Technology for an invisibility cloak hasn’t yet been perfected, but researchers at Duke University have just invented what they’re calling the world’s first “3-D Acoustic Cloaking Device.” When this “cloak” is placed over an object, it reroutes sound waves as if there were no object at all. This could have implications for everything from military design, where it could be used to dodge enemy sonar, to sound design, where it could improve acoustics in concert halls.
“The particular trick we’re performing is hiding an object from sound waves,” Steven Cummer, professor of electrical and computer engineering at Duke University, says in a statement. “By placing this cloak around an object, the sound waves behave like there is nothing more than a flat surface in their path.”
The cloak is constructed from sheets of 3-D printed plastic punctured with repeating patterns of holes and then stacked in a pyramid-like shape. When placed over an object, the structure and the pattern of the holes alter the trajectory of sound waves, making them behave as if they’re unaffected by the presence of the cloak or the object underneath.
To test the design, researchers placed the cloak over a small sphere, which they then blasted with bursts of sound. Using a microphone, they mapped how the sound waves responded and also made videos of those waves as they traveled through the air. Instead being altered by the presence of the sphere and the cloak, the sound waves behaved exactly as they would when pinged across an empty flat surface.
The cloak may look simple, but Cummer says, “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.”
So what can this cloak be used for? “We conducted our tests in the air, but sound waves behave similarly underwater, so one obvious potential use is sonar avoidance,” Cummer says. Research on the cloak was paid for by the U.S. military, and using the cloak to fool enemy sonar could potentially become a staple in futuristic warfare. “But there’s also the design of auditoriums or concert halls–any space where you need to control the acoustics. If you had to put a beam somewhere for structural reasons that was going to mess up the sound, perhaps you could fix the acoustics by cloaking it.”
The design is a breakthrough in the emerging field of metamaterials: artificial materials engineered in ways that give them properties not found in nature–such as the ability to refract light to create apparent invisibility or to reroute soundwaves. The researchers claim the effect works from all directions, no matter where a sound is coming from, unlike previous designs like this Silence Cloak, which only worked from two dimensions.
The results of their research, titled “Three-dimensional broadband omnidirectional acoustic ground cloak,” were published in Nature Materials, March 9, 2014.