BATS and submarines have one thing in common—each uses a form of sonar to locate objects in their path. In its active form, sonar—an acronym of “sound navigation and ranging”—works by firing pulses of sound through air or water, and evaluating the echo that comes back (passive sonar simply listens to sounds given off by other things). If the echo suggests something hawk or rock-shaped, bats and submarines take evasive action. But what if you could render objects invisible to sound waves by making it appear that nothing is in their path?
A team at Duke University has come up with an “acoustic cloak” that does just that. In an article in Nature Materials, Steven Cummer, a professor of electrical and computer engineering, and two Duke colleagues describe a pyramid-shaped device that renders anything underneath it—along with the device itself—invisible to sound. Most impressively, it works regardless of the direction the sound waves are coming from, rerouting the waves to create the impression that nothing is there except the flat surface on which it is placed.
At first glance, the cloaking device looks like nothing more than a layered acrylic pyramid with a repeating pattern of holes drilled in it. Underneath the outer pyramid sits another, inner pyramid, this time without holes, under which you place whatever you want to hide. In reality, the device depends on a complex tangle of mathematical theory, a vast amount of computing power and the use of “metamaterials”. These exotic composites combine two or more distinct materials that, as a result of the precise way in which they are structured, display unusual properties—such as the ability to influence how waves of various kinds pass through them.
Professor Cummer needed a metamaterial that would manipulate sound waves. To take the simplest example, consider a sound wave hitting the device directly from above. It travels through the perforated outer layer of the pyramid, bounces off the unperforated inner pyramid, then passes back through the outer layer. In this instance, the “round-trip” travelled by the wave is roughly half the distance it would have covered had the cloaking device not been on the surface. The device must therefore slow the wave by a factor of two, so that the time it takes the same time to re-emerge as it would if it had simply hit a flat surface. Sound waves coming in at other trajectories must be slowed by different amounts.
The metamaterial that the team came up with is a composite of acrylic and air, structured in a very specific way. Its “unit cell”—or basic building block— is a 5mm cube of air, in the middle of which is a 5mm-square acrylic plate that is 1.6mm thick, with a 0.85mm hole drilled in its centre. The entire outer layer of the device consists of hundreds of these cells (see photo)—which means, intriguingly, that its true “outer edge” is actually a thin layer of air. As Professor Cummer puts it, “things get a bit murky at the edges” when you try to describe the device’s physical structure.
The cloaking device was tested by being “pinged” with short bursts of sound from various angles. A microphone was used to map how the waves responded, and then compared with the response from pinging an unobstructed flat surface. At frequencies between one and six kilohertz, the device made it appear as if the sound waves were reflecting off athat flat empty surface.
The Duke team is now looking at potential applications for its technology. An obvious use is sonar avoidance, which may be why the research was sponsored in part by America's Office of Naval Research. Professor Cummer also sees potential in the field of sound dampening or diffusion: “You can engineer something that has a flat surface but reflects sound like a bumpy surface,” he notes. Because the technology can make sound bounce off an object in ways that do not reflect its shape, it could also be used in concert halls to make structures such as support beams acoustically invisible. And if that doesn’t sound like much, Professor Cummer will be more than happy.
