Mid Side EQ in Mastering: How to Use It Without Wrecking Someone's Mix
Mid side processing in mastering is a genuinely powerful tool, but it gets overhyped. Mid side EQ lets you process the centre channel and the stereo sides independently, which gives you control that a standard stereo EQ doesn't have. The key to using it well is subtlety. Used carefully, it can open up a mix, tighten the low end, or add presence to a vocal without touching anything that shouldn't be touched.
What Mid Side Actually Means
Mid refers to everything in the centre of the stereo image, essentially mono information that appears equally in both channels. Side refers to the difference between left and right, everything that creates the sense of width and space. Reverbs, room sound, and wide-panned elements live predominantly in the sides. Kick, bass, snare, and lead vocal tend to sit in the mid channel.
When you use an M/S EQ, you're EQing these two components independently. Boost the mids in the top end and you pull out the vocal and centre instruments. Boost the sides in the top end and you add air and width to the reverb tails and wide elements. Cut the low end in the sides and you mono-ise the bass, which is often useful for keeping low frequencies tightly controlled.
How to Use M/S EQ at the Mastering Stage
The first rule is to use it sparingly. Half a dB here, 1dB there. M/S EQ is manipulating somebody's stereo image, and if they've worked with a good mix engineer who knows what they're doing, heavy-handed M/S processing will undo decisions that were made deliberately.
There are a few specific applications where mid side processing earns its place. Adding a gentle high shelf to the mid channel around 1.7kHz, maybe half a dB to 1dB, can bring a vocal forward and make the centre of a mix feel more present without affecting the sides at all. Cutting the low end of the sides, anything below 100-200Hz, tightens the low end by making the bass mono. That works particularly well when the mix has a slightly loose or indistinct low end on certain playback systems.
Boosting the sides in the top end adds a sense of width and space, which can help a slightly dry or flat mix feel more three-dimensional. Again, keep it subtle. Pulling the sides up by 1dB in the air band goes a long way. Going further starts sounding gimmicky.
When Not to Use M/S EQ
If the mix is already well-balanced and the stereo image is well-constructed, standard EQ on the full stereo bus will usually be a better choice than reaching for M/S. M/S becomes most valuable when there's a specific problem to address, like bass that's too wide, a vocal that's getting buried in the sides, or a mix that feels narrow and needs some spread in the top end.
Don't use M/S because it's exciting or because it does something obvious when you crank it. The fact that you can hear a dramatic change when you push the sides way up doesn't mean pushing the sides way up is a good decision. Mixes that have been mastered with heavy M/S processing often sound wrong on mono playback systems, where the side information collapses and takes the interesting top-end material with it.
Practical Settings to Start With
For mid side EQ at the mastering stage, a few practical starting points: cut the sides below 100Hz with a low cut to keep the low end mono. Try a shelf on the mid channel at 1.7kHz, no more than 0.5 to 1dB, to add vocal presence. If the mix needs width, add a gentle air shelf on the sides above 8kHz. Check everything in mono before committing to any M/S moves, because the most dramatic-sounding M/S adjustments often make things worse when the stereo image collapses.
FAQ
What is mid side processing in mastering?
It separates the stereo signal into centre (mid) and stereo difference (side) components, letting you EQ each independently. Used to control bass width, vocal presence, or stereo width.
How much M/S EQ should I use?
Half a dB to 1dB. Heavy-handed use undoes mix decisions and creates problems on mono systems.
Good starting points for M/S EQ?
Low cut on sides below 100Hz. Mid shelf at 1.7kHz by 0.5-1dB for vocal presence. Gentle air shelf on sides above 8kHz. Always check in mono.
Does M/S EQ work at the mix stage too?
Yes, on mix buses or stereo instruments it gives independent control over centre and side frequency balance.
For a complete approach to mastering chain order and processing decisions, the Complete Mastering System and Mastering Accelerator at streaky.com cover everything in depth.
