Duke University has built the first 3D omnidirectional acoustic invisibility cloak. The cloaking device, which takes the form of a pyramid of perforated plastic, is hidden from sound waves striking it from any direction — if you fired some sound waves at the device, using sonar for example, you would see nothing; not the cloak, and not the object beneath it. This has obvious applications in the military, but also in architectural acoustics.
Here at ExtremeTech we’ve written a lot about metamaterials — specially fabricated materials that have properties never found in nature — and how they allow for the creation of some seriously outlandish devices, such as invisibility cloaks and superlenses. Metamaterials aren’t necessarily restricted to bending light waves in odd ways, though; they can also be used to manipulate acoustic waves, too.
Case in point: Duke University’s 3D acoustic cloak, which consists of layers of perforated plastic arranged in a pyramidal shape. The exact geometry of the cloak, plus the placement of the holes, very precisely control the propagation and reflection of sound waves. The idea is that, when the cloak is hit by some acoustic waves, it alters the trajectory of the waves so that it’s as if the cloak and the cloaked object were never there.
If you imagine the Duke cloak sitting on the ground, and sound waves moving straight down towards it, they would be reflected by the cloak in such a way that it appears that they’ve bounced off the floor. As you can see in the diagram below, the cloaking is not quite perfect (compare Ground to Cloak), but it’s pretty good for an experimental prototype.
