Checking mono compatibility is one of those things which always comes up when discussing mixing. Some say that mono compatibility is irrelevant in 2022 and that you shouldn’t compromise mixes to account for it, while others feel that mixing should take into account how music is actually consumed, including mono.

I can see merit on both sides and the issues which can arise when stereo material folds down destructively to mono are pretty well understood. Anything from nasty, sucked-out comb filtering effects, all the way to entire elements disappearing altogether can happen. However I want to look at a subject which isn’t spoken about quite so often. Not whether you should check mono compatibility but how you should check mono compatibility.

Consumers Don’t Always Hear Stereo Reproduced

Listeners don’t always hear stereo. That’s a fact. Smart speakers are sometimes entirely mono. Mine are ‘semi-mono’ meaning that they have a pair of HF drivers either side of a mono bass/mid driver. Many phone speakers are stereo too, but only sort-of. However the main way in which audiences aren’t hearing stereo is because of where they are relative to the playback system. The days of domestic listeners being seated in the sweet spot between a pair of properly positioned speakers are largely behind us. The chances are that the fragile, level and time based auditory cues which create a stereo soundstage are unlikely to be being properly presented anywhere other than on headphones, and whether headphone stereo is ‘proper’ is an entire debate on its own.

So retreating from the debate about the merits of mono checks. If you are going to check mono, how do you best do it?

How To Check Mono

The answer very much depends on how important you feel the mono mix is. If you’re just checking that your M/S processing on the synths doesn’t make them null to silence in mono then something as simple as hitting the mono button on your monitor controller, or on any of the plugins which offer mid/side matrixing, solo the Mid channel will suffice. You‘ll have the sum of both stereo channels playing out to you and if there are any major cancellation issues you’ll hear them. However you might be hearing issues caused by your monitoring too.

The problem is that you’re not listening to mono playback, you’re listening to a stereo reproduction of a mono mix, and it’s represented by a phantom mono source halfway between your speakers. This mix is 100% correlated and the more strongly correlated a stereo signal is, the more vulnerable it is to issues with the monitoring system.

Sum vs Difference

If you understand the role of mid/side or sum and difference in stereo you’ll probably know the significance of correlation. In this case what it refers to is the amount of common information between the two stereo channels. Stereo relies on the differences between the two channels to give that sense of width. If the reproduction between the two channels differs because of discrepancies in the monitoring then the more strongly correlated (similar) the two channels of audio are, the more effect these discrepancies will have on the perception of ‘mono’. In short, the stereo information (the difference or sides channel content) of regular stereo material isn’t affected by these issues as much because it isn’t correlated.

Technicalities aside, what kind of discrepancies am I referring to? The first to consider is time of arrival differences caused by the position of the listener relative to the monitors. This idea of a sweet spot where both speakers are identical distances from the listener is nothing new but with 100% correlated audio from both speakers, the audio will be comb-filtered by these time of arrival differences. This also happens with stereo playback but its worse with mono auditioned in stereo. The same goes for mismatches in the performance of the two monitors and the contribution of the room acoustics, at least in so far as how the two monitors are affected differently from each other in the listening room.

So does this matter and what should we do differently if it does? Returning to my earlier point that it depends on how important you feel the mono mix is, if you are “disaster-checking”, making sure your super wide guitars don’t disappear in mono then hitting a mono button is fine. But if you are checking the timbre, that the character of the sounds in your mix aren’t compromised by phasing and comb filtering when summed to mono then they are more significant.

How To Hear Comb Filtering

If you want to check mono then do it from a single speaker. If you want to check the benefits try this simple test. The results will depend on the nature of you room, particularly how dry it is. Try it and see how things sound.

Set up a signal generator plugin creating pink noise on a stereo track. If you’re in Pro Tools the Signal Generator plugin is Multi-Mono and this is important because that means the noise is decorrelated, Check the correlation using a plugin with a correlation meter. Something like Flux Stereo Tool will do if you don’t already have something. With this ‘stereo’ noise playing back move from side to side in front of your monitors. You’ll probably hear a little bit of tonal variation, a bit of a ‘swoosh’ as you move and the noise will sound a little different everywhere you stop.

Try this again but with the output summed to mono. Move between the speakers again and you’ll probably hear a more pronounced ‘swoosh’ This is comb filtering caused because the uncorrelated ‘difference’ between the channels is no longer masking the mono element. This is why mono mix checks like this are more vulnerable to phasing.

Lastly, pan both sides over to one side, or mute one speaker from your monitor controller. Although the shift over to one side will be distracting, if you move from side to side the ‘swoosh’ will be much reduced. because it is mono reproduced by a single speaker. One channel, one speaker, one path from speaker to listener.

This is compromised in the real world by the contribution of indirect sound from reflections in the room and you will probably find that the drier the room, the more pronounced the difference will be.

Mono Checks, In Mono

So what to do if you want to check you mixes in mono ‘properly’? Using a dedicated centre monitor is a great solution. Exactly how you plumb this in is up to you but the advantage of a mono check is that, while it’s nice if the mono monitor is the same model as the LR monitors, it doesn’t have to be.

If you don’t want to install an extra monitor then programming a way of accessing a mono mix via a single speaker from a single button push is a good solution, and involves no additional gear. Asking around the team James Richmond suggested using the amazingly flexible DADman software for his Avid MTRX to set up exactly this function. In fact he set it up while on the phone to me in less time than it probably took for you to read this paragraph.

Luke Goddard suggested doing something very similar with his programmable Audient Nero, interfaces like the iD44 can do the same. For software-only solutions SoundFlow seems an obvious contender and even some clever routing in your DAW’s mixer could achieve the same.

Do you mix for mono? If do do you monitor in stereo while you’re doing it? Share your thoughts in the comments.