What Does Summing To Mono Mean? (Audio & Mixing)

If you're interested in producing and mixing music, then you're surely aware of stereo and mono signals. With stereo still reigning as the standard for modern music production, it's important to understand both stereo and mono signals and how they interact.

That's the short answer. In this article, I'll dive deeper into what really goes on when summing to mono, along with its relevance and importance in modern music production.

---

Differences Between Mono And Stereo Signals

Before we get to our deeper discussion on summing stereo signals to mono, I should lay some foundational groundwork. Feel free to skip ahead a few sections if you only want to read about summing to mono.

Mono audio has a single channel of audio, whereas stereo audio has two channels.

A channel is effectively a distinct audio signal/waveform.

Upon playback, a mono mix can be sent to one or more speakers, and each speaker will receive and attempt to produce the same sound waves. Conversely, when playing back a stereo mix, we can send the left and right channels to separate speakers (often a stereo pair) and have each speaker (including headphones) reproduce the sound waves represented by their respective channels.

Most modern music is mixed in stereo, though many of our tracks will be in mono within our mix sessions. It's through panning and stereo effects that we can adjust the balance of our mono tracks between the left and right channels of the greater stereo mix.

Stereo mixes are the standard largely because having two channels of audio coincides with our auditory system — we have two ears. Having tracks panned out around the stereo panorama gives them an added sense of directionality and dimensionality (particularly width) that isn't really obtainable in mono mixes.

Now, in order for a mix to take advantage of the spatial benefits of stereo, there need to be differences between the audio signals in the left and right channels. Otherwise, we could achieve the same result by duplicating a mono signal onto the left and right channels, which would sound the same upon playback as a simple mono audio file.

When we're listening in an ideal environment, we can hear stereo mixes as they're intended to be heard, with all the width and directionality choices made by the producer and/or mixing engineer. Perhaps the best way to listen to stereo mixes is through a nice pair of headphones, where getting into the ideal listening position is easy.

The idea of the “ideal listening position” was actually the source of pushback when stereo mixes were first becoming popular. The issue was that it put too much power or responsibility on the end listening to set up their listening environment correctly, and that mono mixes were still much more universal in the way they'd be listened to.

It turns out that stereo ultimately won, though many end listeners still listen in environments that wouldn't give them the maximum enjoyment of the stereo image. Some even listen in mono.

---

What Is Phase In Audio Signals?

Phase is the position of a point in time along a waveform cycle, defined in degrees (or radians) with a corresponding amplitude (either positive or negative).

Phase is a useful metric of repeating waveforms, but in audio, we're more concerned with general phase relationships. We like to know how “in phase” or how “out of phase” two or more signals are with each other.

To get two perfectly in-phase signals, we could duplicate a mono signal. The two waveforms are exactly the same, and there are no differences at any given point.

To get two perfectly out-of-phase signals, we could duplicate the same mono signal but flip the polarity of the copy. Now we have a situation where when one signal is at its maximum peak, the other is at its negative peak and vice versa, throughout the entirety of the waveform.

Two signals that fall anywhere between these two extremes will lie somewhere on a phase correlation scale, which spans continuously from -1 to +1 (or from 180º to 0º).

Phase correlation meters 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 how the stereo audio is heard under normal or ideal listening conditions. It will also have a detrimental effect if the audio is summed to mono, which brings us to the main point of this article.

For more information on phase, check out my video:

---

What Does Summing To Mono Mean?

Summing to mono, as the name would suggest, means summing multiple channels of audio to a single mono channel. In order to maintain relative output levels, summing devices generally divide the output by the number of summed channels.

In most instances, we'll be summing a stereo mix to mono, so I'll write the rest of this article as if we're dealing with stereo mixes.

So, to reiterate, in the case of stereo audio, summing to mono means combining the left and right channels and dividing the output level by two.

Power amplifiers will sometimes offer a sum-to-mono feature in order to drive a single speaker or speaker array with a mono signal from a stereo source. There are also cables and DI boxes that effectively sum stereo signals to mono.

Furthermore, this summing can be done digitally, either within our digital audio workstations or before the DAC on digital playback devices that generally connect to a mono speaker.

Now that we understand the basics, let's dive into what actually happens to the audio when a stereo mix is summed to mono. Audio waveforms from stereo mixes are awfully complex, so let's seek to understand the extremes and then extrapolate our ideas for everything else from there.

More specifically, let's consider the following prospects of a stereo mix:

1. Information panned to the centre (equal information in the left and right channels).
2. Information that is completely equal but out of phase between the left and channels.
3. Information panned to the hard left (information only in the left channel).
4. Information panned to the hard right (information only in the right channel).

In the first case, summing to mono would have no effect on the audio. Remember that we're combining the left and right channels together and then dividing the output level by, so in this case, we'd be doubling the level only to half it again. In an ideal listening environment, the information panned to the centre would be perceived as being in the centre, which would be no different once it's summed to mono.

In the second case, having two signals that are completely out of phase with each other would lead to their absolute cancellation when summed together. In other words, this information would completely disappear when the mix is summed to mono.

In the third and fourth cases, the information hard-panned to the left or the right would still be present in the summed-to-mono mix. However, their levels would be halved. Remember that part of summing stereo mixes to mono is halving the output, so if we have information that's only present in a single channel, it will be reduced by half. Furthermore, the hard-panned information that would be heard as very directional will now be heard as being centred (assuming ideal listening positions).

From this information, we can extrapolate that stereo information with varying amounts in the left and right channels will be centred and reduced in level relative to the information that is already strictly centre-panned.

Stereo elements and effects will suffer in terms of level, clarity and strength (due to phase cancellation), and the centre image will be unaltered, thereby sounding “enhanced” in relation to the sides information.

---

The Importance Of Mono Compatibility

I believe that mono compatibility is a sign of a strong stereo mix, and I believe it's something to strive for in many cases.

First and foremost, there will be situations where your stereo mix will be listened to in mono, and it's important that the mix doesn't completely collapse when that happens.

Smartphone speakers, club PA systems and portable Bluetooth speakers are just a few examples of playback systems that often playback audio in mono.

Additionally, we have the issue of non-ideal listening positions, where the “mono” information or that which is equally distributed between both channels, is likely the most important to get right so the listener can hear the most essential information of the song.

So then, mono compatibility, the ability for your stereo mix to translate well to mono playback systems, is important. I'd argue it's as important as getting a nice, wide stereo mix, though these two goals seem theoretically at odds with one another.

An easy way to check for mono compatibility while mixing is to insert a plugin on the mix bus that can sum the mix to mono. Personally, I use the stereo version of the Gain plugin that comes stock with Logic Pro X (see the featured image of this blog post).

When I want to check my mix in mono, I hit the “Mono” button and reference back and forth between mono and stereo a few times to get an idea of what needs to be done to improve my mix and the mix's mono compatibility.

It can even be worthwhile to get the initial balance of mix (including the panning) done with the mix bus is summed to mono to help maintain a stronger balance in mono and to help get a wider mix (more energy in the sides) if you naturally mix panned tracks a bit low.

To learn more about mono compatibility, check out my video below:

---

Related Questions

What is phase, and why is it so important to get right in mixing? Phase is the position of a point in time on a wave cycle, though, in mixing, it refers to the alignment of positive and negative amplitudes between non-identical waveforms. Having “better phase” means better alignment and less destructive interference between tracks, yielding better mix results.

How can I make my mixes wide? Stereo width, in mixing, is largely about the phase coherence between the left and right channels. We want differences between the stereo channels while also having a strong centre image, so contrast is key when it comes to panning our tracks.