I'd like to show you an example of
levitating small objects with sound
waves let me just remind you how a
loudspeaker works it's just a diaphragm
that pushes repeatedly pushes on the air
producing high pressure waves that move
away and that's what we have here with
this little gizmo it does the same thing
except it's at a very high frequency
twenty-eight thousand Hertz which is way
above our human hearing range so these
sound waves move upward and up above I
have a glass plate which is going to
reflect the sound back down again
and when the distance between the glass
plate and the loudspeaker is just right
then those sound waves that are moving
down will interfere constructively with
the sound waves coming up and give us
regions of high pressure and low
pressure which don't move around they
stay in one spot and it's in those high
pressure zones that I'm going to try to
levitate a small ball so let me turn on
the power and even though I can't hear
it I'm still going to use ear protection
as I said before the distance between
the reflector and the loudspeaker is
just right if I change it and which I'm
going to do you'll see that this
standing wave disappears and the Falls
are no longer suspended there we've now
set up the little loudspeaker and the
reflector in front of this mirror which
is part of our clear knob take setup and
purpose of that is to be able to
actually see where there are regions of
high pressure and low pressure in the
air especially the air between the
reflector and the loudspeaker so let me
turn up the volume and before I do that
I will put on my protective hearing
and we if I adjust the height of the
reflector there's a point at which you
see bands of white light and dark this
is the standing wave and it only happens
when the distance between the reflector
and the speaker is some multiple of a
half a wavelength so every single one of
those bands is half a wavelength if I go
up a little higher
then it disappears again but if I
continue to go higher then it will come
back when I let another half a
wavelength multiple of a half a
wavelength away now it's in these bands
that I'm going to place the little balls
and you can see the little ball settles
down where there's a bright band and
that is the high-pressure area and I
know it's a high pressure because you
see there's a white band right next to
the reflector which is where the highest
pressure will be if I again change the
height the standing wave goes away and
the little balls will fall down