MAKING SOUND VISIBLE
Schlieren Photography
W. Beaty         Dec 1999

In a "Schlieren" optical system, we can make small variations in the density of air become visible by using a point-source illuminator, a razor blade, and large lens (or a large telescope or searchlight mirror.) The "Schlieren" setup is also called "the Foucault Mirror Test" because it can be used to test the curvature of homemade telescope mirrors. The razor blade is adjusted so that it partially blocks the light... the shadow of the "test object" is cast upon the viewing screen... and if some heated air near the "test object" should slightly deflect the light, then the light will either miss the razor blade completely, or it will be blocked. As a result, any regions of the air which deflect the light will cause dark and light patterns to appear on the screen. An image of the moving air will appear on the screen! A warm hand will cast a hand-shaped shadow, but it will be surrounded by a plume of rising warm air which resembles flames.

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 5-page article [pages: 2, 3, 4, 5 ] mentions two other simpler "Schlieren" setups, the first giving a huge field of view. In one, a bright pointsource is placed on a camera lens, facing outwards, and the camera is aimed at a distant retroreflective screen (reflective safety cloth, find on Amazon.) The light from the LED normally bounces back and hits the LED, so the camera sees a dim field of LED light. Any density patterns will bend the light so it comes from the wrong place, then returns to shine in the lens, making bright and dark patterns. A second technique: a large wall is painted with a fine vertical grating, a still-camera is aimed at this wall, and an opaque photo of the grating is inserted in the 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 ideas. 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 standing-waves, yet the required wattage would be fairly small. Build a sealed aquarium or a clear plexiglas chamber and drive it at high frequency with a signal generator, an audio amp, and a small 30-watt piezo-tweeter bought from Radio Shack. Or even drive it with narrow, high-power pulses, so the loudspeaker reinforces a thin sheet of high pressure rather than a long wide sine-wave. Pulse-mode standing waves are weird.

Dr. Winston Kock's "VISIBLE SOUND"

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.

In the late 1980s (or early 90s?) the Exploratorium science museum built a large version of a "Winston Kock Scanner". This was built by one of their Artist in Residence people, MIT student Thomas Zimmerman (who also was the inventor of the VR data-glove.)

NEW ADDITION 2024: Apparently we can use our phones to create wide-area Schlieren videos, based upon a paper printout of find vertical lines. It's the Moire-patterns version, where the phones pixels act as one Moire screen, the paper printout as the other. Atmospheric lensing then distorts the smooth gray pattern of a single wide Moire-pattern stripe.

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