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.”