Washington, January 10 : Duke University engineers say that they have a theory on how to develop a three-dimensional sound cloak, which may hide submarines in the ocean from detection by sonar, and even improve the acoustics of a concert hall by effectively flattening a structural beam.
"We've devised a recipe for an acoustic material that would essentially open up a hole in space and make something inside that hole disappear from sound waves," said Steven Cummer, Jeffrey N. Vinik Associate Professor of Electrical and Computer Engineering at Duke's Pratt School of Engineering.
The researchers, however, conceded that the properties required for a sound cloak are not found among materials in nature, and that they would have to develop artificial, composite metamaterials for creating it.
In a previous research, the Duke team has already shown that an "invisibility cloak" can allow microwaves or light to travel seamlessly around it, and emerge on the other side without distortion.
They reported in The New Journal of Physics a theory showing that an acoustic cloak could be built, but their theory relied on a "special equivalence" between electromagnetic waves (microwaves or light) and sound waves, which is only true in two dimensions.
Another research team also suggested that a 3-D acoustic cloak couldn't exist.
Cummer, however, was not convinced with those propositions.
"In my mind, waves are waves. It was hard for me to imagine that something you could do with electromagnetic waves would be completely undoable for sound waves," he said.
Cummer attempted to derive the mathematical specifications required to prevent a shell from reflecting sound waves, a key characteristic for achieving invisibility.
He said that his mathematical specifications appeared to work, at least on paper.
"We've now shown that both 2-D and 3-D acoustic cloaks theoretically do exist," Cummer said.
"It (the theory) opens up the door to make the physical shape of an object different from its acoustic shape," he said.
Cummer said that the existence of an acoustic cloaking solution also indicates that cloaks might possibly be built for other wave systems, including seismic waves that travel through the earth and the waves at the surface of the ocean.