What is mono compatibility?

Mono compatibility in plain English

A stereo mix is two channels, one for each speaker. Play it on something with a single speaker and the two have to become one, so the device adds left and right together. That's called summing to mono, and a mix is mono compatible when it survives it.

Surviving doesn't mean nothing changes. Some of the width goes, because width is made of the differences between the two channels and a single channel can't hold a difference. What has to stay is the vocal, the kick, the snare and the bass, at the level you set them and with the tone you gave them.

The trouble starts when a part is out of phase between the two sides. In stereo each speaker plays its own channel, so nothing gets subtracted on the way out and the problem can hide. Add the channels together and every moment where one side pushes while the other pulls turns into cancellation. The phase lesson explains phase itself; this page is about what summing does to a whole mix, and how to keep it working.

Does mono compatibility still matter?

Most music is heard in stereo now, but a lot of it still gets summed on the way to someone's ears:

• Phones with a single loudspeaker, and laptops and tablets whose two speakers sit so close together that the channels blend in the air.
• Single Bluetooth speakers and smart speakers. Most play a mono sum, or put their drivers close enough that the result is nearly the same.
• Club and venue PA systems, many of which are run in mono so that someone standing by the left stack still hears the whole song. Ceiling speakers in shops, gyms and restaurants are usually mono too.
• The Mono Audio setting in the accessibility options on iPhone and Android, which people with hearing in one ear rely on.
• FM radios, which drop to mono when the signal gets weak, and AM radio, which is almost always mono.

Listening on one earbud is a related case. You hear one channel only, and a device won't sum the two for you unless its mono setting is on, so a part panned hard to the other side simply isn't there.

There's a second reason to care. A mix that cancels in mono nearly always has a phase problem that affects the stereo version too, heard as a vague centre or as bass that feels weaker on speakers than it does in headphones. The Mono button is the quickest way to find it.

What happens to a mix in mono?

Summing to mono plays (L + R) ÷ 2 through every speaker. Engineers call that sum the mid, and the difference between the channels, (L − R) ÷ 2, the side (the mid/side lesson covers both). Mono keeps the mid and throws the side away, which is the most useful way to think about the whole subject: anything that lives only in the side signal disappears in mono.

How much each part of a mix changes depends on how alike its two channels are, which is what a correlation meter measures. A centred vocal is identical in both speakers, so the sum is the same vocal at the same level. Two different takes of a guitar panned hard left and right have nothing in common, and each one comes out 3 dB down. A part that's the same in both channels but with one side flipped upside down cancels completely.

The first three rows are the normal life of a mix, and they explain why a good mix still sounds a bit different in mono. The centre keeps its level while the wide parts lose 3 dB, so the vocal and drums step forward and the stereo reverb behind them drops back, which makes the mix sound drier. Pan laws play into this too. In a DAW using a constant-power pan law, a centred part and a hard-panned part sound equally loud in stereo, and in mono the centred one comes out 3 dB louder. The pan laws lesson goes through why.

The last two rows are where mixes break. A late copy cuts a row of notches out of the tone, a comb filter with its first notch at 1 ÷ (2 × delay); a flipped copy takes out everything the two sides share. For comparison, the lab's full mix loses 0.9 dB in mono with nothing cancelling, which is a healthy result.

Reading the lab

The lab plays a stereo loop through a short chain: a delay and a polarity switch on the right channel, then width, then pan, then the Mono button. Every screen is measured from that same chain. The copy controls are locked on this page, because they work inside each channel and affect stereo and mono alike.

The figure to watch is dB in mono, in the Right now panel. It compares the loop summed to mono with the loop in stereo, so 0 dB means nothing was lost and a figure like −0.9 is the normal cost of some width. The SUM meter next to the goniometer shows the same thing while the loop plays; if it falls well below the L and R meters, something is cancelling.

The wide screen labelled "What mono does to the tone" sends a click through your settings, adds left and right together and draws the frequency response. A flat line at 0 dB means mono leaves the tone alone, and each dip is a frequency that cancels, labelled in Hz. The dashed line is the left speaker on its own, for comparison. Underneath, the correlation meter reads from −1 to +1, with a strip showing the reading across the whole loop.

The drums, lead, guitar and full mix come from the Wonky Percussion and Flow packs; the acoustic guitar and vocal are public-domain recordings from Freesound. You can use your own sound too. Upload a mix, or record eight seconds, and it plays through the lab in place of ours without leaving your device. A bounce of your own track is a good thing to try: press Mono and read the panel.

What causes mono problems?

Almost every mono problem comes down to the same sound arriving in both channels with something different about it. One side might be flipped, delayed or shifted in phase, and each of those gets into a mix in its own way.

One side flipped

If one channel of a stereo track has its polarity reversed, every push in the left meets a pull in the right. In stereo it sounds wide and strangely placeless, with weak bass, because the two speakers partly cancel each other in the room. In mono, whatever the two sides shared is gone. On a sound that was identical in both channels, like the acoustic guitar loop, that's nearly all of it: the lab measures more than 50 dB of loss.

It's rarer than the other causes, but it does happen. A miswired cable or patch, a stereo sample exported with one side inverted, or a widener that works by flipping one side will all do it. The correlation meter makes it easy to spot, because it sits below zero for the whole song. The polarity lesson covers the switch in detail.

A delay between the sides

This one is easy to do by accident. Delay one side of a sound by a few milliseconds and in stereo it seems wider, leaning towards the earlier speaker; that's the Haas effect. In mono the two sides add together as a sound and its own late copy, which is a comb filter. At 10 ms the notches start at 50 Hz and repeat every 100 Hz, right through the body of a guitar or a vocal.

The same thing happens when you copy a part, nudge the copy a few milliseconds later and pan the two apart. Spaced pairs of microphones do it too, since each mic hears the source at a slightly different time and the gap between them changes as the player moves. Coincident pairs such as XY and mid/side put both capsules in the same spot, which is why they hold up better in mono.

Stereo wideners

Wideners work in very different ways, and the difference decides what they do in mono. One that turns up the side signal, like the width knob in the lab, leaves the mono version untouched. One that creates width by delaying one side, shifting its phase or flipping part of it can leave comb filtering, a phasey swirl or a straightforward drop in level when you sum. The stereo widener vs mid/side lesson compares the two kinds.

Chorus and other stereo modulation

A stereo chorus usually runs a differently moving delay on each side. In stereo that sounds wide and lush. In mono the two sides add together, and their moving delays comb-filter against each other and against the dry sound, so the chorus loses its width and gets a more obvious, phasey swirl. Stereo phasers and ensemble effects behave the same way.

Wide synths and stereo samples

A synth patch with wide unison, a stereo oscillator mode or a chorus on the output can arrive with its sides partly opposed. The lead loop in the lab reads −0.18 across the loop and loses 3.8 dB in mono. It still plays its tune and nothing vanishes, but it falls back a long way behind where it sat in stereo. Whether that's a problem depends on the mix. A lead that has to carry the song probably needs fixing, while a wide pad that's there for atmosphere can take it.

Stereo reverb

A stereo reverb is designed so its two sides are mostly unrelated, because that's what makes it sound spacious. Unrelated sides lose about 3 dB in mono, so the reverb drops back against the dry parts and the mix gets a little drier. That's expected and usually fine. If the vocal ends up sounding too dry in mono, the reverb was probably set in stereo at a level that only just worked.

Width that's safe in mono

Panning is the safest way to make width. A pan knob only changes how much of a track goes to each speaker, without delaying anything, so it can't cause a comb filter. A mono track panned hard to one side comes out 3 dB down in mono, which the lab measures exactly, and keeps its tone.

Real double-tracking is safe too. Two separate takes of a guitar or a backing vocal, panned apart, are different performances, so there's no fixed delay between them and nothing cancels in a regular pattern. Each loses 3 dB in mono.

Mid/side width can't change the mono version at all. Push the full mix to 200% in the lab and the correlation falls from +0.64 to +0.06, but the mono version is the same sound you heard at 100%. The dB in mono figure grows from 0.9 to 2.8 dB only because the stereo version got louder. A very wide mix loses more of its character when it's summed even when nothing cancels, and the stereo width lesson is worth reading before you reach for 200%.

