How to check your mix in mono

What happens when stereo becomes mono

A mono sum adds the left and right channels together. Whatever is identical in both sides (the centre of the mix) adds up cleanly. Whatever differs between the sides behaves differently:

• In phase (the same signal in both channels): adds up and stays solid.
• Uncorrelated (different signals, like two separate takes): adds up with less gain than the centre, so it drops a little relative to it.
• Out of phase (the same signal, inverted or delayed in one side): cancels. A fully inverted signal disappears completely; a short delay cancels some frequencies and not others, which is comb filtering.

That last case is the problem. A part that sounds huge and wide in stereo can turn thin, hollow or silent in mono.

What typically disappears

Stereo wideners and chorus

Many stereo-widening plugins work by making the two sides different in phase or time. Push them hard and the widened part cancels in mono. Chorus and ensemble effects with wide stereo spread do the same.

Haas-effect doubles

Copying a part to the other side with a delay of a few milliseconds makes it sound wide. In mono, that short delay causes comb filtering: the part gets a hollow, phasey tone and can lose level badly at some frequencies.

Stereo reverbs and room mics

Reverb returns often carry a lot of out-of-phase content. In mono the reverb usually gets smaller, which can make a mix sound drier than intended. Spaced room mics and wide stereo recordings can also shift in tone.

Polarity mistakes

A stereo track with one side polarity-inverted, a mis-wired mic or a reversed plugin setting will cancel almost completely in mono. It sounds odd but not obviously broken in stereo, which is why it slips through.

Low end

Stereo bass, wide sub layers or a kick with a stereo effect can lose weight in mono. Because low frequencies carry so much energy, even a small cancellation there changes the whole mix.

How to check

1. Sum to mono at the end of the chain. Use a mono switch on your monitor controller, your DAW's master or a utility plugin on the master, set to mono only while checking.
2. Listen for disappearing parts. Toggle back and forth. Does a synth, a double or the reverb drop out? Does the vocal suddenly jump forward because everything around it got quieter?
3. Listen for tonal changes. A hollow, flanged or thin sound in mono points to time or phase differences between the sides.
4. Listen on one speaker. Mono through both monitors is a phantom centre; mono through one speaker is closer to what a smart speaker does.
5. Check the side channel. Anything that's loud in Side is what drops in mono.

How to fix mono problems

• Back off the widener. Most wideners sound fine at a lower setting. Find the point where the part is still wide in stereo but stays present in mono.
• Use real doubles instead of copies. Two separate performances, panned apart, are uncorrelated rather than out of phase. They survive mono far better than a delayed copy.
• Lengthen or remove Haas delays. If you need a delay-based width effect, try it on a less important part, or check which delay time causes the least damage in mono.
• Keep the low end centred. Mono the bass and kick, or use a utility or mid/side EQ to make everything below roughly 100–150 Hz mono.
• Fix polarity at the source. Flip the polarity of the offending side or mic, then check both stereo and mono again.
• Balance for both. If the reverb vanishes in mono, you may be relying on it too much. A little more in stereo, or a less wide reverb, can work in both.

Checking with Listen as Mono on your iPhone

MixOutpost streams your master bus to your iPhone and has a Listen as control with Stereo, Mono, Side, L and R. Switching to Mono sums the stream on the phone, so you can check mono on the speaker, AirPods or anything else the phone plays through, while the session plays.

1. 01

   Stream the master

   Put the MixOutpost plugin last on your master output and press Start Sharing. Pair your phone once with a QR code and tap Listen.

2. 02

   Switch to Mono

   In Listen, set Listen as to Mono. Flip back to Stereo to compare, then try Side to hear exactly what's dropping out.

3. 03

   Fix on the Mac, hear it on the phone

   Adjust the widener, the reverb or the polarity in your DAW and hear the result immediately. Nothing on the phone changes what your DAW outputs.

Listen as only changes playback on that phone, and the app shows an Altered playback badge while it's on, so you don't forget you're in mono. Loudness readings still measure what your Mac sends, and recordings are made before Listen as. The proposed free version of MixOutpost includes Mono, Side, L and R.