Acoustic Cloak Designed

Objects coated in a new material would be "hidden" from noise.

City dwellers, rest easy. Engineers have designed a material that redirects sounds and could be used in buildings to shield them from noises. The sound-shielding material, which, if actually made, would be the first acoustic cloaking device, could also be useful in hiding military ships and other vessels from sonar.

Sound shield: An acoustic cloak comprising alternating layers of sound-scattering materials should make objects invisible to sonar--and insulated from sound. In this computer-generated image, a cylinder (green circle) is coated with 200 layers of such a material, which was found to be the optimal design. Sound waves moving from left to right (their peaks and troughs are represented by red and blue lines) flow past the object and reform on the other side with no distortion. Credit: New Journal of Physics

Acoustic cloaking materials, which direct sound waves around an object so that they re-form on the other side with no distortion, do not exist in nature. But engineers led by José Sánchez-Dehesa at the Polytechnic University of Valencia, in Spain, have created a plan for making them, using alternating layers of two different materials. These materials would comprise arrays of sonic crystals--patterns of small rods made of aluminum or other materials that allow some sound waves to pass while blocking the passage of others.

The design of the cloaking materials, published in the New Journal of Physics, shows that making an acoustic shield "can be done in a straightforward and simple way," says Steven Cummer, an electrical engineer at Duke University who was involved in the construction of the first light cloak in 2006.

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Building on the theoretical work of John Pendry at Imperial College in London, a group at Duke University led by David R. Smith and including Cummer created a shield that makes objects invisible to a particular frequency of microwave light. They used metamaterials, artificially structured composites designed to have properties unmatched by natural materials. For about 10 years, engineers have been designing metamaterials to manipulate light in the hope of creating new display technologies, microscope lenses, and computer chips dense with transistors. The new acoustic-cloak recipe builds on Cummer's recent theoretical work on acoustic materials, and it shows that metamaterials can be used to manipulate sound waves as well as light waves. Cummer, who was not involved in Sánchez-Dehesa's work, says that it should now be possible to fabricate an acoustic cloak.

In order for a material to work as an acoustic cloak, the speed of sound passing through it must be direction dependent. That is, sound waves traveling through the shielding material from one direction must move at a different speed than waves traveling in a perpendicular direction. These differences create scattering effects that should direct sound waves to flow over a shielded object like water flowing around a rock. Because the waves return to their original conformation after passing such a shielded object, the object effectively becomes invisible to sonar. And a listener inside such a shield wouldn't hear the sounds flowing around.