What is the difference between Mono & Stereo audio?

The short answer: Mono audio signals contain one channel. Stereo audio signals contain two channels.

It’s what that extra channel allows us to do for music, movies, and other media that is the most important difference.

By the end of this post, you’ll understand the benefits and drawbacks of each.

What Is Mono Audio?

Let’s start with mono audio. A mono audio signal contains only a single channel.

A mono recording can be played out of one speaker, but it could also be played out of two speakers, five speakers, or any number of speakers and still be considered mono. 

If you sent a mono signal to 5 speakers, each of the 5 speakers would play the exact same thing.

What Is Stereo Audio?

Stereo audio signals, on the other hand, contain two channels – left and right.

Each speaker can play something different. This requires two amplifiers – one for the left speaker and one for the right speaker.

To optimize stereo audio, it’s best to have your speakers set up in a specific way. A good rule is to separate your speakers to form an equilateral triangle between the speakers and your listening position.

The Benefit of Stereo Audio

Most media today is mixed for at least two speakers for stereo, if not more for surround sound systems. Here are the benefits of stereo audio.

More Immersive Listening Experience

When a listener is set up with a stereo system like this, they feel more immersed.

Listen to the difference between a mono and stereo version of the same song:

With two speakers, it becomes possible to create a phantom image – you don’t perceive sound as emanating from one speaker or the other, but from the space between them.

Depending on the blend between the left speaker and the right speaker, the perceived location of the sound source can be adjusted anywhere between the two speakers. This is called a stereo image.

We’re able to create stereo images by playing on a few cues that we humans use to localize sound, or determine which direction a sound comes from.

Interaural Level Differences

One of these cues is an interaural level difference, or ILD – a difference in the level of a sound between your left ear and your right ear.

As we intuitively know, the force of a sound wave is attenuated, or reduced, over distance. The further a sound travels, the quieter it becomes.

If a sound originates directly in front of you, the sound will take the same length path to your left ear and your right ear. So, the sound will be equally loud at both ears.

If it originates over to your left side, the sound travels further to the right ear and is quieter in the right ear because of that. This indicates to the brain that the sound originates from the left side.

The most common method for creating a stereo image – panning – works on this principle.

If I send a signal to the left speaker and the right speaker, but send it to the right speaker at a lower level, it fools your brain into thinking my voice is coming from the left side.

Interaural Time Differences

Another cue humans use to localize sounds is an interaural time difference, or ITD. An ITD is the difference in a sound’s time of arrival at each ear.

If a sound comes from your left side, we know that it travels a shorter path to the left ear. Travelling a shorter path, it will reach your left ear first before reaching your right ear. Our brains tell us that the sound must have originated from the left side.

The Haas Effect

In fact, the interaural time difference cue is so powerful that even if a sound is equally loud in both ears, if it reaches the left ear first, it will still appear to have originated from the left side. This is called the Haas Effect, and there is a mixing technique that works on this principle.

To use this technique, I’ll send the same signal to both speakers evenly. Then I’ll start to add delay to the right channel. 

Now the signal from the left speaker reaches your ear first before the signal from your right speaker, causing you to perceive the sound as though it were coming from your left side – even though the sound is equally loud in both speakers.

Stereo Microphone Techniques

Up until this point, the examples of stereo imaging have included stereo speaker systems. There are also stereo recording techniques that work on these same principles.

Read this other post that I wrote to learn about stereo microphone techniques.

Reduced Phase Interference Between Speakers

Another benefit of mixing audio in stereo is that it can be used to prevent phase interference between speakers. 

When the same sound originates from multiple locations, the two sound waves will interfere with one another. A mono signal played through a stereo system would be especially susceptible to this, because the signals are exactly the same.

By mixing in stereo, you could pan all instruments so that each signal predominantly comes from only one speaker. The left and right signals would interfere less and the problems caused by phase interference between the speakers would be greatly reduced.

You can read this post I wrote to learn more about phase interference. 

The Drawback of Stereo Audio

While stereo makes for a much more immersive experience, it does have it’s drawbacks.

Mono Compatibility

One important problem with stereo audio has to do with mono compatibility. Although stereo audio can prevent phase interference between speakers, it can actually cause phase issues in other ways.

It’s important to consider that some listeners will be using smartphones or other mono playback systems. When your stereo mix is played through a mono system, the left and right channel will be mixed together – perhaps resulting in interference between the left and right signal.

Complexity & Cost

The other drawback of stereo audio is that it’s expensive and more complex to set up. You need separate signal chains, separate amplifiers, and separate speaker circuits.

For things like spoken word, it is sometimes not practical to utilize stereo systems.