You haven’t heard it all

Experimental cloak makes objects sound like they aren't there

A new cloak that hides objects from sound could really confuse a bat trying to echolocate in a cave. Sound waves striking a bump covered by the cloak bounce back as if they hit a flat wall instead.

A HOLEY SOUND An acoustic cloak made from holey strips of plastic metamaterial bends sound waves, making the object beneath acoustically undetectable. Bogdan-Ioan Popa

SOUNDS FLAT This series of images depicts measurements of how a sound pulse propagates when hitting (1) a flat, reflective surface; (2) an object placed on this surface; and (3) the object covered by an acoustic cloak. Bogdan-Ioan Popa

This sound-bending trick could smooth out the acoustics of concert hall walls. If adapted for water, it might also be useful for hiding submarines from sonar.

“The successful demonstration of a cloak that works for sound waves in air is a triumph,” says Che Ting Chan, a physicist at the Hong Kong University of Science and Technology, who was not involved in the research.

The acoustic cloak is made of a metamaterial, researchers at Duke University report in the June 24 Physical Review Letters. For half a decade, metamaterials have been fashioned into invisibility cloaks that bend light or other kinds of electromagnetic radiation in unnatural ways — hiding tiny objects and inspiring endless references to Harry Potter.

Using metamaterials to make a cloak that can guide sound waves was first proposed by Duke team member Steven Cummer, an electrical engineer, and a colleague in 2007 in the New Journal of Physics.

“Waves are waves,” says Cummer, who also helped to make the first metamaterial cloak, which bent microwaves. “The equations that describe electromagnetic waves are similar to those that describe acoustic waves.”

The first acoustic cloak for sound traveling through water was reported in the January 14 Physical Review Letters by a team at the University of Illinois at Urbana-Champaign led by Nicholas Fang. The engineers placed a flat disk into a shallow pool of water. Intricate channels crisscrossing the disk guided waves of ultrasound around the structure, hiding both the disk and a steel cylinder sitting in a hole at the disk’s center.

To manipulate sound waves in air, Cummer’s team designed and built a cloak that sits atop an object like a piece of draped carpet. By layering simple metamaterial building blocks — ordinary strips of perforated plastic — the researchers hid a triangular wooden block a couple of inches high and more than a foot long at its base.

Sound waves over a range of high but audible frequencies slowed and changed direction cleanly after striking the holey plastic. Most reemerged appearing to have traveled all the way down to the flat surface beneath the block.

The prototype is two-dimensional — both the speaker generating the sound and the microphone recording it must be in the same plane above the object. But Cummer believes he could make a 3-D version that would cover an entire bump on a log, not just a slice.

“We are still a long way from a full acoustic cloak, but this carpet cloak is a step in the right direction,” says Andrew Norris, a physicist at Rutgers University in Piscataway, N.J.

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