A mid/side width control can only scale the difference a sound already has. That makes it safe in mono and useless on a mono source. A widener that adds a delay, chorus or phase shift between the channels creates a difference out of nothing, so it can spread a mono synth or guitar, and you pay for it with a comb filter when the channels are added back together.
Use mid/side on anything that is already stereo. Save the other kind for a mono part that needs width, and check that part in mono every time you use it.
At a glance
| Property | Mid/side width | Delay widener | Chorus and phase wideners |
|---|---|---|---|
| What it does | Turns the side signal up or down | Plays one side a few milliseconds late | Detunes, modulates or phase shifts each side differently |
| Widens a mono sound? | No | Yes | Yes |
| In mono | Exactly the same as before | A fixed comb filter | Moving notches with chorus, fixed dips with phase shifts |
| In stereo | Same tone, more or less spread | Spread, with the sound leaning to the early side | Spread with some movement or shimmer |
| The main control | Width, in % or as side level in dB | Delay time, usually 5 to 30 ms | Depth, rate or amount |
| Best for | Stereo pads, mixes, buses, narrowing | Mono synths and guitars, as an effect | Mono leads and pads that can take some movement |
Is a stereo widener the same as mid/side?
Sometimes, and that's why the two get confused. Mid/side is a way of describing a stereo signal. The mid is what the two channels share, (L + R) ÷ 2, and the side is how they differ, (L − R) ÷ 2. Any stereo sound can be turned into mid and side and back again without losing anything, and the mid/side processing lesson walks through the maths.
A stereo widener is a product category: any plug-in with a knob that makes things wider, and different plug-ins get there in different ways. The simplest just turns the side up. Inside, that's mid/side width with a friendlier label. Ableton's Utility does this with its Width control, and so does Logic's Direction Mixer with Spread. Others build width out of a short delay, a pitch-shifted copy or a chain of phase-shifting filters, and those behave like a different tool altogether.
So the useful question about any widener is whether it scales a side signal that's already there or creates one.
Reading the lab
The lab puts both kinds of width on one panel. Width is a mid/side width control, where 100% leaves the sound alone, 0% removes the side and 200% doubles it, and Delay R plays the right channel later than the left, which is the simplest widener that creates width. The polarity, pan and copy controls belong to the phase lesson and are greyed out here.
Keep an eye on the screen titled What mono does to the tone. The lab sends a click through your settings, adds the channels together and draws the result, so a flat line means mono leaves the tone alone and every dip is a frequency that cancels. Beside it, the correlation meter tells you how alike the channels are, and the Right now panel gives the measured correlation, the level lost in mono and the first notch for the loop you've picked.
You can load your own sound with + Your sound. It stays in your browser, and it's the quickest way to find out what your favourite widener has been doing to a part.
How does mid/side width work?
A mid/side width control leaves the mid where it is and multiplies the side. Write the left channel as mid plus side and the right as mid minus side, and widening by a factor k gives you L = M + kS and R = M − kS. Add those two together for mono and the kS cancels, leaving 2M. Whatever k is, mono is the mid and nothing else.
As supplied, the full mix in the lab has its side 6.5 dB below its mid and a correlation of +0.64, and it loses 0.9 dB when summed to mono. Set Width to 160% and the side goes up by 20 × log10(1.6), which is 4.1 dB, so it's now only 2.4 dB below the mid. The correlation falls to +0.28 and the stereo version gets 1.1 dB louder, because the extra side adds level to both speakers. Press Mono and you hear the original mix's mono version again, sample for sample.
That 1.1 dB is easy to miss. Mid/side width makes the stereo version louder as it makes it wider, and louder tends to win a quick comparison, so turn the widened version down to match before you decide it helped.
The same control narrows. A synth patch or a stereo sample that is wider than you want can be pulled back to 60 or 70%, and the tone of the part stays as it was, because the mid is untouched. A delay or chorus widener can't narrow anything.
How does a delay widener work?
The oldest way to create width is to play one channel a little later than the other. Somewhere between about 5 and 30 ms, your ears still hear one sound, but it seems to spread out and lean towards the side that arrived first. That's the Haas effect, and it's what a lot of simple widener plug-ins and doubler presets are built on.
In stereo it works. On the full mix in the lab, a 15 ms delay on the right drops the correlation to +0.07, close to what 200% mid/side width gets you. The trouble starts when the channels are added. A sound plus a copy of itself 15 ms late is a comb filter: any frequency where the copy lands half a cycle out of step cancels. The first one is at 1 ÷ (2 × 0.015 s), which is 33 Hz, and there's another every 67 Hz from there all the way up the spectrum.
