'Invisibility Cloaks' Could Break Sound Barriers

Steven Cummer

Contrary to earlier predictions, Duke University engineers report in Physical Review Letters that a three-dimensional sound cloak is possible, at least in theory.

Such an acoustic veil would do for sound what the "invisibility cloak" previously demonstrated by the research team does for microwaves -- allowing sound waves to travel seamlessly around it and emerge on the other side without distortion. Steven Cummer, Jeffrey N. Vinik Associate Professor of Electrical and Computer Engineering at Duke's Pratt School of Engineering said that such a cloak might hide submarines in the ocean from detection by sonar, he said, or improve the acoustics of a concert hall by effectively flattening a structural beam.

As in the case of the microwave cloak, the properties required for a sound cloak are not found among materials in nature and would require the development of artificial, composite metamaterials. (For more about metamaterials, click this link) The engineering of acoustic metamaterials lags behind those that interact with electromagnetic waves (i.e. microwaves or light), but "the same ideas should apply," Cummer said.

In 2006, researchers at Duke and the Imperial College London used a new design theory to create a blueprint for an electromagnetic invisibility cloak. Only a few months later,the team demonstrated the first such cloak, designed to operate at microwave frequencies.

Cummer and David Schurig, a former research associate at Duke who is now at North Carolina State University, later reported in "The New Journal of Physics" a theory showing that an acoustic cloak could be built. But that theory relied on a "special equivalence" between electromagnetic and sound waves that is only true in two dimensions. A report by another team had also suggested that a 3-D acoustic cloak couldn't exist. It appeared they had reached a dead end.

This time, he started instead from a shell like the microwave cloak his team had already devised and attempted to derive the mathematical specifications required to prevent such a shell from reflecting sound waves, a key characteristic for achieving invisibility. On paper, at least, it worked.

Image Description: Analytical computation of the interaction of an acoustic wave with the ideal cloaking shell. The peaks and valleys in the color scale represent the instantaneous pressure field of a uniform plane wave traveling from left to right as it impinges on a rigid sphere surrounded by a cloaking shell whose thickness is equal to the radius of the interior sphere. The shell smoothly bends the acoustic wavefronts around the interior rigid sphere so that they smoothly pass around the object. No acoustic wave is scattered back toward the source, nor does the object cast any acoustic shadow. From the pressure field outside the shell, there's no way to know whether there is an object there or not.

Although the theory used to design such acoustic devices so far isn't as general as the one used to devise the microwave cloak, the finding nonetheless paves the way for other acoustic devices, for instance,those meant to bend or concentrate sound. 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.

Reference
"Scattering Theory Derivation of a 3D Acoutic Cloaking Shell"
S. Cummer, B.-I. Popa, D. Schurig, D.R. Smith, J. Pendry, M. Rahm and A.Starr,
Physical Review Letters, 100, 024301 (2008), Abstract Link.