Using computer simulations, a group of researchers has determined that it's possible to build objects that will allow sound waves to slip past undisturbed. If the concept is proved experimentally, it could pave the way not just for military applications such as stealthier submarines but also for auditoriums with less reverb and perhaps even smoother flights for airplanes.
Radar stations and bats detect targets such as planes and moths by emitting radio and sound waves that bounce off the objects. The key to the concept of acoustic cloaking is directing the flow of these sound waves so that waves will pass around an object as though it doesn't exist, and thus no waves bounce back to the listening ears. But no one has been able to achieve this property or even know if it is feasible.
But Duke University electrical engineer Steven Cummer and his team think they have identified the acoustical properties needed to design such devices. Some of their colleagues, working with light waves, have revealed some of the secrets of invisibility (Science , 20 October 2006), and now Cummer's group has discovered certain similarities with that research he says have relevance to suppressing the transmission of sound. Reporting online 14 January in Physical Review Letters, the team says its calculations and computer simulations suggest how to achieve a property called sonic anisotropy, which allows sound waves to be channeled into one direction no matter from which direction they originate.
The key is synthesizing special materials, not found in nature, that can create anisotropy. Such materials could be, for example, made of precisely fabricated metallic cylinders several centimeters in diameter, or smaller than the wavelength of most audible sounds and therefore able to avoid reflecting them, Cummer says. Not that finding the necessary materials would be a snap. "You'd need a degree of anisotropy that's more than conventional materials can provide," he explains. "We've only just now shown that this kind of sound shaping is technically feasible," he adds, so the researchers have just begun "thinking in earnest about where it might be useful in practice."
Along those lines, Cummer says the research might be beneficial in areas such as reducing turbulence by allowing designs for aircraft surfaces and ship's hulls that impede the flow of air or water to a much smaller extent than today's technology can achieve. Mechanical engineer Nicholas Fang of the University of Illinois, Urbana-Champaign, says the research "clearly indicates" that achieving anisotropy is critical to building an acoustic cloaking shell. Although that technology remains challenging, he says, "I am glad to see we now have more tricks to play with sound."