'Cloak' could make objects invisible to sound waves: study

Research team tests formula, has yet to try it out for real

Last Updated: Friday, January 11, 2008 | 10:37 AM ET

CBC News

A device that prevents sound waves from reflecting off objects is a scientific possibility, a group of researchers has announced.

"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," Steven Cummer, a professor of engineering at Duke University, said in a news release on Thursday.

Cummer and a team of researchers wanted to disprove a previous theory that suggested a three-dimensional sound cloak, which would surround an object and allow sound waves to travel around it without distortion, was a scientific impossibility.

Researchers from Duke successfully created an engineered material that rendered objects imperceptible to microwaves in 2006. By placing an electromagnetic cloak over a copper cylinder, they were able to cause microwaves to pass around the object without obstruction.

After the microwave experiments, a separate group of scientists claimed the technology would not work with sound waves. Cummer was not convinced.

"In my mind, waves are waves," he said. "It was hard for me to imagine that something you could do with electromagnetic waves would be completely undoable for sound waves."

His team started with a cloak like the one used in the microwave experiment and figured out the mathematical formula that would prevent it from reflecting sound waves.

There hasn't been a practical test yet, and Cummer hasn't said when there might be one.

According to the team's theory, a large enough cloak could even render submarines undetectable to sonar.

The same technology could also be used to dramatically improve the acoustics in a concert hall, by flattening structural beams, for example, and allowing undistorted sound to reach the audience in spite of architectural obstacles, he said.

The team's report is scheduled to appear in Physical Review Letters on Friday.