Mid/side EQ in plain English
A normal EQ on a stereo track changes both speakers by the same amount. Cut 300 Hz and you cut it everywhere, from the kick in the middle to the guitars panned out to the edges. Most of the time that's what you want.
Mid/side EQ gives you a different pair of handles. It splits the sound into two parts. The mid is everything that's the same in both speakers, which you hear as the centre of the stereo picture, and the side is everything that differs between them, which you hear as width. Each part gets its own EQ, and then the two are put back together into left and right.
So you can brighten only the wide parts of a mix, or clear rumble out of the edges without thinning the kick and bass in the middle. Both are hard to do with an ordinary EQ, and they're the two jobs people use mid/side EQ for most.
The maths of mid and side
The sums are short. The mid is the two channels added together and halved, and the side is the left minus the right, halved:
Mid = (L + R) ÷ 2 and Side = (L − R) ÷ 2
Going back is just as easy: left is mid plus side, and right is mid minus side. Nothing is lost along the way, so with both EQs flat, a mid/side EQ gives you back exactly the sound you put in. Some plug-ins skip the halving or use 0.707, which only changes the levels. Here's where a few simple sounds end up, with each level written as the height of the wave in that speaker:
| The sound | Left | Right | Mid | Side |
|---|---|---|---|---|
| Dead centre, like a lead vocal | 1 | 1 | 1 | 0 |
| Left speaker only | 1 | 0 | 0.5 | 0.5 |
| Right speaker only | 0 | 1 | 0.5 | −0.5 |
| Panned part of the way left | 1 | 0.5 | 0.75 | 0.25 |
| Same in both, one side upside down | 1 | −1 | 0 | 1 |
A centred sound lives entirely in the mid, and a hard-panned one is split evenly between mid and side. Reverb, stereo pads and doubled guitars differ between the speakers from moment to moment, so a lot of them ends up in the side. The last row is the odd one: two copies with opposite polarity are pure side, which is why they vanish in mono.
A mono speaker plays (L + R) ÷ 2, which is the mid. So whatever you do to the side, a mono listener never hears it.
Inside a mid/side EQ
A mid/side EQ encodes left and right into mid and side using the sums above, runs each one through its own EQ, and decodes them back to left and right. Plug-ins hide the encoding and decoding, so all you see is an EQ with a Mid/Side switch, or a setting on each band that says Stereo, Mid or Side.
A boost on the side makes that range of frequencies wider, because the side gets louder compared with the mid. A cut on the side narrows it. The mid works the other way round: boost it and that range moves towards the centre; cut it and the range sounds wider, since the side now makes up more of it. You can widen something with a mid cut and never touch the side at all.
Say a mix has a lot of energy at 2.5 kHz in the centre and you cut 3 dB there on the mid. The parts in the middle lose a little presence and the parts at the edges keep all of theirs. A normal EQ making the same cut would dull both.
Reading the lab
The lab plays a stereo loop through a real mid/side EQ. The big screen shows the spectrum of the mid in teal and the side in violet, measured on the whole loop after the EQ, with thin dashed lines for how they looked before. Both are on the same scale, so on most mixes the violet shape sits well below the teal one, because the side is quieter than the mid. On top of them are the two EQ curves, and the numbered points belong to whichever EQ you're editing.
The Edit buttons choose which EQ the controls change, and the band buttons pick one of four bands: a low cut at 24 dB per octave, a low shelf, a bell and a high shelf. The Listen buttons choose what you hear. Stereo is the normal result, Mid only is the mono version, and Side only plays just the difference between the speakers.
The screen underneath shows width by range. Each bar is the side's share of the energy in one range of frequencies, from sub-bass up to air. At 0% that range is mono. At around 50% the two speakers are playing unrelated things, which is about as wide as a mix gets before parts start cancelling. The dashed outline is the sound before the EQ and the filled bar is after it, and the panel beside it explains your last change with numbers measured from the loop.
Whatever you listen to, the lab plays it at the loudness of the original. On the full mix the side is 6.5 dB quieter than the mid, so Side only gets turned up while you listen, and A/B switches both EQs off at the same loudness so a boost can't win by being louder. You can load your own stereo file with Your sound, and it stays on your device. A microphone recording is mono, so its side is silent and only the mid EQ does anything to it.
What is mid/side EQ used for?
