MAKING SOUND VISIBLE
A "Schlieren Camera" can photograph the warm air rising above any hot object. This type of camera is commonly used by researchers to photograph shock waves in hypersonic wind tunnels. But think: a shock wave is a kind of sound wave. Perhaps a Schlieren system can photograph other sorts of sound waves? But sound waves MOVE, they move at around one foot per millisecond, so in order to capture them, we'd have to set up some sort of high speed flash photography. Maybe not: if we could flash the light source in synch with the sound waves, then the sound waves should become "stroboscopically frozen", and if these density-waves in the air are dense enough, the shadows of the waves should be visible on the Schlieren Camera screen!
A superbright LED would be a good pointsource illuminator if it was much
brighter than normal. However, Charles Yost has discovered that LEDs work
fine in this system if the frosted screen in the above diagram is replaced
with a television camera with the camera's lens removed. The camera is
positioned close to the razor blade, so that the image of the "test
object" is very small yet partially covers the CCD sensor within the
camera. Simply view the result on a television screen.
Other possibilities: this article
mentions two other simpler
"Schlieren" setups. In one, a bright pointsource is placed on a camera
lens, facing outwards, and the camera is aimed at a distant
retroreflective screen. The light from the LED normally bounces back and
hits the LED, so
the camera sees a dim field. Any density patterns will bend the light so
comes from the wrong place, then returns to shine in the lens. A second
technique: a large wall is painted with a fine vertical grating, a camera
is aimed at this wall, and an opaque photo of the grating is inserted in
camera at the location of the grating's real image. If the test object is
at a different distance, the camera can focus on this object and blur the
grating. Yet any deflections in the light will make more/less light hit
the opaque grating, causing brightness changes.
I have not yet tried the above idea. Will it work? Maybe not, because
sound waves are typically 100,000 times less intense than 1 ATM of
pressure. Sound waves are density waves, but the changes in
density might be too small to photograph. (Photos of bursting
balloons apparently work OK though.)
To guarantee that the variation in pressure is maximized, we
must use VERY LOUD sound. Also, we must use sound of fairly high
frequency, since low-freq waves will be too big to fit into the viewing
field. Sound moves at about 1100 feet per second, therefore a one-inch
sound wave would be 13,000 cycles per second. A low frequency could be
used if the sound consisted of brief pulses. A 100 microsecond pulse
would give a sound wave that's a little under an inch thick. If
the same 100uS pulse was applied to the LED as a 1A current, the
viewing field should be quite bright. We should pulse the LED in
synch with the camera frame-rate in order to avoid visible "beats"
caused by the video scan.
Another idea: use a resonant chamber. That way the sound could be
extremely intense, yet the required wattage would be fairly small. Build
a clear plexiglas chamber and drive it at high frequency with a signal
generator, an audio amp, and a small piezo-tweeter bought from Radio
Shack. Or even drive it with narrow, high-power pulses, so the
loudspeaker reinforces a sheet of high pressure rather than a long wide
Around 1982 I tried to build a real-time version of the above system. I
attached a row of microphones to an old vinyl record album, with each
microphone feeding an op-amp chip which drove an adjacent LED. When the
whole assembly was spun at about 500 RPM, a glowing band of light
appeared. The sound from a tiny loudspeaker would create a bright region
in the band of light. When I played a constant tone into TWO tiny
loudspeakers (headphones, actually), the dark bands of an interference
pattern appeared in the band of light from the spinning LEDs.
A few years ago the Exploratorium
science museum built a large version of a "Winston Kock Scanner".