Stem Mastering is the process whereby an audio engineer takes grouped stereo mixes from a final mix, enhances the sound and creates a technically excellent releasable version of the track.
The original goal of mastering was to take a collection of songs (album/EP) and make sure they played back at consistent level whilst taking the listener on a musical journey. The purpose of mastering has evolved in a number of ways. Firstly, releasing a single is far more common than releasing an album or EP. So an engineer frequently deals with just one song rather than a collection of songs. Secondly, mastering is no longer simply a technical formality. Artists expect mastering to drastically improve the sound of their mix. We mastering engineers have adapted to fulfil the needs of our clients.
There are two types of widely accepted mastering practices. The first is stereo mastering. This is where the audio engineer enhances and prepares just one single audio file for commercial release. The second is stem mastering. Stem mastering gives the audio engineer a greater amount of control over the master as the sounds are separated into groups of similar sounds. This allows the engineer to make discreet changes to individual sounds within the mix as well as applying the standard processing of stereo mastering. The goal of both are to get the music sounding as high quality as possible.
Lets address the elephant in the room before we get started…
Is it a bad idea to master my music myself?
A lot of producers master their own music. Some do it exceptionally well, and some don’t. Having heard your song a million times, your brain is unable to detect and fix those little issues that make or break a track. Hiring a pro with a fresh pair of ears that you can trust is usually a recipe for success. However, if you have your final mix ready and you’re determined to master your own music, the best thing you can do is give your ears and brain a break from the track. Get your mix as close to the final sound as you possibly can. Then take at least a week long break from the track. Don't listen to it at all, no matter how tempted you are. When you come to master your track you’ll do a much better job with fresh ears.
The ‘Technically’ Excellent Final Mix
This isn't a mixing tutorial so I wont go into too much depth about the final mix here. However, its crucial that you get the technical aspects of you mix on point to set yourself up for success when mastering. Address each of the bullet points below to get that technically excellent final mix.
- Mix peaks at -6dB [decibels]. This gives you the space or ‘headroom’ that you will need to make EQ adjustments.
- Mix has a good dynamic range (no limiters or compressors on the master/output). This keeps the transients in tact giving your music punch and clarity.
- The balance of the instruments is comparable to other tracks in the same genre.
- The high and low frequencies are comparable to other tracks in the same genre.
- You’ve listened to your final mix in deferent environments (Studio monitors, car speakers, sound dock, earbuds, headphones, iPhone speaker… the more the better)
As a mastering engineer, I consistently saw that a lot of music producers struggled to get a great final mix that ticked all the technical boxes. I teamed up with plugin developer 29 Palms to provide a solution to relieve their pain. We created LEVELS to empower producers to get a great final mix in their home studio. It comes with a 15 day trial and a free mixing eBook. Check it out here.
Creating Stems For Mastering
So at this stage you should be totally happy with your mix. Now you want to create the stems that you will use in your mastering session. You are going to break your mix up into 6 categories that will become your 6 stems. It’s crucial that you group similar sounds together. This will give you more freedom to make adjustments during mastering.
Bad grouping example - Lets say you have a sweep with a piercing top end in the same stem as a dull lead. You wont be able to boost the top end of the lead as it will make the sweep even more piecing. If you want to tame the sweep, it will make the lead even more dull.
Good grouping example - Lets say you have a dull lead and a pad in the same stem. You should be able to slightly boost the top end without the pad becoming horribly prominent in the mix.
Below is an example of an effective way to break up a mix into stems. It’s important that you bounce your kick and bass to separate stems. The low end frequencies are the trickiest to get perfect. By separating them you’ll be able to tweak and fine tune until they sound just right. Be sure to bounce your bus processing and effects with its dry signal. For example, include the reverb and delay of your lead vocal in the Vocal stem.
- Vocals - lead and backing.
- Effects - sweeps, sub drops.
- Music - pad, synth, guitar, piano, strings, lead,
- Drum tops - snare, hi hats, cymbals, toms, percussion.
- Bass - all bass elements.
- Kick - by itself.
I like to move the channels around in my mix so they’re adjacent to other channels in their stem category. I then the colour code the categories so I can easily see the groups.
Final checklist before exporting your stems…
- Make sure the loudest part of the song is peaking at -6dB.
- Make sure there are no limiters or compressors on your master channel.