How to fix a mix that fails in mono

Start by finding the part that changes most when you press Mono, then fix it at the source. Narrowing the whole mix until a meter looks happy treats the symptom and costs you width you didn't need to lose.

The lab shows why narrowing doesn't fix much. Take the lead down to 50% width and its correlation rises to +0.48 while the gap between stereo and mono shrinks from 3.8 dB to 1.3 dB, yet the mono version hasn't changed at all. Narrowing makes the stereo version sound more like the mono one. It can't put back what cancels, because the cancelling happened before the width control ever saw the sound.

What works depends on the cause:

• For a flipped side, flip it back. If a whole track reads below zero on a correlation meter, the polarity switch on one of its channels will usually cure it.
• For a Haas delay or a copied part, take the delay out, or replace the copy with a real second take.
• A widener that delays or phase-shifts can be swapped for one that works on mid and side, or turned down until the mono version stops changing.
• Stereo bass should go mono below about 100 to 150 Hz. Ableton's Utility has a Bass Mono switch for this, and most mid/side EQs can high-pass the side signal.
• With a wide synth or sample, look in the patch for unison spread, a stereo chorus or a phase setting and narrow it there. Another option is a mono copy of the part tucked under the wide one, so the mid has something solid in it.

After each fix, press Mono again and listen to the same part. The comb filtering lesson has more on getting rid of notches you didn't ask for.

How to check your mix in mono

This takes a couple of minutes and needs nothing you don't already have.

1. Put a mono switch last on the masterIn Ableton Live, Utility has a Mono switch. In Logic, the Gain plug-in has a Mono button. It has to come after everything else, including any widener or limiter on the master.
2. Add a correlation meterLogic's MultiMeter has one alongside a goniometer, and plenty of free metering plug-ins do too. Put it before the mono switch, or it will read +1 whenever mono is on.
3. Play the busiest sectionChoruses and drops are where stereo effects and layers pile up, so they're where things cancel.
4. Switch between stereo and mono and listen to the anchorsThe kick, snare, bass and lead vocal should keep their level and tone. Some reverb and width dropping back is normal.
5. Find the part that changesIf something drops or goes hollow, solo groups of tracks in mono until you find it. Check effect returns as well as tracks.
6. Fix the cause, then check againUse the fixes above, then press Mono once more on the same section.
7. Listen on a real single speakerA phone speaker or a small Bluetooth speaker at low volume will tell you whether the vocal and bass still come through.

Common mono compatibility mistakes

Only ever listening on headphones

On headphones each ear gets one channel and the two never meet, so a Haas delay can sound great and a flipped side only a little odd, with no warning of what mono will do to them. Press Mono, and check on speakers when you can.

Narrowing everything to pass the meter

A correlation meter that dips below zero for a moment on a reverb tail isn't a fault. Squeezing the whole mix narrower to keep the needle positive throws away width the mix could have kept, and as the lead in the lab shows, it doesn't bring back anything that cancels.

Checking in the wrong place

A mono switch on one track tells you about that track. Put the check at the very end of the master chain, after any widener, so you hear what a mono speaker would play.

Widening the low end

Chorus, wideners and Haas delays on a bass line or a kick sound huge in headphones and lose their weight in mono. Keep the sub in the middle and widen only the upper part of the sound.

Turning a part up to make up for mono

If a synth disappears in mono and you push its fader, it gets too loud in stereo. Fix whatever makes it cancel first, and set the level afterwards.

Train your ears for mono

The quiz under the lab plays a loop with hidden settings, all in stereo, and asks whether it will survive being played in mono. Some rounds play the loop as it is or a little wider or narrower, which is safe; others have one side flipped or delayed by under 2 ms, which isn't. You have to hear the stereo version and predict what summing will do. A flipped side tends to feel as though it's inside your head with no clear centre, and a short delay pulls the sound towards one speaker, so listen for where the middle is.

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