World's first 3-D acoustic cloak hides objects from sound

US researchers have demonstrated the world's first three-dimensional acoustic cloak which reroutes sound waves to create the impression that the cloak and anything beneath it are not there.
The acoustic cloaking device works in all three dimensions, no matter which direction the sound is coming from or where the observer is located, and holds potential for future applications such as sonar avoidance and architectural acoustics, researchers said.
"The particular trick we're performing is hiding an object from sound waves," said Steven Cummer, professor of electrical and computer engineering at Duke University.
"By placing this cloak around an object, the sound waves behave like there is nothing more than a flat surface in their path," Cummer said.

To achieve this new trick, Cummer and his colleagues turned to the developing field of metamaterials - the combination of natural materials in repeating patterns to achieve unnatural properties.
In the case of the new acoustic cloak, the materials manipulating the behaviour of sound waves are simply plastic and air.

Once constructed, the device looks like several plastic plates with a repeating pattern of holes poked through them stacked on top of one another to form a sort of pyramid.
To give the illusion that it isn't there, the cloak must alter the waves' trajectory to match what they would look like had they had reflected off a flat surface.

Because the sound is not reaching the surface beneath, it is travelling a shorter distance and its speed must be slowed to compensate.
"The structure that we built might look really simple," said Cummer.
"But I promise you that it's a lot more difficult and interesting than it looks. We put a lot of energy into calculating how sound waves would interact with it. We didn't come up with this overnight," Cummer said.
To test the cloaking device, researchers covered a small sphere with the cloak and "pinged" it with short bursts of sound from various angles. Using a microphone, they mapped how the waves responded and produced videos of them travelling through the air.

Cummer and his team then compared the videos to those created with both an unobstructed flat surface and an uncloaked sphere blocking the way.
The results clearly showed that the cloaking device makes it appear as though the sound waves reflected off an empty surface.
The study is published in the journal Nature Materials.