Pyramid Shaped ’Invisibility’ Device May Hide Things from Sonar
Source: livescience.com
Cloaking devices, a staple of science fiction (think Harry Potter), are getting closer to reality. Researchers at Duke University have built a structure that would hide anything under it from sonar — at least in air. Made of sheets of perforated plastic, the pyramid-shaped cloak changes the shape and speed of sound waves as they hit it. Those changes make the sound waves appear to reflect off the surface the pyramid is standing on, as though it wasn’t there.
To build the cloak, Lucian Zigoneanu, Bogdan-Ioan Popa and Steven Cummer modeled the way sound waves act on a computer. They tried several simulated shapes, and eventually came up with the pyramid design, made with sheets that have holes in them.
Holes are key
To test their idea, they put a sphere inside the pyramid, and then placed both in a largely empty room with a sound generator and a microphone. The sound generator made a "ping" that would bounce off the pyramid. A single microphone on a gantry-like apparatus recorded the sound from hundreds of different positions.
The holes had to be exactly the right size, adjusted according to the wavelength of the sound hitting them. Without the holes the sound waves would just bounce off and reveal the presence of the pyramid. With them, some sound waves were slowed down. The slower waves followed a longer path back to a detector, just as they would if the pyramid wasn’t there. The shape of the reflected wave looks just as it would if it hit a flat surface, fooling any sonar into thinking the cloak and anything under it doesn’t exist.
When sonar equipment sends a "ping," or bats or dolphins use echolocation, they time the sound to see how long before it is reflected back. Simply absorbing the sound wouldn’t work for the same reason absorbing all light wavelengths wouldn’t — instead of an invisible object you’d have a black shape.
"In [the] cloaking problem you can’t have the sound reflect in a different direction, and you can’t just absorb, because it casts a shadow," Cummer told Live Science.
That fact makes cloaks tricky to design: The waves of sound or light must come back to the detection device — be it sonar microphones, human eyeballs or radar — with the same shape and frequency they’d have with no object in the way.
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Read the full article at: livescience.com
