Controlling the spread of sound and how it bounces off objects is not easy. But by using a few perforated sheets of plastic and a complex algorithm, researchers at Duke University have developed the world's first 3D acoustic cloaking device.
The device escapes acoustic detection by rerouting sound waves in such a way that both the cloak and anything beneath it appear invisible. In future, a scaled-up version of the cloak could be affixed to ships, submarines, or aircraft to help these large objects avoid sonar detection. And in fact, the study was supported by grants from the Office of Naval Research and from the Army Research Office. Alternatively, it could also be used by architects to reduce acoustic effects in buildings or other structures like auditoriums or concert halls.
"The particular trick we're performing is hiding an object from sound waves," explained Steven Cummer 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." Cummer is an electrical and computer engineer at Duke University. His paper now appears at Nature Materials.
Unlike previous acoustic cloaking devices, this one works in all three dimensions. It doesn't matter which direction the sound is coming from or where the observer is located. To achieve this, metamaterials were arranged in repeating patterns to achieve specific properties. In this case, plastic materials were able to manipulate the behavior of sound waves in air. The device itself looks fairly strange — a pyramidal structure consisting of several plates, each with a repeating pattern of holes poked through them.
Computers were used to calculate the exact configuration required to give the illusion that the object isn't there. The trajectory of soundwaves are made to match what they would have looked like had they reflected off a flat surface. The sound doesn't reach the surface beneath, so it's traveling a shorter distance; the speed must be slowed accordingly.
The researchers were able to test the device by having it cover a small sphere and then "pinging" it with short bursts of sound from various angles. After mapping the resulting waves, and comparing it to a non-cloaked sphere, it was clear that the device was making it appear as though the sound waves were bouncing off an empty surface.
The challenge now will be to refine this small-scale model in such a way that it can be made practical for large scale applications.
Read the entire study at Nature Materials: "Three-dimensional broadband omnidirectional acoustic ground cloak."