Mixing: What Is Mono Compatibility & Why Is It Important?

While stereo audio is certainly the most common for modern mixes, some playback systems still only produce sound from mono signals. So, if you want your stereo mixes to translate to as many systems as possible, ensuring proper mono compatibility is essential.

In this article, we'll discuss mono and stereo audio and dive deep into what mono compatibility is and how to achieve it.

If you prefer video format, please watch my video on the topic of mono compatibility below:

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Mono Vs. Stereo Audio

The main difference between mono and stereo is the number of channels used to record and playback audio signals. Mono signals are recorded and played back with a single audio channel. Stereo signals are recorded and played back with two audio channels (the left and right channels).

From a listener's perspective, the most noticeable difference is that mono signals cannot create a perception of width. Conversely, when played back through a stereo system (i.e., a stereo pair of speakers or a pair of headphones), stereo audio signals can produce perceived width.

The stereo width or “stereoness” of stereo audio is a product of the differences between the two channels. Differences in the waveforms cause contrasts in sound between the left and right channels, which ultimately coincide with perceived differences at each of our ears. We can then experience directionality from stereo audio.

If the two channels of a stereo audio file are identical, we'll effectively hear it as mono since there are no differences between the left and right channels. In this case, we'd have perfect phase correlation, and the stereo version would sound the same as the summed-to-mono version. This wouldn't really be considered “mono compatibility,” as we'd basically have the same thing.

In the case of mono compatibility and stereo signals being played through mono systems, the two stereo tracks (left and right) will effectively be summed together into a single mono channel. Depending on the pan law/rule of the mixer or DAW (the amount a signal is attenuated at the centre relative to the extreme left or right), the process of summing may also introduce or require some attenuation as the stereo signal is summed to mono.

Mono audio signals/files use only a single channel to convert signals into sound. Therefore, even if several speakers are connected, the same audio channel will drive all the speakers.

The playback systems which use two speakers are known as stereo systems. Stereo audio files contain both left and right channel information that commands the left and right speakers of the system to pull and push air.

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What Is Mono Compatibility?

Mono compatibility, as the name would suggest, is having stereo audio that is “compatible” (i.e., still sounds great) in mono.

As I discussed in the previous section, the sense of “stereo” we get from stereo audio is due to the differences between the left and right audio channels.

Summing stereo audio files to mono requires us to combine these two stereo channels into a single mono channel. Any differences in phase between the left and right channel waveforms will interact and cause both constructive and destructive interference.

If this interference due to phase differences is too great, we can have significant changes to the balance of the mix as we sum to mono.

Proper mono compatibility means having a great stereo mix that doesn't completely fall apart when summed to mono.

In other words, mono compatibility means having the necessary phase differences (between the left and right stereo channels) to create a compelling stereo mix while simultaneously having a strong enough phase correlation between the left and right stereo channels that they don't interfere too destructively with each other when summed together.

Of course, this is easier said than done, and we'll discuss strategies for achieving mono compatibility in an upcoming section of this article.

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Why Is Mono Compatibility Important For Stereo Mixes?

Mono compatibility is crucial for stereo mixes because of the likelihood of the mix being played back on mono playback systems.

We can spend all the time, energy and money crafting a perfect stereo mix in a perfect studio space. However, we have no control over how the end listener will listen to the audio.

There's a running joke of the million-dollar studio producing music to be listened to on a pair of $10 earbuds. Another example is that radio will heavily compress the audio and introduce noise in cases of bad reception (not to mention that radio speakers, whether they're in the car or the home, are usually far from ideal playback devices).

It's also the case that the playback of the mix would be down through mono systems. Many public address systems, smartphone and laptop speakers, portable Bluetooth speakers and more are mono by design.

Playing back a stereo mix through these systems means the audio will be summed to mono. If we don't have proper mono compatibility, our mix will sound different in the worst way possible.

So, that's why it's important to spend some time working on the mono compatibility of a stereo mix.

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How To Achieve Great Mono Compatibility In A Stereo Mix

Now that we have a grasp on what mono compatibility is and why it's worth striving for in our stereo mixes, let's go over a few strategies for achieving great mono compatibility in our mixes.

• Understand phase issues and correct them as need be
• Mix in mono
• Mix the “skeleton” in mono
• Mix the bass in mono
• Consider LCR panning
• Consider mid-side processing

Understand Phase Issues And Correct Them As Need Be

This first “strategy” is more so about understanding the fundamentals of phase.

Phase refers to the amount that a wave has passed through its cycle. Because sound and audio are waves, phase is important. Two identical waves/signals in-phase sum together perfectly, while two identical waves/signals out-of-phase cancel each other out perfectly.

Due to the complex nature of sound and audio, we'll rarely ever have perfectly in-phase audio waveforms unless they're duplicated.

So when we're discussing phase in mixing, we're generally talking about how well two or more signals line up with each other over time. How well do they match up in both positive and negative amplitude?

Again, if a stereo audio file's left and right channels are identical, that file will be heard as if it's mono (even though it has two channels). All the differences in phase between the right and left stereo channels are effectively what make the audio sound stereo.

So, we want to be able to understand the potential phase issues in our mixes and know how to fix them.