Those notches are what make a delay-widened part sound hollow and phasey in mono. In the lab the full mix comes out 2.7 dB down in mono with the delay on, which is only a little worse than the 2.0 dB at 160% width. The difference is in the tone. The mid/side version loses its edges and keeps its sound, while the delayed one turns into a thinner, coloured version of itself.
Changing the delay only moves the notches. At 1 ms the first notch is at 500 Hz with one every 1 kHz after it, right across the middle of a vocal or guitar. At 20 ms they start at 25 Hz and sit 50 Hz apart. No delay time gets rid of them.
Other ways wideners create width
Chorus and detune
A chorus widener sends each side through its own slowly changing delay, so the copies drift against each other in pitch and time. Pitch-shift wideners detune the two sides by a few cents in opposite directions; Soundtoys MicroShift does this with a short delay as well, and varies both over time. In mono you get notches that keep moving instead of a fixed comb. That can be less obvious than a delay, but it adds a wobble or smear to the tone. The chorus lesson covers how the modulation works.
Phase shifts
Some wideners pass each channel through different allpass filters, which shift the phase of each frequency by a different amount while leaving its level alone. In stereo the two sides no longer line up, which spreads the sound. In mono, wherever the two shifts end up half a cycle apart, that frequency cancels, so the damage depends on the design and you can only find it by listening.
Splitting frequency bands
Logic's Stereo Spread splits the sound into frequency bands and sends them alternately to the left and right. Each band is whole in one speaker, so when the channels are added back the bands sum to something close to the original. It's one of the gentler ways to widen a mono source, though a part can sound slightly uneven as its notes move between bands.
A side made from the mid
A widener can also take a delayed copy of the mid, often filtered as well, and use it as a new side signal, adding it to the left and subtracting it from the right. Because exactly the same thing is added on one side and taken away on the other, it cancels perfectly in mono and the mono version comes out untouched. You pay in stereo instead: each speaker on its own now has a comb filter, with peaks on the left where the right has notches. If a widener spreads a mono sound and still passes the mono test, listen to one speaker at a time as well.
Widening a mono sound
This is the job where mid/side runs out. A mono recording has identical channels, so its side signal is silence, and 200% of silence is still silence. Pick the nylon guitar in the lab and set Width to 200%: the goniometer stays a vertical line and the correlation stays at +1.00.
A 15 ms delay does widen it, taking the correlation down to +0.21, and the notches arrive with it. Every widener that creates width comes with some version of that cost. A second recorded take panned opposite the first gives you a wide part with no fixed comb filter in mono, which is why double tracking is still the usual method for rhythm guitars and backing vocals. When a second take isn't possible, a stereo reverb or a ping-pong delay on a send adds width around the part while the dry signal stays in the centre.
Can narrowing undo a delay widener?
Say a synth preset came with a Haas delay baked in and it sounds hollow in mono. Pulling a mid/side width control down to 0% looks like the obvious fix, but listen to what happens when you try it on the delayed guitar in the lab. The correlation goes to +1.00, so the meters say it's mono, and the notches are still on the screen.
The reason is in the maths. The mid of a delayed pair is the sound plus its own late copy, halved, and that is already a comb filter. Width at 0% plays only the mid, so it plays the comb in both speakers. Mid/side can remove the width a delay made, while the delay itself stays in the sound. If you have the dry sound, take the delay off. If it's printed into a stereo file, try keeping only the early channel and using it as mono, since that one has no copy in it.
How to test what a stereo widener does
Plug-in manuals don't always say how a widener works, and some have several modes that work differently. This test takes a couple of minutes.
- Put it on a mono soundUse a mono recording, or a stereo one with Ableton Utility or Logic's Gain plug-in set to mono before the widener. A vocal or a guitar works well.
- Turn it upIf nothing gets wider, the widener is scaling a side signal, so it's mid/side width and safe in mono. If the sound spreads out, it's creating width.
- Check the sumPut a mono button after the widener and switch the widener on and off. With mid/side width you'll hear no difference at all. With anything else, listen for hollowness, a thinner low end or a tone that wobbles.
- Solo each speakerPan the output hard left, then hard right. A widener that keeps mono intact by building a side from the mid shows its comb filter here instead.
- Look for a crossoverSome wideners can leave everything below a frequency you choose alone. Set it to around 150 Hz and the widener can't put a comb filter on the kick and bass.
Which should you use?