When bouncing your stems…
- Solo each channel in a group.
- Bounce each group one at a time.
- Bounce the audio at 24bit, and whatever sample rate the session is in.
- Bounce In Stereo (even if you kick is in mono).
- Do not dither or normalise.
Preparing The Mastering Session
As Benjamin Franklin said ‘By failing to prepare, you are preparing to fail.’ Preparation is key to set yourself up for a successful mastering session. Import your 6 stems into the project. Also import a bounce of this final mix as it is. Think of 2 reference tracks that are the perfect example of the sound you are hoping to achieve. They should be in the same genre and ideally have fairly similar sounds, instruments and balance. Import these reference tracks into the project.
At this point you should have your 6 stems, your original mix and the 2 reference tracks in your project. We are going to create a bus for your mastering chain rather than simply having the mastering chain on the output. Creating a bus for your mastering chain allows you to hear your reference tracks without them going through your mastering chain. This would dramatically change their sound and make them a useless reference.
Select your 6 stems and set their output to a bus channel.
Awesome Mastering Chain
In its most basic form, your mastering chain should have 5 plugins in the following order. A gain plugin, an EQ, a compressor, a limiter, and a metering plugin. There are lots of awesome and expensive mastering plugins, but your DAW will also come with all of these plugins as standard. The additional tools you use will completely depend on the goals of your mastering. For example, if you wanted to get a more dirty vibe you might chose to insert a tape emulation plugin.
My mastering chain usually looks something like this…
Fabfilter Pro Q2 (Clean EQ for minor and precise frequency adjustments)
Fabfilter Pro MB (Multi-band compressor to shape the sound and control the punch)
UAD Shadow Hills Mastering Compressor (Analogue emulation compression)
UAD Manley Massive Passive EQ (Tube EQ to add smooth and rich frequency adjustments)
UAD Oxford Inflator (Harmonic Distortion to increase perceived loudness and energy)
Fabfilter Pro L (Limiter to control the output and tame transient peaks)
LEVELS (Metering to understand what my adjustments are doing to the audio)
I’ll disable any plugins that aren't necessary for the session but i have them ready if i decide to use them. My goal whilst mastering is to make minor adjustments that have a positive affect on how the song sounds. If your track needs major adjustments, make the changes in your mix session.
Now that you’ve set up the session and have the mastering chain in place, now is the time to take a break from your music. When you come back to the session with fresh ears you will be totally prepared to focus your energy on the master itself rather than having to create the project from scratch.
The techniques that I discuss will give you an insight into my approach to mastering. Don’t take everything I recommend as gospel, tweak my ideas to suit your style. Each mastering session is unique in its needs and should to be approached with an open mind. Below is the chronological process I go through to stem master a track.
Set The Loudness
I begin my session by checking how loud my reference tracks are. I use the LUFS meter on LEVELS to find the integrated LUFS value. Lets say it's -11LUFS. I then run my stems through the mastering chain and adjust a few of the settings to make sure my tracks loudness sits around -11LUFS. My limiter usually has an output of -0.5dB (which minimises inter-sample peaks and distortion post transcoding) and a make up gain of around 3dB. I personally like having the Oxford Inflator working at 100% as I find it gives a track amazing energy. If I still need a bit more volume I’ll increase the gain on the Pro Q2, as it is a 'clean gain' which adds no extra noise. If I need less volume I’ll pull back on the limiter as that will increase my dynamic range as well as decrease loudness.
Once i’m happy with the loudness of the track I get to work on any EQ adjustments I want to make. I solo my reference tracks and l listen carefully to the balance of the low and high frequencies. I don't try to match them exactly, but I use them as a guide as to how I want the balance of my track to sound. I use the Pro Q 2 to make small clinical adjustments to my overall sound. I use my Manley Massive Passive to add top end sparkle and warm up the mids. If I think the mix is feeling a little muddy, i’ll identify which stem is causing the issue and use the Pro Q 2 to take out any problematic frequencies around 100-400Hz. The beauty of having stems is that I can remove any mud without my adjustments affecting the whole mix.
