A simple plastic shell has cloaked a three-dimensional object from sound waves for the first time. With some improvements, a similar cloak could eventually be used to reduce noise pollution and to allow ships and submarines to evade enemy detection. The experiments appear March 20 in Physical Review Letters.
“This paper implements a simplified version of invisibility using well-designed but relatively simple materials,” says Steven Cummer, an electrical engineer at Duke University, who was not involved in the study. Cummer proposed the concept of a sound cloak in 2007.
Scientists’ recent efforts to render objects invisible to the eye are based on the fact that our perception of the world depends on the scattering of waves. We can see objects because waves of light strike them and scatter. Similarly, the Navy can detect faraway submarines because they scatter sound waves (sonar) that hit them.
So for the last several years scientists have been developing cloaks that prevent scattering by steering light or sound waves around an object. The drawback of this approach, however, is that it requires complex synthetic materials that are difficult to produce.
José Sánchez-Dehesa, an electrical engineer at the Polytechnic Institute of Valencia in Spain, and his colleagues pursued a different method: Instead of preventing sound waves from hitting an object — in this case an 8-centimeter plastic sphere — they built a cloak to eliminate the scattered waves left in the sphere’s wake.
Using computer algorithms, the researchers came up with a design made up of 60 rings of various sizes that form a cagelike structure around the sphere. Simulations indicated that sound waves scattering off the sphere and the ringed cloak would interfere with each other and cancel out. (Noise-cancelling headphones exploit this phenomenon by emitting sound waves that minimize ambient sounds in a room.)
Because the cloak did not need to steer sound waves in complicated ways, Sánchez-Dehesa and his team built it out of plastic with the help of a 3-D printer. They hung their creation from the ceiling of an echo-free chamber, pointed a speaker at it and played a range of sound frequencies. For most frequencies, the sphere scattered an easily detectable amount of sound. But at 8.55 kilohertz — an audible high pitch — the cloaked sphere became imperceptible to the sensors behind it.
The study marks the first time scientists have ever cloaked a three-dimensional object from sound. That’s probably music to the ears of the U.S. Office of Naval Research, which partially funded the study to explore the possibility of sonar invisibility.
However, this cloak is just a small step toward stealth submarines. It has to be custom designed and built for each object, and it works only for a narrow frequency range coming from one direction. If the speaker had been set up anywhere else, the cloak would not have worked. Sánchez-Dehesa’s team plans to develop broadband and multidirectional cloaks.
But Cummer points out that even a limited cloak can have useful applications. He suggests that structures capable of manipulating a specific sound frequency from one direction could help minimize noise pollution from a congested highway. “The cloak does one thing quite well, with a very simple structure,” he says.