The on-trend process of 'stem mastering' simply means bouncing stems for mastering rather than a stereo mix in a single file.
The ability to independently tweak each main element of the mix - drums in one stem, vocals in another, etc - gives the mastering engineer (be it you or a hired gun) far more control over the sound of the final master than a traditional stereo premaster ever could.
It's a technique that's well worth trying, so here are half a dozen tips to get your started.
1. Be prepared
2. Don't be a prima donna
As a rule, creative input decreases as a track goes through the various stages: the artist and/or producer provide the artistic vision, the mixing engineer sweetens and sculpts the mix’s sonics, and the mastering engineer optimises it for final output. Note - stem mastering isn’t mixing, so respect the artist’s vision!
3. Know your limits
When dialling in master bus limiting, overly spiky or dynamic elements can trip gain reduction too early, resulting in less-than-ideal loudness levels. When stem mastering, simply head to the offending stem and apply limiting earlier in the chain. Multiple stages of subtle limiting give a more controlled overall sound.
4. Carry that weight
If a particular stem is getting lost in the overall song, you can easily turn it up. If it’s not quite ‘dense’ enough, try mixing in some transparent RMS weight with parallel compression.
5. Easy housekeeping
Just as you would with a single-file premaster, it’s a good idea to run through a few basic routines to make sure your stems are delivered to the engineer correctly. First, make sure all your stems are of identical length, and that all ‘lead-in’ elements and reverb tails aren’t lopped off, so that they all line up perfectly when the mastering engineer drops your audio files into their project. To retain maximum resolution and headroom, export them out in 24- or 32-bit, at the same sample rate as your mix project. Also, bounce out the master channel along with the individual stems.
Before you send, open a fresh session in your DAW and import the stems. Listen and compare the stem mix to that master export, ensuring that everything sounds identical and looking out for any weird exporting errors.
6. Master that master
As with any mastering process, when stem mastering, try to use as little processing as possible. 0.5-2dB of EQ boost or gain reduction should do. If you need to go further, it probably means the mix needs addressing at source.