14 DAY TRIAL // A group of investigators from the Duke University in Durham, North Carolina, announce the creation of the world's first three-dimensional acoustic cloak. While invisibility cloaks have been under investigation for many years, acoustic cloaks have received slightly less attention. Now, using nothing more than math and perforated sheets of plastic, Duke experts finally achieve this objective.
The operating mechanism the cloak is based on is fairly simple – its plastic sheets are arranged in such a pattern that any object located underneath the device is made to feature no acoustic signature. Even more interestingly, the cloak itself features no such signature. In effect, it hides both itself and target objects from acoustic detection efforts.
Duke University researchers say that their innovation works in 3D, meaning that the same effect applies regardless of where the sound waves are coming from. In addition to tremendous applications in architectural acoustics, the new asset could also be employed in submarine countermeasures, such as sonar avoidance.
The investigation was led by Duke professor of electrical and computer engineering Steven Cummer and graduate student in electrical and computer engineering Bogdan Popa. Details of the innovation were published in a recent online issue of the top scientific journal Nature Materials.
“The particular trick we’re performing is hiding an object from sound waves. By placing this cloak around an object, the sound waves behave like there is nothing more than a flat surface in their path,” the team leader explains. He says that newly-developed metamaterials were essential for this study.
Metamaterials are materials that do not exist naturally, but are rather synthesized piece by piece to answer a particular demand. Usually, experts assemble these pieces into repeating patterns that allow the end product to exhibit properties invisible in nature. In the particular case of the 3D acoustic cloak, only air and plastic are used, but based on extensive mathematical calculations.
The structures created at Duke resemble some type of pyramid, made up of layered sheets of plastic that feature several holes poked through them. How wide these holes are, and how far apart they are spaced from each other, is a function of complex calculations, the group explains.
“The structure that we built might look really simple. 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 explains.
“We conducted our tests in the air, but sound waves behave similarly underwater, so one obvious potential use is sonar avoidance. 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,” he concludes.