Acoustic Cloaking

Computer simulations and the use of wave scattering theory have demonstrated that, contrary to earlier predictions, it should be possible to produce a 3-dimensional material shell which is invisible to sound waves, analogous to “optical cloaking,” the process in which light waves are guided around an object and then refocused on the far side and in the same direction (with no reflected light to betray position) so as to make the object seem invisible. Full optical cloaking has not been achieved yet, but researchers expect to be able to do it.

Can the same thing be done with sound waves? In principle there is no reason why it couldn’t be done. The leader of a group of scientists examining this issue, Steven Cummer at Duke University, says that many of the principles that pertain to the channeling of light waves around an object also apply to sound waves. To be sure, there are differences. Sound waves oscillate in the direction of their motion while the electric and magnetic fields composing light waves oscillate perpendicularly to the wave motion. In the optical case, cloaking will require a material (actually a meta-material) tailored, highly anisotropic (varying widely according to the direction through the material) index of refraction.

In practice, the index of refraction for electromagnetic waves depends on the permittivity, a measure of the material's response to an applied electric field, and permeability, its response to an applied magnetic field (for an account of the demonstration of negative-index materials, see http://www.aip.org/pnu/2000/split/pnu476-1.htm). The acoustic equivalent of these two parameters are the mass density and the bulk modulus (the springiness) of the background fluid (usually air or water) in which the object sits. Cummer (919-660-5256, cummer@ee.duke.edu) says that in the short run acoustic cloaking might be more practical than optical cloaking.

A limitation of electromagnetic cloaking, he says, is that it requires portions of the wave to move faster than the speed of light (in full accordance with special relativity); this can be done for very limited frequency ranges but not for wider ranges, limiting the applicability of optical cloaking. This limitation does not apply to sound waves moving through matter. Furthermore, the acoustic properties of most materials means that sound waves might not be absorbed as readily in acoustic cloaking as light waves are absorbed in optical cloaking (in which case the cloaking would be something less than perfect).

Applications of acoustic cloaking come easily to mind: hiding submarines from sonar, for example. Another potential practical application might be in architecture, where acoustic considerations (reducing noise) might not have to be sacrificed in the interest of structural integrity. Among Cummer’s collaborators are David Smith of Duke (one of the early pioneers in the field of negative-index materials) and John Pendry of Imperial College (the early theorist of negative-index studies). (Cummer et al., Physical Review Letters, 18 January 2008; considered an editor’s Suggested article in PRL)