Compression and Balance
The majority of the compression will have been done during mixing. During mastering i’ll compress lightly to get a rounded feel to the overall sound. I’ll set a low threshold of 1.2, a long attack, and a short release. These settings are just my personal preference to get a nice open sound. I know many producers love the CTF (Compressed to F***) sound and go for hotter settings. I use a multi-band compressor on every master I do. The control it gives me to shape the sound is perfect for what I want to achieve. When setting the bands, make sure the crossovers aren't set right in the middle of an important sound, such as the middle of the kick punch frequency. I also like to have consistency in the thresholds of each band. I find this gives a more natural sound. I’ll use my ears and reference tracks to find the perfect balance and punch for the track. I'll use the dynamic range feature in LEVELS to make sure I don't over compress the music.
Level Matching The Master To The Original Mix
This is a crucial stage that many people miss out because they don't quite understand the importance of level matching. Your new mastered track will sound a fair bit louder than your original mix. When you A/B test them (switch between the two tracks to hear the difference in sound) the louder one will have more clarity and a richer sounding bass. However, this may simply be because its louder. The Fletcher Munson concept stipulates that the human ear will perceive louder sounds as having more bass and more top end, thus fooling you into ‘preferring’ the master over the mix. Head over to my blog to read more about level matching.
To match the loudness of your master to your original mix you will need to lower the volume of your mastering channel fader. Usually anything between 5dB to 15dB depending on the loudness of the mix. I use the integrated LUFS meter in LEVELS to give me an accurate reading of the perceived loudness of the mix. I then reduce the volume of the master channel (post processing chain) until it gives me the same reading on the LUFS meter. I can then objectively A/B test between the mix and the master to see if I have made complimentary improvements to my sound. During the level matching process I might make a few more adjustments to my master.
Don’t forget to bring the mastering channel back up to 0dB before you bounce the final version!
The output channel within the mixer of DAWs uses a Peak Sample meter. This type of meter isn't accurate enough to display what we actually hear through our speakers. An analogue to digital conversion takes place when our audio goes from being a digital file (WAV MP3 AAC) to sound waves coming through our monitors. During this conversion, the sample blocks go through a reconstructing filtering process that gives us our smooth sounding end result. However, when the sample blocks are hitting close to 0 dBFS (zero on the output meter) The reconstruction process can induce a small amount of clipping.
Inter-sample peaks can be avoided by using a quality true peak meter to make sure your audio won’t distort when converted from digital to analogue. This will give you the information you need to leave the correct amount of headroom between the peaks of your master and 0 dBTP (decibels true peak). I use the true peak meter in LEVELS.
Mastering Tips And Tricks
Release On Limiter - If you’re trying to get your track sounding really loud, it might start to get crunchy. Try increasing the length of the release on the limiter to lessen this distortion.You could also try increasing the attack. By increasing the attack you might start affecting the punch of your music so make conscious and informed decisions when playing with the settings.
Dynamic Phase On Multi Band Compressor - A multi band compressor can induce phase shift and pre-ringing as it uses filters to split the audio into the separate bands. A dynamic phase or linear phase setting will minimise these sonic issues.
Bass Space - Use the Bass Space feature in LEVELS to make sure there are no unnecessary low end frequencies in channels other than your kick and bass. This will help you get punchy, rich and clear lows in your master.
Harshness Control - Brainworx Bx_ Refinement is an awesome tool that subtly removes the harshness from audio. I sometimes use this in my mastering chain, or on the individual channels that are inducing the harshness. I would recommend that you use delicate settings as it can make the mix sound a bit thin if you over do it.
Getting Your Master LOUD - Loud masters are best achieved when there is moderately heavy compression on the individual stems during the mix down stage. Remember, the louder you go, the more your dynamic range suffers. Check out this post on why it’s better to go for dynamic range over loudness.
Limitations Of Home Studio Mastering
Why is it so difficult to get that ‘chart topping’ sound?
Many music producers are disappointed with their home studio mastering results as they have unrealistic expectations of what they can achieve without acoustic treatment and high quality monitors. In a nutshell, it's impossible to make good mix decisions without a flat listening environment. It's amazing how much of an effect the room can have on how you perceive the sounds of your mix. You might pump up the bass to what seems to be the perfect level in your home studio. Only to find that you've made your track sound muddy and unbalanced when you listen elsewhere. In your home studio you might feel like Quincy Jones, then you hear you mix in the car (or worse... the club) and it can be a nasty shock to hear how different it sounds.