7 Tips for Mono Compatibility in a Stereo Mix

A great mix shouldn’t just sound good coming out of your fancy stereo speakers, it needs to work out of a mono boom box too. Learn how to take care of phase issues early on, so you can focus on getting a big sound.

By Charles Hoffman

How many times have you created a mix that sounds amazing in stereo, but when you collapse it to mono it falls apart? Mono compatibility is something that you need to take into serious consideration while mixing because this is how many of your listeners will hear your song. If the guitars panned out to the edges of your stereo image disappear on a mono Bluetooth speaker, it could vastly change the way they experience the track.

At a basic level, it’s essential that you understand the concept of phase. Phase issues are the leading cause of poor mono compatibility in mixes, so nailing down this concept is critical. Phase issues may occur at the recording stage when placing multiple mics at various distances from a single instrument. How to prevent this during the recording stage is a separate topic of discussion. In this article, I’ll deal mainly with how to identify phase issues, and how to fix them while mixing.

Following my explanation of phase, I’ll walk you through some techniques you can apply to achieve stereo width while maintaining mono compatibility. The key phrase here is “maintaining mono compatibility,” because I believe the skeleton of your mix should be created in mono and then built outwards into stereo, not the other way around. It’s much easier to preserve something that works than to fix what’s broken.

What Is Phase and Why Is It Important?

It’s important to understand the concept of “phase” because it plays a significant role in mono compatibility. Elements that are out of phase with one another will drop in level or cancel each other out when summed into mono. This means potentially catastrophic mix problems on mono playback systems.

To understand phase, you first need to understand the basics of studio monitor operation. Sound waves are emitted from a studio monitor when its cone pushes and pulls air. These waves travel towards your ear, and your brain interprets them as sound. Keep in mind that this process is entirely physical, whether you can see these waves occurring or not.

As a simple example, imagine a speaker playing a sine wave. Each peak in the sine wave tells the speaker to push out, while each trough tells it to suck back into the speaker’s frame. Duplicating the sine wave will cause the amplitude of the signal to increase. The sine waves are in phase with one another and their wave cycles are completely in sync. They’re both telling the speaker’s cone to push out and suck in at the same time, generating a physical sound wave with twice as much energy.

If you take the second sine wave and invert its phase, it’s now telling the speaker to suck in and then push out. One sine wave is telling the speaker to do one thing, while the other sine wave is telling it to do the complete opposite. In this situation, the speaker is being fed two completely different instructions, and the result is complete silence. It's almost like a game of tug-a-war is happening, and both teams are evenly matched. The resulting phenomenon is known as phase cancelation and needs to be taken into consideration while mixing and mastering audio.

You can test this phenomenon yourself using a little experiment. Create two tracks, and place the same mono audio loop on each one. Pan one track hard left and the other hard right. Invert the phase of one of these using any utility or EQ plugin that has a ‘phase invert’ button; the result is a stereo image that appears incredibly wide. Before you get too excited about this, you need to check for mono compatibility. Place a utility plugin on your master channel and engage the mono switch. There’s now no sound passing through the utility, due to complete phase cancelation.

If you remove the utility and apply a phase correlation meter (more on this below), you’ll see that the meter’s pin is constantly touching -1. This is the worst thing that can happen if you’re trying to achieve mono compatibility.

1. Identify Phase Issues

In practice, you’ll find that phase issues manifest themselves in the form of reduced signal amplitude or complete silence when multiple parts are summed together. Luckily, it’s easy to check for phase issues, and in many instances, correct them. A phase correlation meter can be helpful for this purpose. The Waves InPhase plugin includes a phase correlation meter, as well as certain phase correction parameters that we’ll discuss briefly.

Phase correlation meters are easy to read as they’re generally a line with a moving dot. One side of the meter is typically labelled with a “-1” and the other with a “+1.” If the dot pulls towards the “-1” side it means that the signal is out of phase, and if the dot pulls towards the “+1” side it means that it’s in phase. It’s okay if the dot dips below 0 occasionally, but the goal is to reach, or at least get close to, +1 most the time.

Identifying phase issues can be difficult if you aren’t methodical about it. It’s helpful to start by checking that the kick and bass are in phase with one another. I generally like to build up my mix from these two elements, so making sure that they’re in phase with one another sets up my mix for success. I’ll also check that the different elements that make up each of my instrument groups are in phase with one another. Finally, I’ll check to see that my groups are in phase with one another.

