I'm going to show you the sound of a
clap ...
and I don't mean some digital depiction
of a clap. I mean that when this man's
hands come together you're going to see
something that is normally invisible.
You're going to see the actual
sound wave leaving his hands and
traveling outward at 761 miles per hour --
the speed of sound!
And here it is again.
How is this possible?
Well, I'll start the explanation not with
sound but with the heat
from a lighter. There's a puff of butane,
sparks fly,
and the fuel ignites. But that shape
billowing up from the flame isn't smoke --
That's normal air that has been expanded by heat.
We're able to see the density change thanks to a technique called
Schlieren Flow Visualization.
Here's how it works.
You start off with a light, shining through a single slit.
If you reflect that light of a parabolic
mirror all the rays become parallel
and then you can use another parabolic
mirror to refocus the light down to a single focal point
and then in through the lens of a camera
to make a picture.
Now here's the trick -- you place some 
sort of barrier right at the focal point.
Now you add something that will distort
the air -- like a candle.
The candle will block light rays making a
silhouette, and the flame will make light.
But rising heat will change the density of
the air above the lighter
and that will bend the light rays. The
bent light ray won't pass through the focal point.
it will be blocked by the barrier ... and
the picture will be darker.
This technique can be used to see
anything that distorts the air.
The heat from a hair straightener for
example.
Even the heat coming off a human hand.
Epidemiologists used to study 
sneezes and coughs.
Engineers use it to study 
aerodynamic flow.
And sound? Well -- that's just another
change in air density.
A traveling compression wave. So Schlieren Visualization
along with a high speed camera can be
used to see it as well.
Here's a book landing on a table.
The end of a towel being snapped.
A firecracker.
An AK-47.
And of course, a clap.
