What's the difference between mono and stereo audio? Well the short answer is that mono signals
contain only one channel whereas stereo signals contain two channels. But the most important
difference is what that extra channel allows us to do to improve the listening experience
and by the end of this video, you'll understand the benefits and drawbacks of each.
But if you're new to this channel, I'm Kyle. If you want to learn audio
production online, subscribe to Audio University.
Let's start with mono audio. A mono audio signal contains only a single channel of audio.
That single channel could be played out of a single speaker
but it could also be played out of two speakers, five speakers, or any number of speakers. If
a system only allows you to send the same signal to each speaker, it's a mono system.
A stereo audio signal contains two channels - a left channel and a right channel. Each
speaker can play a different signal, but this requires a separate amplifier
for each speaker - one for the left speaker and one for the right speaker.
To optimize stereo audio, you should set your speakers up in a specific way. A good rule is
to form an equilateral triangle between the speakers and your listening position.
When a listener is set up with a stereo system like this, they can feel more immersed in what
they're listening to. Take a listen to the difference between a mono and stereo version
of the same recording. With two speakers in stereo it becomes possible to create a phantom image. You
don't hear the sound as emanating from one speaker or the other, but rather from the space between
the speakers. Depending on the blend of the left speaker and the right speaker, a sound source can
be perceived as coming from anywhere between the 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 sounds, or determine the location of a sound source in space.
One of these cues is an ILD, or an interaural level difference. It's based on the difference
in level between the left ear and the right ear. As we all know, sounds get quieter as they travel
further in distance. So, if a sound source is directly in front of you,
the distance it travels to each ear is the same and so the level in each ear will be identical.
However, if you move the sound source over to your left side, the sound travels further to your right
ear than it does to your left ear and because of that it's going to be louder in your left ear,
and it will be perceived as coming from the left side. The most common method for creating
a stereo image is called panning and it works on this principle. If I send my voice to both
of your speakers evenly, my voice will sound like it's coming from directly in front of you,
even though it's coming from both speakers evenly. If I send less of my voice to the right
speaker than I do to the left speaker, my voice will sound like it's coming from the left side.
Another cue humans use to localize sounds in space is an interaural time difference, or an ITD.
ITDs describe the difference in the time of arrival at the left ear versus the right ear.
We know that if a sound comes from the left, it travels a shorter path to the left ear than
it does to the right ear, and because of that it takes longer to get to the right ear than it does
to the left ear. The brain determines that the sound must have come from the left side. In fact
ITDs are so powerful that even if a sound is evenly loud in both ears, if it reaches
one ear first before reaching the other, we'll determine that the sound came from that side.
This is called the Haas effect and there's a mixing technique that plays upon this principle.
To demonstrate this technique, I'll send my voice to both of your speakers evenly. My voice is
evenly loud in both speakers and it's reaching both of your ears at the same time. Because of
that, it sounds like I'm directly in front of you. But now I'll start to delay the right speaker,
pushing it back in time. Now my voice reaches your left ear first, before reaching your right ear,
and even though both speakers are evenly loud, it still sounds like I'm coming from your left side.
Up until this point, all of the examples of stereo imaging have described stereo
speaker systems. But there are also stereo recording techniques. I wrote a whole post
about stereo microphone techniques that I've put a link to in the description of this video.
Another benefit of mixing audio in stereo is that it can help to mitigate the phase
interference between speakers. If the same signal originates from two locations in space,
those two sound waves will interfere in the air.
A mono signal is especially susceptible to that because the two signals are exact copies
of one another. You can learn more about phase interference by watching this video.
One important problem with stereo audio is mono compatibility. Although mixing in stereo can help
prevent phase issues between speakers, it can also cause phase issues in another way. It's important
to remember that some listeners will be listening on a Bluetooth speaker, a smartphone, or some
other mono playback system. When that happens, your left and right stereo mix will be combined
into a mono signal. Any phase issues that exist between the left channel and the right channel of
your mix will be summed together, and that'll cause cancellation when played back in mono.
Another drawback of stereo audio is that it's more complex and more expensive because you
need two of everything - separate signal chains, separate amplifiers, separate speaker circuits.
For many things, like spoken word, it's just impractical to do this.
It's also impractical for background music in a gym, or a bar, or a store because very
rarely will the listener be perfectly between the left and the right speakers.
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