BOYD F. EDWARDS: OK,
this is a demonstration of interference patterns
produced by two speakers. This signal generator
is generating a tone that's at about 2,000
cycles per second, 2000 hertz, and it's sending the same signal
to both of these speakers. And if you're out in front
of those two speakers, then what matters is
the distance from you to one speaker compared
with the distance from you to the other speaker. If those two distances
are the same, then the sounds
that are produced by these two speakers overlap
and interfere constructively. They add together to
form a louder sound. But as soon as you've moved
a little bit in such a way that the difference between
the two paths between one speaker to you and the
other speaker to you is half a wavelength,
then instead of the peaks and the valleys overlapping
with each other, you get a peak where there's
a valley in the sound.

and the sound cancels. That's called
destructive interference. And so what we're
going to do next is to use this microphone
attached to this meter and move through
the sound field, and you'll be able to
see, with your eyes on the meter, the
level of the sound as well as hear the sound picked
up by this microphone that shows this constructive and
destructive interference pattern. [METER BEEPING] OK, right now, we
have this microphone that's picking up the sound
from those two speakers. And we're symmetrically located
in the center, between the two speakers. So what we'll do now is to
move away from the center. And this readout here
gives the loudness of the sound that's picked
up by this microphone. And as we move through the
interference pattern produced by these two speakers, you'll
be able to both hear the sound from this microphone, which
will be louder, then softer.

then louder again
as we move through constructive interference
to destructive interference. And then you'll also be able
to see that the readout will show that loud-soft pattern. So I'm going to be quiet now so
that the microphone will just pick up the sound
from the speakers. [BEEPING CONTINUOUSLY] OK. We went through about three
or four different minima and maxima, and
hopefully you were able to hear that the sound
is changing in loudness.

Interference Demo: Speakers

Interference Demo: Speakers