Tightening the low end
This is the most common use. A low cut on the side, somewhere between about 80 and 200 Hz, takes the bass out of the edges of the stereo picture and leaves the bass in the centre alone. Below that frequency the mix becomes close to mono.
There are good reasons to want that. Your ears are poor at telling where very low notes come from, so stereo bass adds little you can hear as width, and many club and live systems feed their subwoofers a mono signal anyway. On a vinyl record the side becomes up-and-down movement of the groove, so loud low-frequency side content can make the needle jump. Engineers cutting vinyl often fold the low end to mono for that reason, with a filter called an elliptical EQ.
The full mix in the lab has 11% of its energy below 150 Hz in the side. A 24 dB per octave low cut at 150 Hz on the side brings that down to about 2%, while the bars from 500 Hz up barely move. Listen in Side only while you set the frequency. You'll hear the rumble go, and if you push it too high you'll hear the body of the wide parts go with it.
Some tools build this in. Brainworx's bx_digital EQ has a Mono Maker control and Ableton Live's Utility has a Bass Mono switch, and both fold everything below the frequency you pick into mono, which is the same job as a side low cut.
Adding air to the sides
A high shelf of 1 to 3 dB on the side, starting somewhere between 8 and 12 kHz, makes the top of a mix spread wider. Cymbals and reverb tails get brighter at the edges, while a centred vocal keeps the tone you gave it. On a master it widens only the top end, and it can't cause trouble in mono, because the shelf disappears there.
Making space in the centre
A lead vocal usually sits in the middle and carries a lot of its clarity between about 2 and 5 kHz. If the music is busy in that range, a cut of 2 or 3 dB on the mid of the instrument bus makes room for the vocal and leaves the wide parts at the edges as bright as they were. The same idea works lower down: a mid cut around 250 to 400 Hz clears mud out of the centre, where the kick, bass and vocal pile up.
Fixing a problem in one part of the picture
Sometimes a problem lives only at the edges. A pair of hard-panned rhythm guitars might be harsh around 3 to 5 kHz, or a stereo drum room might have too much cymbal in it. A cut on the side deals with that and leaves anything centred alone, like the snare or the bass. Going the other way, a boxy tone that only shows up in the centre of a stereo piano is a job for the mid.
How to use mid/side EQ, step by step
This is the order the lab's lesson follows, and it works on a mix bus, a master or a single stereo instrument.
- Listen to the mid and the side on their ownSolo each one before you change anything. You'll hear which parts live where, and it's usually obvious straight away what's in the side that shouldn't be.
- Clear the low end out of the sidePut a low cut on the side and raise it from 20 Hz while you listen in Side only. Stop when the rumble has gone and before the wide parts start to sound thin. On a full mix that's often 100 to 150 Hz.
- Make one move at a timeChange one band on one channel, then listen in stereo. Mid/side moves are easy to overdo because each one changes width as well as tone.
- Keep boosts smallA decibel or two on the side is plenty on a whole mix. If you need 6 dB, the problem is probably in the mix itself.
- Check in monoSwitch to Mid only, or your DAW's mono button. The side moves disappear, and the mix should still sound balanced without them.
- Compare at the same loudnessBypass the EQ with its level matched. If you can't say what improved, take the move out.
Starting settings for common jobs
These are places to begin, set so you can hear the move clearly. Adjust from there by ear.
| Job | Channel | Band | Starting point |
|---|---|---|---|
| Mono the low end | Side | Low cut | 100 – 150 Hz, 12 – 24 dB/oct |
| Air at the edges | Side | High shelf | 1 – 3 dB boost from 8 – 12 kHz |
| Room for a vocal | Mid | Bell | 2 – 3 dB cut at 2 – 4 kHz, Q 1 |
| Mud in the centre | Mid | Bell | 2 – 3 dB cut at 250 – 400 Hz, Q 1 |
| Harsh wide guitars or cymbals | Side | Bell | 2 – 4 dB cut at 3 – 5 kHz, Q 1.5 |
| A mix that's too wide | Side | Wide bell or shelf | 1 – 2 dB cut where it sounds too wide |
Mid/side EQ and stereo width
Every mid/side EQ move is a width move too. Turn the side up across all frequencies and you have the simplest kind of stereo width control; turn it up in one range and only that range gets wider. That gives you width by frequency, which is why mid/side EQ is so handy on a master, where you can't reach the individual parts.
