First You Don't See It, Now You Don't Hear It

January 23, 2008

Specially fabricated materials can do for sound waves what they already do for electromagnetic radiation—achieve invisibility. In principle these so-called meta-materials bend sound waves in such a way that objects become acoustically invisible, say mathematicians and scientists from the University of Liverpool and the Indian Institute of Technology in Kanpur.

A meta-material gains its properties from its structure rather than from its composition. Using mathematical modeling, the researchers showed that a flat lens made out of a meta-material has unusual properties and can behave like a silencer.

Liverpool mathematician Sebastien Guenneau explained: “We found that at a particular wave frequency the meta-material has a negative refraction effect, which means that the image produced in the flat lens appears at a high resolution in exactly the same way it appears in reality." In other words, the image is erect rather than upside down, as it would be with a conventional convex lens.

The surprise, Guenneau said, was that "at the point where negative refraction occurs, the meta-material becomes invisible.” Used in sonogram technology, the lens could make the image appear in mid-air like a hologram rather than on a computer screen.

“We also found that if we arranged the meta-material in a checkerboard fashion, sound became trapped, making noisy machines, for example, quieter,” Guenneau said.

Greater control of sound using meta-materials could lead to improved medical scans and pinpoint locating and drilling of oil at high sound frequencies.

The research appeared in the January 2008 issue of the New Journal of Physics in the article “Acoustic metamaterials for sound focusing and confinemen.”

Source: University of Liverpool, Jan. 7, 2008.