- [Narrator] When the surface of a speaker beats
the air, it starts from a resting position,
moves out to increase the air pressure,
then moves back through its resting position
into the speaker cabinet to create low air pressure
before returning to its resting position.
And that whole movement is one full cycle.
Professional studio monitors can complete
tens of thousands of cycles per second.
The precise timing is called the phase.
Because the movement is cyclic,
it can be measured in degrees just as
you might measure the angle of a hand
moving around a clock.
Zero is the resting position,
90 is fully out, 180 is back to
the resting position, 270 is fully into
the speaker cabinet and then 360
is back to the resting position again.
Ideally to maximize amplitude
you will want two speakers in a stereo configuration
to move out and in exactly in sync.
If the speakers are producing the same frequency
at the same amplitude, for example
if they're producing a mono signal shared equally
between both speakers, then they can cancel out
each other's air pressure waves
if they have out of sync phase.
You may find there's a spot where
the timing is just right to create silence.
Move to a different position in relation
to the speakers and you'll hear the sound again.
You can create this effect by reversing
the signal wires into one of your speakers
and playing a mono sound through them both.
This principle is used
to create noise canceling headphones.
The headphones have a built in microphone
that listens to the ambient sound
and at the simplest level the headphone plays
the same sound but with inverted phase.
If the sound coming in is 10 db
and you produce a sound exactly with reversed phase
that's also 10 db at exactly the same frequency,
one cancels out the other exactly.
Ambient sound is complex and can change
pretty quickly, plus there are limitations
in the microphones used.
So the system isn't 100% effective.
Microphones also have a phase.
They are kind of like a speaker in reverse.
As long as microphones capture independent sound sources
there's usually no problem.
However, if you record audio on two mics
where the subjects are sitting close together,
sometimes the voice of one speaker
will be loud enough to be captured
by both microphones.
However, in the time it takes for the sound
to travel from the subject to the second microphone,
the precise timing of the air pressure wave
that is the sound signal can be out of sync.
Now just as a high pressure wave hits
their own microphone, the low pressure part
of that wave is hitting the second microphone.
Now we have a problem because both recordings
contain the same voice at the same volume
at the same frequency, but the signals
are out of phase.
When this audio is played through two speakers
or blended together in an audio mix,
the voice might cancel itself out.
The problem is so common, audio editing applications
usually have an easily accessible button
to reverse the timing of the phase in a signal.
You can do some pretty clever things
with the perception of sound using phase adjustments.
But easily the most common adjustment
is simply to reverse the timing in this way.