It's also safe in a way some wideners aren't. Wideners that use short delays between the speakers can cause comb filtering when the mix is summed to mono. A side boost only changes a level, and in mono the side vanishes completely, so nothing cancels. The risk is a different one: if the mix only sounds right with a big side boost, the mono version loses that boost and can sound small.
To see what your changes do to width, watch a correlation meter. Boosting the side pulls the correlation down towards 0, and cutting it pushes the reading up towards +1, which is mono. The mid/side processing lesson covers the same split used with compressors, saturation and reverb, and mono compatibility goes further into checking a mix in mono.
Common mid/side EQ mistakes
Boosting the sides too far
A wider mix sounds impressive for a few seconds. Push the side much past 3 dB on a whole mix and the centre starts to sound hollow: the vocal and the kick lose weight, and the edges can turn harsh. When the centre sounds weak, lower the side before you think about raising the mid.
Never checking in mono
Most single Bluetooth speakers play in mono, and so do many club systems. Mono is just the mid, so everything you balanced with the side EQ is missing there. Check with Mid only, and if the mix falls apart, the side moves are doing too much.
Cutting the side too high
A side low cut at 300 Hz removes more than rumble. It takes the width out of the lower half of every stereo part. The strings loop in the lab has 38% of its energy below 150 Hz in the side, and a high side cut takes that width away. Pick Strings, set the side low cut and listen in stereo while you raise it.
Using it on mono sounds
If a track is mono, its side is silent and a side EQ has nothing to change. The acoustic guitar loop in the lab is effectively mono, with less than 1% of its energy in the side, so the side EQ does nothing you can hear on it. A stereo reverb on a mono sound puts something back in the side, though, so mid/side EQ after the reverb still works.
Judging by loudness
A boost on either channel makes the whole sound louder, and louder sounds better at first. Compare at matched level, the way the lab does, before you decide the EQ helped.
Training your ears to hear mid and side
Hearing where in the stereo picture an EQ move happened takes practice. The quiz under the lab boosts 2.5 kHz by 9 dB on the mid, on the side, or on both, covers the screens and matches the level. A mid boost brightens the centre and a side boost brightens the edges, while a boost on both is an ordinary EQ. Use headphones, or sit with your speakers evenly either side of you, because the clues are all about position.
For more on EQ itself, start with the EQ lesson, which explains bells, shelves, Q and the frequency ranges this page refers to.
Questions people ask
Is mid/side EQ the same as a stereo widener?
They overlap. Turning up the whole side channel is the simplest kind of stereo widener, and a side boost on an EQ does the same to one range of frequencies. Many wideners work differently, with short delays or phase tricks that can cause comb filtering in mono. A side boost can't do that, because the side simply disappears in mono.
Should I use mid/side EQ on the master?
It can help in small amounts. A low cut on the side around 100 Hz and a decibel or two of high shelf on the side are common mastering moves. Bigger fixes, like a vocal that sits too far back, are usually better made in the mix, where you can reach the vocal itself.
What frequency should I cut the sides at?
Start around 100 to 150 Hz on a full mix and listen in Side only while you move it. Go lower if the mix has wide parts with low notes you want to keep, like a stereo synth bass or a string section, because the cut takes their width away along with the rumble.
Does mid/side EQ affect mono compatibility?
The side EQ can't change the mono version at all, because the side cancels when left and right are added together. The mid EQ is what mono plays, so every mid move is heard in mono. What does change is how mono compares with stereo: if you boosted the sides a lot, the mono version loses everything you added.
Can I do mid/side EQ in any DAW?
Yes. Ableton Live's EQ Eight has an M/S mode, FabFilter Pro-Q lets you set any band to mid or side, and iZotope Ozone's EQ has mid/side processing. In other DAWs, a free encoder and decoder such as Voxengo MSED turns left and right into mid and side, so you can put any EQ between them.
Should I use a linear-phase EQ for mid/side work?
For big moves on a master, it's worth trying. A normal EQ shifts the timing of the frequencies it changes a little, and when the mid and side get different shifts, sounds near that frequency can move slightly in the stereo picture. A linear-phase EQ delays every frequency by the same amount, at the cost of extra latency and some pre-ringing.