This starts with the individual tracks. Going about aligning them correctly (especially with tracks with significant low-end and transient information) will pay dividends down the line. This can be done by flipping the polarity of some tracks that seem out-of-phase with others or by actually nudging specific tracks in the timeline to align their phase better.

If two tracks are better aligned phase-wise, panning them to opposite sides can lead to better mono compatibility since they'll have less destructive interference when they're brought together in mono.

Next, we should be aware of the phase differences between the left and right channels of the entire mix.

Your digital audio workstation should have some sort of stock phase correlation meter that can tell you how your left and right channels match up phase-wise.

Phase correlation meters span continuously from -1 to +1, or from 180º to 0º. They can be put on stereo tracks or the stereo mix bus to meter the phase relationship between the left and right stereo waveforms.

At +1, we have a 100% correlation between the channels (they are exactly the same).

At 0, we have the “widest permissible left/right divergence” or the widest permissible stereo image.

Having the mix bus correlation meter moving between 0 and 1 is ideal. Smaller variations mean smaller differences in width.

At -1, our left and right channels are completely out of phase and will completely cancel each other out.

Mix bus correlation meter values between -1 and 0 mean that significant phase issues are present that will interfere with the stereo audio and definitely with the summed-to-mono audio.

Now, even at 0, we're likely to encounter some mono compatibility issues. Any time we strive for maximum width in a stereo mix, we will have phase cancellation when summing to mono. It's inevitable. Stereo width is primarily caused by phase differences between the left and right channels.

So it's worth monitoring the phase correlation between the mix's left and right channels. The closer we are to +1, the more correlated we are. However, it's important also to have stereo width. Use your discretion!

Mix In Mono

This seems counterintuitive for stereo mixing, but mixing in mono is an effective way to get better results in both our stereo mixes and, of course, in our mono compatibility.

It's often best to begin mixing with the mix bus summed to mono. Mixing in mono from the very start will focus our efforts on the level and frequency balance of the tracks and force us to achieve separation without panning.

Panning is an excellent tool for separating elements within a mix. However, the separation caused by panning will fall apart when the mix is summed to mono. We can often rely too heavily on panning separation and forego using EQ, compression (notably sidechain compression), and other processes and effects to achieve separation that will better translate in mono playback.

Getting the initial mix completed in mono will help tremendously in achieving great mono compatibility, and it will help improve the separation between elements when we open the mix into stereo and begin panning.

Speaking of panning, we can even go about panning while summed to mono. Again, this will give us a solid idea of how the stereo mix will sound when summed to mono.

From there, we may even opt to rough in some stereo effects while still monitoring in mono.

Of course, we'll have to begin mixing in stereo at some point, but it's a great idea to get the bulk of the mix balance done in mono before getting to the more advanced mix decisions in stereo.

Mix The “Skeleton” In Mono

The “skeleton” of the mix is made of the most critical elements. For example, the kick, snare, bass guitar, bass synth, lead vocal, lead instrument, etc., can be mixed mono (centre panned) for improved mono compatibility.

Having the most important elements in the direct centre means they won't be affected when the mix is summed to mono. They'll actually be enhanced relative to the stereo information (including all panned tracks) when the mix is summed to mono.

So, if there's an important element in the mix, it's worth considering panning it to the centre image!

Mix The Bass In Mono

In addition to the most important elements being mixed mono/centre, the low-end frequencies should nearly always be centred in a stereo mix.

Low frequencies have long waveforms. Any phase differences are more noticeable as the longer waveforms interact with each other over longer timeframes.

So, if there are phase issues in the low-end between the left and right channels, there will be destructive interference in the stereo mix that will translate to the mono mix as well.

We naturally hear low-end frequencies as omnidirectional or non-directional as their wavelengths are longer than the distance between our ears. We should opt to emulate this acoustic reality in our mixes (as is par for the course in many mixes).

Consider LCR Panning

LCR panning is a mixing strategy that pans tracks either hard left (L), centre (C) or hard right (R), with nothing in between. This is different than an intermediate approach that allows the panning of tracks across the entire panorama.

Note that we can then rely on stereo effects (delay, reverb, etc.) to fill the space between the middle and the extremes.

Going back to mixing the “skeleton” or most important elements in mono, the LCR system goes hand in hand with that tip. If we have an important element (or a bass element), we aren't going to be hard panning it, so the only option is to mix it right down the centre in mono.

The hard-panned tracks can be doubled and panned to opposite sides (this is common with guitar). Any tracks that we can afford to have sound different in the mono mix can be panned hard left or hard right as well.

Consider Mid-side Processing

Mid-side processing effectively splits stereo information (left/right) into the sum and difference (mid/side). The sum or “mid” channel is the same as what we'd have when summing to mono. The difference or “side” channel is the information that's different between the left and right that is lost when summing to mono.

With mid-side processing, we can process the mids independently from the sides. This way, we can effectively mix the summed-to-mono version in reference to the entire stereo mix. We can also mix the sides information to adjust the stereo mix, knowing that these changes will disappear entirely when the mix is summed to mono.

My advice is to use mid-side processing sparingly, as it can easily be overdone. However, making small changes to the mids and sides can help us hone in on our mixes' mono compatibility.