Sound waves moving from left to right flow past the object and reform on the other side with no distortion.
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.
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.
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.
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. According to Einstein’s theory of special relativity, light shields can only work at one wavelength. “As a wave moves around a [cloaking] material, it has to go faster than it does through the air,” explains Cummer. According to the laws of physics, it’s not possible to do this at more than one frequency at a time. The speed of sound, however, is not a universal constant, so it should be possible to craft broadband acoustic cloaks.
According to Sánchez-Dehesa’s design, the thicknesses of the alternating layers making up the sound shield must be very carefully controlled. Cummer says that this will present an engineering challenge, but not an insurmountable one. Indeed, says Cummer, the design for a sound shield is “giving engineered acoustic materials a big push forward.”