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June 18, 2008

Acoustic shield design for sonar invisibility


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

From Technology Review: 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. Engineers led by José Sánchez-Dehesa at the Polytechnic University of Valencia, in Spain, have created a plan for making an acoustic shield, 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. This is follow up work related to metamaterials that are being developed for superlenses and optical invisibility.

Sánchez-Dehesa has modeled a two-dimensional acoustic cloak but says that extrapolating his work to three dimensions should be straightforward. "We're proposing a cloak for any shape," he says. Hiding warships from sonar is one possible application. But Sánchez-Dehesa is interested in the problem of noise generally. "In principle," he says, "it's possible to make this cloak very thin," on the order of centimeters. "If we're able to design a wall to put in a house to screen external noise, it would be very nice." Cummer imagines columns for concert halls that do structural work but, acoustically, are effectively not there.

Unlike light cloaks, which can shield objects from light of only one frequency, acoustic cloaks should be able to shield an object to a broad range of frequencies. The speed of sound, however, is not a universal constant, so it should be possible to craft broadband acoustic cloaks. [Speculation: Similar principles could work for shielding against earthquake waves through the ground]


Acoustic cloaking in two dimensions: a feasible approach

This work proposes an acoustic structure feasible to engineer that accomplishes the requirements of acoustic cloaking design recently introduced by Cummer and Schurig (2007 New J. Phys. 9 45). The structure, which consists of a multilayered composite made of two types of isotropic acoustic metamaterials, exactly matches the conditions for the acoustic cloaking. It is also shown that the isotropic metamaterials needed can be made of sonic crystals containing two types of material cylinders, whose elastic parameters should be properly chosen in order to satisfy (in the homogenization limit) the acoustic properties under request. In contrast to
electromagnetic cloaking, the structure here proposed verifies the acoustic cloaking in a wide range of wavelengths; its performance is guaranteed for any wavelength above a certain cutoff defined by the homogenization limit of the sonic crystal employed in its fabrication.

They present an acoustic cloak that could be physically realizable. In brief, the proposed cloak is based on a multilayered structure consisting of two layers with the same thickness and made up of two different acoustic isotropic metamaterials. These metamaterials are built with sonic crystals (i.e. periodic arrays of sonic scatterers) based on two types of elastic cylinders that have to accomplish certain requirements on their mass density and effective sound speed. Numerical experiments based on multiple scattering method are presented to support the exact performance of the proposed cloak.
The paper is organized as follows. First, in section 2, we review the solution in the previous paper and report our approach to get the acoustic cloaking. Numerical experiments demonstrating the performance and properties of the proposed cloak are also presented and discussed. Section 3 describes the recipe to build the metamaterials needed to fabricate the multilayered cloak making it physically feasible. Finally, the work is summarized in section 4.

FURTHER READING
Jose Sanchez-Dehesa is one of the researchers that developed the new acoustic cloak design

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