Why the Difference between Mono and Stereo Audio Files is Important for Your Game
Today we’ll be covering Unity’s “Force To Mono” option in the Audio Import Settings, mono & stereo audio files, and why the difference is important for your game.
This blog post accompanies the video tutorial we’ve created for this subject, which can be viewed here:
(Guideline for which audio assets should be mono/stereo at the end of this blog)
Mono & Stereo
Before we get to Unity, let’s talk a little bit about the terms mono and stereo. We often hear these terms, but what exactly do they mean?
In the audio world, these terms are relevant in two different contexts:
- Format and
- Content
Format
Mono and stereo as a format implicate the number of channels in an audio file or container.
- Mono format refers to the fact that the file contains only one channel of audio
- Stereo format means that the audio file contains two channels, which will be mapped as left and right (L/R)
When viewing audio files in an audio or video editing program, the difference between mono and stereo will look something like this:
Content
Mono and stereo content proves to be a little more complicated to explain. The standard playback system of the day is stereo, i.e. two speakers to play audio through. When a mono audio file is mapped to play through such a system, it will play the one channel of audio content equally through both speakers. With stereo, the two channels are separated and played discreetly through the left and right speakers.
However, when we have a stereo file where the two channels of audio are exact replicas of each other, it will be audibly no different than the same content in a mono container, because either way, the playback result is the same.
Stereo Content
So what then is “stereo” content exactly?
“Stereo”, in the everyday use of the word, means you have differences in either level/loudness, time of arrival, or frequency content in the two channels within your audio file, which gives you the effect, should you be sitting between two speakers or listening with headphones, that sounds come from the left, the right, or somewhere in between, often referred to as the stereo field.
These differences, when used correctly, can give one the impression that there is a panorama of sound between the two speakers, or in the case of headphones, between your ears. With the addition of just one extra speaker, we have tricked the brain into perceiving a horizontal field of sound. Pretty cool, right?
Phase Cancellation
The last and most important point that we want to discuss before moving to Unity is phase cancellation.
Phase cancellation occurs in audio if you sum (i.e. combine) two or more channels of audio that have phase and/or time of arrival differences into a mono audio container.
The most classic and stark example of phase cancellation is that of a pure sine wave.
If we duplicate this sine wave and combine it with the original, we just get a louder sine wave, 6dB louder to be exact. This will always happen when two exact sounds get combined together. Now, if we reverse the phase of the second sine wave (i.e. flip the sound wave upside down) the resulting audio is…. nothing!
Why does this happen? Because the combination of the original sine wave and a perfectly inverted copy of itself cancels out the sound completely. Think of it like shooting a cannonball out of the rear end of a truck at the same speed as the truck is driving forwards. The speed of the cannonball and the truck cancel each other out, as shown here:
This is the most extreme example of phase cancellation, but phase cancellation can also occur partially and unevenly across the frequency spectrum. That means, when multiple channels of audio are combined, certain frequencies that will add together and become louder and others will cancel each other out. Depending on the original frequency content, the summing of these channels could have a very subtle or a very drastic effect on the quality of the resulting mono audio file.
Let’s move on to the Audio Import Settings in Unity and talk about why this information about stereo and mono is important for your game.
Force To Mono in Unity
When you click on an audio asset and take a look at the inspector, you will see “Force to Mono” as the first option in Unity’s Audio Import Settings. By checking this box, we tell Unity to do what we just did in the previous section, which is to force the combination of two or more audio channels into a mono audio container, hence the word “force”.
There are two reasons to consider forcing sounds to mono in your game:
- Optimizing for Performance and
- Optimizing for Unity’s Audio System.
Force to Mono for Performance Optimization
Forcing stereo/multichannel audio files to mono is a great way to save space in your build and optimize your game’s performance. Since a stereo file has two channels instead of one, it needs twice the storage space and twice the computer resources for playback during runtime.
You might be thinking, “but audio doesn’t normally take up that much space or performance compared to other aspects of the game”, and you would be right. However, if you are dealing with hundreds or even thousands of audio files, or making games for machines with low-performance specs (mobile, for example), the difference is often not so trivial.
Force to Mono for Unity’s Audio System
The other reason Force to Mono is so important has to do with Unity’s audio playback, specifically what Unity does when you switch a sound from 2D to 3D in the audio source component.
When you tell Unity to playback stereo sounds in 3D space, be it for 3D games or VR, both left and right channels will originate at the same point in 3D space, making them essentially a mono sound. That means that there is no benefit to having a stereo audio file if it will be used in a 3D setting and leaving it in stereo is just a waste of space and resources.
When to use Force to Mono
So how do you best move forward with this knowledge?
Our main recommendation would be: make sure your audio assets are delivered in the correct channel format so that you don’t have to worry about this!
Experienced game sound designers will know which format certain sounds should be in and design them that way from the beginning.
If you’re doing your own sound or working with a sound designer who doesn’t have much experience in games, here are a few situations to look out for where “force to mono” can be either helpful or harmful:
Mono Content in a Stereo Container
Stereo is the most standard of audio formats. This means that many sound effects you might find online or from the Unity Asset Store may be packaged in a stereo container even though there is no real stereo content.
As you can hear from the Wilhelm Scream example, there is no difference in sound after forcing it to mono. In this situation, it is safe to say that this was effectively already a mono sound effect, just packaged in a stereo container (i.e. the two channels of audio had the exact same content). In our experience, this is a very common occurrence so always double-check.
Loudness Issues after Force to Mono
Another thing to notice when forcing to mono is the loudness of the newly created mono audio file. It’s somewhat quieter than the original. Unity is aware of this issue, and you can remedy it by selecting the normalize option.
Phase Cancellation
Forcing a stereo file with stereo information to mono can cause phase cancellation issues like those we encountered in the examples from the last section. If there is a noticeable difference in the quality of the audio after forcing to mono, you’ll have to decide for yourself whether it is acceptable for that sound and its role in your game.
Now that we’ve discussed this issue in-depth, do we need to go through all of our audio assets and make this decision for each one individually? Luckily, no.
How to Choose
There are always exceptions to the rules, but the guidelines below should help you make some basic decisions regarding the audio assets in your game.
Final Words
Let’s summarize the most important information:
- Force to Mono is extremely important for optimizing the performance of your audio
- For 3D sound sources, Unity will force to mono automatically so there is no reason not to force stereo sounds to mono in this situation
- Mono SFX often comes packaged in stereo containers. Leaving these sounds as stereo files waste your game’s resources
- Be careful that phase cancellation doesn’t degrade the sound quality of your stereo SFX when forcing them to mono
That’s it! We hope this has helped you to understand the difference between mono and stereo audio, the “Force to Mono” option in Unity’s Audio Import Settings, and why it is important for your game.
If you want to learn more about how audio functions in Unity, be sure to check out our other tutorials (linked below). If you have a question, leave it in the comments or contact us directly through our website:
