Invisible to the Ears


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Bogdan Popa shows off the 3D acoustic cloak he helped design and build as a member of Steven Cummer’s laboratory. Image: Duke.edu

The Invisible Man, Harry Potter, The Martian Manhunter, The Voice: All these characters have some way to make themselves disappear. But that doesn’t mean that they go unnoticed when they go unseen. Footsteps, falls, bumped objects, and their very breath can give them away. To make themselves truly undetectable they’d have to find a way to hide the sounds they make as well as the light they reflect.

Now researchers at Duke University have discovered a way to mask sound, for once giving us in the real world a power not found in fiction. Their acoustic invisibility cloak is a pyramid of perforated acrylic and looks something like the headgear found on Devo, circa 1980. Put an object inside it, and you’ll have no idea what it sounds like. The pyramid reflects whatever sound is sent its way, from any direction, as if it weren’t there. That is, if you say hello to it, your echo will sound as if the pyramid and what it hides are merely a flat surface.

Not unlike the invisibility cloaks of the optical world that have appeared as of late, the sound cloak relies on metamaterials, where repeating patterns allow waves of various forms to behave unnaturally.

“All of the invisibility work starts at the same theoretical point,” says Steven Cummer, a professor of electrical and computer engineering at the university, and the lead researcher for the project. “It’s a concept of coordinate transformations, where you take a particular wave and stretch it or twist it, open holes or bend it in a way you can describe mathematically. Then you can write down a set of idealized material parameters that will effect how the wave propagates.”

Schematic representation of the square pyramid cloak and its orientation. Image: Duke.edu

The cloak disrupts the two properties that effect how waves travel through a material: mass density and bulk modulus - or the stiffness of a material when subjected to compression. “Those material properties need to be different in different directions to implement these bends and twists and opening holes,” says Cummer.

The pyramid is really a layer cake of alternating acrylic and air. When sound is travelling parallel to the acrylic sheets, it passes through air undisturbed. But when it’s coming at the pyramid, hitting the point first, “the flow has to go through just a small fraction of the available space, slowing the sound wave in that direction.”

When Cummer and colleagues sent a ping at the structure, the sound waves bounced back as if there were no structure at all. “We can fool bats and anyone who does echo location in air,” says Cummer.

A similar cloak could also fool anyone using sonar. But underwater metamaterials would have to be a bit more advanced than a few pieces of acrylic with some strategically drilled holes. The difference in solidity between air and a solid is so great that the material used on land to make a sound cloak hardly matters. It could just have easily been metal. In water, things are different. “Metal and plastic are only a little bit denser than water, a ratio of 2 to 5 instead of a factor of 1000.” says Cummer. Sonar deceivers would have to be made of an extraordinarily dense material.

The acoustic cloak is likely to find a more immediate application in the concert hall. “People spend a lot of effort designing the acoustic response of concert halls and studios,” says Cummer. “How sound waves reflect off of surfaces in is an important part of that design.” The cloak could hide architectural elements that reflect sounds in an undesirable way. It could also add acoustic texture to otherwise flat-sounding fixtures. “We’ve shown that we can take a bumpy surface and reflect it like it was a perfectly flat surface,” says Cummer. “You can also do the inverse of that. You can engineer a flat surface that reflects like a bumpy surface.”

Though it took some serious number crunching and engineering to design the cloak, actually making one is a rather simple affair. “There can definitely be variability in how precisely the layers are separated, and the placement of holes­, as long as the average over a few building blocks is homogonous. So it’s very tolerant of manufacturing errors.” Interested amateurs could 3D print one in an inaudible heartbeat.

And those disappearing superheroes may finally extend their invisibility to that other sense.

Michael Abrams is an independent writer.

We can fool bats and anyone who does echo location in air.

Prof. Steven Cummer,
Duke University

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April 2014

by Michael Abrams, ASME.org