control is a great thing to experiment with. A limiter is a great option for making mixes louder, for example when mixing and delivering to a band.
You can bet your bottom dollar that bands will be comparing your mixes to Spotify and Apple Music, which are mixed, mastered and normalised files, so it can be helpful to know how to deliver mixes at a reasonably loud level—even if it crushes your mix!
This becomes complicated when a band signs off on a mix and the files go to a mastering engineer, who generally don’t want to receive an overly compressed or limited file. For this reason, it can be good to print a limited and unlimited mix all at once! For example, you’d mix to your mix bus, send this to an additional auxiliary track with a limiter on it, printing to an audio track. This way you’ll monitor the limited mix, but print an unlimited version at the same time, ready for delivery to mastering.
Limiting works well at an instrument bus level as well, to crush drums, bass or other low end instruments.
Mid-side processing
Mid-side processing is accessible on a lot of plugins, or tools like HOFA 4U Meter, Fader & MS-Pan can decode the sides and middle for processing with whatever plugin you like! Other options like FabFilter Pro Q-3 and Pro Q-4 allow you to right-click and select ‘Stereo Placement’ for EQ moves, allowing you to process the middle or sides.
Mid-side processing can get really complicated really quickly, so let’s tread lightly. Mid-Side processing refers to processing the middle and the side of the mix separately which can cause some issues. As most of us understand, sources playing out the middle of the mix are actually playing equal volume from both stereo sides, summing in the centre of the stereo image. Because of this, affecting the sides and middle separately can shift and affect the phase relationships of your mix. Mid-Side processing doesn’t just affect the hard left and right panned sources, but everything in between that’s playing out both the centre and the sides, i.e. a tom that is panned 45% left.
Alas, carefully used, mid-side can add depth and definition to your mix. You might hear people talking about ‘monoing the bass’, and this refers to cutting low frequencies from the sides of a mix, leaving the lower, sub frequencies alone in the middle for clarity and definition.
While yes, you could also just high-pass filter the other elements at an instrument level, mid-side processing has a more cohesive effect!
Mid-side compression is another great trick to have up your sleeve, and is part of what made the Fairchild 670 so famous. In “Lat/Vert” mode, the Fairchild compressed the middle, Vertical, or the sides, Lateral. This helps to bring together the vocal, snare, kick and bass, usually panned centre, leaving the sides like guitars, horns, synths and more a little less compressed on the sides.
EQ
EQ is a really popular way to treat your mix bus, boosting entire frequency areas to, again, build a layer of cohesion into your mix. Boosting low end to a carefully crafted and shaped kick and bass can make them feel larger than life, without ruining the relationship between them or with other elements.
Pultec
A super common mix bus EQ is the Pultec, used for the ‘Pultec Trick’. Eagled-eyed readers will notice in the thumbanil outlined above that the Pultec EQP-1A allows you to both cut and boost a pre-selected frequency. Surely that would render it useless?
What makes the Pultec so interesting is that the bell curve from the boost and cut controls are not symmetrical, so you can boost and cut somewhere like 60Hz and the curve slightly boosted and focused before being cut. Neve EQ is similar, boosting causes a little dip either side of the selected frequency, while cutting adds a little boost. This makes your moves feel bigger and more focused, rather than just boosting or cutting a frequency with a clean, transparent EQ.
Where clean, transparent EQs can be useful is for a trick not dissimilar to multi-band compression, but instead is more like band-passed compression.
Multi-band compression and notch EQ
What do you think would happen if you added a huge boost at 100Hz of a kick drum and added a compressor after it? Well, that boost would hit the compressor’s threshold first, effectively compressing only 100Hz or thereabouts. You could do an inverse notch EQ after the compressor and voila, you’ve tamed 100Hz in a kick while leaving the rest largely unaffected.
Multi-band compressors work similarly, though more succinctly, and allow you to compressor certain areas of a stereo mix, rather than processing the entire thing. Reign in your low end elements while allowing you to let the high end breathe and retain dynamic and clarity. Or, do the opposite! A lot of this is about experimentation, and all of these tricks and tips have been discovered while gear isn’t used how it’s meant to.
Within reason, you can’t break a plugin, so digitally you can go even harder experimenting!
API produce some stellar stereo bus processors, one famous one being the 2500 compressor, recently revitalised in the API 2500+ hardware that adds a Mix knob to the original design!