2. Fix Phase Issues

Correcting phase issues is usually just a matter of aligning the waveforms of different elements together. This can either be done by nudging audio clips in your arrangement or by using track delays, which essentially delays the audio playback by the desired number of samples. The Waves InPhase plugin achieves this correction for you in a less time-consuming and more accurate manner, by perfectly aligning the waveforms. In the video below, audio educator Michael White demonstrates how to align acoustic drums together using InPhase:

3. Start Your Mix in Mono

There’s a lot of controversy over mixing in mono. Some people believe you should start your mix in mono and later add stereo width. Others think you should simply check your mix in mono throughout the mixing process, and address phase issues as you find them. Truthfully, there’s no correct way of doing this as it comes down to personal preference and whichever method provides you with favorable results.

Starting a mix in mono is beneficial because it forces you to set your levels appropriately and utilize EQ to massage different elements together. The idea is that you get your song sounding phenomenal in mono, and only then begin panning elements and applying other stereo widening techniques. I’ve used this method many times in the past and it has almost always paid off.

Starting a mix in stereo can still work, but you need to be conscious of the mixing decisions you’re making and how they will affect mono compatibility. It’s also critical that you frequently check your phase correlation meter. Many people mix this way, although it could be considered a riskier, more advanced way of mixing.

4. Pan Elements in Mono

Most people pan the elements of their song while listening in stereo, which makes sense because that’s how you’ll be hearing those elements playback on a stereo system. However, when you have a dense mix and you’re tight on space, panning something like your hi-hats in mono can help you find space for it. By panning the out-of-place elements across the stereo field while listening in mono, you’ll more easily find the point where they sit neatly; this is where you’ll leave the element.

5. Record Multiple Takes

None of this should scare you away from panning elements hard left and right in your mix. Phase issues will only occur if the panned element is out of phase with the opposite channel. If you pan the exact same element left and right in your mix, you’ll end up with a sound that plays back centered in the stereo field.

To create width that does a good job of translating into mono, try adding a slight delay to a mirrored track, and apply something like Waves UltraPitch to modulate its pitch and time.

Recording a second take is another great method that will allow you to create stereo width. Using a second recording of a vocal, guitar part or synth patch that’s slightly modified from the one panned left/right is a great way to produce a wider stereo image. This tends to sound much more natural than attempting to modulate the pitch and time of a duplicated signal to create stereo width.

6. Mono Your Low End

Sub frequencies aren’t directional, meaning you won’t be able to tell if they’re coming from the center or sides of your stereo image. These are frequencies that are “felt” more than heard. Because of this, there’s no reason to pan the low end of bass elements out to the sides. At worst, doing this could cause potential phase issues when your mix plays back in mono.

I’m sure you’ve heard super-wide sub basses before, but this is actually somewhat of an illusion. It’s possible to use a multiband stereo imager to mono elements below 80–100 Hz, and progressively widen upper bands. The result is a bass that appears to be quite broad, when in reality the sub frequencies are dead-center. Picturing your mix like a tree that’s narrow at the bottom, and then opens gradually as it moves up the frequency spectrum, can be especially useful when trying to visualize how your mix will sound.

7. Embrace Phase Cancelation

Sometimes it’s okay to lose stereo information as long as you accommodate for it. As you already know, panning two identical signals left and right, but entirely out of phase with each other, will result in silence when summed to mono. In stereo, however, this placement will create a broad stereo image. It may seem like you can’t have it all… or can you?

Placing a third recording dead center in your mix will ensure that there’s still something present when you collapse your song into mono. If the elements placed out to the sides are guitars, you could easily tuck a third guitar, playing the same part, into the center of your stereo image. This will let you get the incredibly wide stereo image you want and ensure that there’s still guitar present in mono. If you understand what phase is and why phase cancelation occurs, you can play around with it however you want.

Conclusion

At the end of the day, the only way to make sure that your mix translates well into mono is by listening to it and adjusting as necessary. A phase correlation meter will help you identify phase issues, and so will using your ears. Being diligent about identifying phase issues before they get out of hand will pay off by the end of your mix.

Want more on stereo mixing? Get 12 tips on making a wider-sounding stereo mix.
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