- [Instructor] Our brains do a lot of calculating for us
before information reaches our consciousness.
For example, you've probably had the experience
of being in a conversation at a party
and suddenly hearing your name spoken from across the room.
Somehow, in amongst the noise of everyone speaking,
you managed to hear just your name.
This isn't actually what happened.
In all likelihood, your brain was listening
to everything in the room all at once
and filtering out noise that didn't seem important.
But someone saying your name is important,
and so that part came through.
The filter was removed on purpose
without your conscious mind knowing anything about it.
This is one of the many so-called autonomic processes
in your body,
like breathing when you're not thinking about it.
But of course, now you are.
When we hear a sound, we quickly build an image or a model
of the source of the sound in our minds.
This is possible partly
because there's so much information included with the sound,
like the location relative to us,
the acoustics of the surroundings,
and of course a great deal of information
about the source itself.
We can all tell the difference
when a happy person or a sad person says hello.
One of the ways we work out the location of a sound source
is by the varying amplitude coming into our ears.
Technically, it's the amount of high and low air pressure,
but let's just stick with amplitude for now.
When a microphone records a sound,
it captures a single channel,
one signal that's like an electronic ear
capturing the amplitude of sound waves that hit it.
That mono signal can give us a lot of information
about the sound source and its context,
but it's impossible to tell the direction it came from
because it's mono.
The only direction information you're going to get
is the location of the single mono speaker
creating the sound,
or I suppose the single headphone that plays the sound.
The easy solution is to capture sound using two microphones.
Each mic captures a separate signal,
which is based on the particular location of the microphone.
If the microphones are facing away from each other,
just as your ears are,
speakers can create a convincing representation
of the sound location based on those signals,
simply using the relative amplitudes.
For example, if you recorded the sound of a car passing by,
you'd hear the car engine
getting louder and louder on one side,
pass across the middle of the speakers
as if it were next to you,
then get quieter and quieter on the other side.
As you probably know already,
the placement of speakers is important
for the accurate recreation of stereo space.
And just for the record,
I'm using the term speaker here
rather than the more popular professional term monitor,
because I've been describing monitors
while talking about visual displays
and I don't want to confuse the two.
The relative amplitudes
coming from correctly-positioned speakers
can tell your brain where a sound was coming from,
in a range of perhaps 150 degrees,
perhaps a little less in front of you.
Remember, most of our perception is relative.
So if a sound gets louder, it seems to get closer too,
though there are other factors
that acoustically shape a sound
to give us telltale indicators
of where a sound is coming from.
Two microphones can capture two signals,
which will do a pretty good job of recreating our experience
of the location of sound sources as if we were present.
If the amplitude from one speaker goes down
and the other speaker goes up,
we'll have the impression
that the sound is moving across the space
between the speakers.
Though there are microphones
that will record stereo in a single device,
it's not really a single microphone.
It's two microphones in a single body,
with separate signals recorded,
listening in different directions,
and separate wires connecting them
to separate recording channels.
So one channel is one separate signal.
To increase the nuance of spatial positioning
when recording and replaying sound,
you can add more microphones.
A 5.1 sound mix is made up of two regular stereo;
two rear stereo
intended to come from speakers positioned behind you;
a center speaker which is intended
primarily to clearly present speech,
which is usually right in front of the camera
and is a mono source;
and then there's a subwoofer,
which adds power to the low frequency sounds.
Below about 80 hertz, it's nearly impossible
to know which direction a sound is coming from,
so it's not as important
where you place the subwoofer in the room.