Mid/side width, for anything that's already stereo. It widens with no change to the mono version and no colour in stereo, and it narrows too. Use a widener that creates width when the part is mono and you can't record a second take, and treat it as an effect you've chosen to hear.
| Job | Mid/side width | Delay, chorus or phase widener |
|---|---|---|
| Widen a stereo pad or string section | Best choice, 120–150% | Adds movement, costs mono |
| Widen a mono synth or guitar | Does nothing | Works; check it in mono |
| Narrow a sample that's too wide | Best choice, 50–80% | Can't narrow |
| Keep the low end in the centre | Cut the side below 100–150 Hz | Use the crossover, if it has one |
| Width on a mix or master | Gently, 110–120% | Avoid |
| Lead vocal, kick, bass | Leave at 100%, or narrower | Avoid |
The one exception is a part that only ever plays in stereo, like a sound for a headphone-only game or installation. There a delay or chorus widener costs you nothing, since nobody will hear the mono sum.
Mid/side does more than width
A widener has one knob. Once a signal is split into mid and side, you can put any processor on either half, which is how a lot of mixes get their width without a widener at all. A high shelf of 2 or 3 dB on the side above about 5 kHz opens up the top of a mix while the low end stays where it was. A high-pass filter on the side at 100 to 150 Hz puts the bass in mono. A compressor on the mid alone can hold a vocal and kick steady while the side, where most of the reverb's width lives, stays uncompressed.
Processing on the side never reaches the mono version, because mono throws the side away, so of those moves only the mid compressor changes what a mono listener hears. The mid/side EQ lesson covers the EQ moves in detail.
Common mistakes
Using a delay widener on a mix
A whole mix is already stereo, so mid/side width can do everything a widener could. A Haas or chorus widener on the master comb filters the kick, bass and vocal in mono, which are the parts a mono listener can least afford to lose.
Turning up a mid/side widener on a mono track
When a width knob does nothing, it's tempting to push it further or stack a second widener. Neither will help, because the track has no side signal to scale. Switch to a widener that creates width, or use a stereo effect on a send.
Judging width at a higher level
In the lab, 160% width makes the full mix 1.1 dB louder in stereo. Turn the wider version down until it matches before you compare, or the louder one wins whether the width helped or not.
Widening the bass
Low frequencies carry most of a mix's energy, and a delay widener's first notch on a 15 ms setting sits at 33 Hz, right in the sub. Keep everything below about 150 Hz in the centre with a crossover or a side low cut.
Trusting the correlation meter alone
After narrowing a delayed sound to 0%, the meter reads +1.00 and the comb filter is still there. A good correlation reading tells you the channels match; it can't tell you whether the sound they share is still the sound you started with.
Train your ears
The quiz under the lab plays a loop with hidden settings, covers the screens and asks whether it will survive mono. Some rounds only change the width, which never hurts the mono version; others add a short delay between the sides or flip one of them. Every round is matched in loudness and plays in stereo, so listen to the centre of the sound. Mid/side width keeps it solid and in the middle, a short delay pulls it towards the left speaker, and a flipped side leaves no clear centre at all.
Questions people ask
Is mid/side the same as a stereo widener?
Mid/side is a way of splitting a stereo signal into what the channels share and how they differ. A stereo widener is any plug-in that makes a sound wider. Many wideners use mid/side inside and turn the side up, but others use delays, chorus or phase shifts, which behave very differently in mono.
Are stereo wideners bad for mono?
Only the ones that create new differences between the channels. A widener that just raises the side signal leaves the mono version exactly as it was. Delay, chorus and phase-based wideners change the mono version, and on a bass or a lead vocal that change is usually easy to hear.
Why does my widener do nothing to a mono track?
It is almost certainly a mid/side widener. A mono track has identical channels, so its side signal is silence, and turning silence up leaves silence. You need a widener with a mode made for mono sources, a stereo effect, or a second take.
Can I use a widener on the master?
A mid/side widener, gently: 110 to 120% with the low end kept in the centre is plenty for most masters. Avoid delay and chorus wideners on a whole mix, because they comb filter everything in mono, the kick and vocal included.
What is a safe amount of stereo widening?
It depends on how wide the sound already is. With mid/side width, mono is never affected, so the only limit is how the stereo version sounds to you. With other wideners, push it until the part goes hollow in mono, then back off.
Does narrowing with mid/side fix phase problems?
It removes width and hides the problem in stereo, but it can’t remove a comb filter that is already in the mid. If two channels are out of time, line them up or take the delay off; if one is flipped, flip it back.