June 16, 2008 Research into the cloaking properties of “left handed” metamaterials is continuing, with the latest news coming from scientists at the Polytechnic University of Valencia who have proven that these man-made substance can make objects impervious to sound waves. A proposed "acoustic cloak" would use sonic crystals, a class of metamaterial, to bend sound waves around an object, and could be used to render vessels Sonar-invisible... perhaps even bring to life that staple of spy technology: the Cone of Silence.
Metamaterials have a negative refractive index, meaning that waves can be diverted to the left of the incidence beam, rendering objects invisible in certain frequencies. The sound cloak could be made by arranging two types of acoustic isotropic metamaterials in a multilayered configuration. The metamaterials would be built with sonic crystals, (periodic arrays of sonic scatterers), based on two types of elastic cylinders. For a wide range of frequencies, 50 layers are able to reduce the backscattered field by an order of magnitude, in comparison to a naked rigid cylinder. A test with 200 layers was described as “near perfect.”
The research, conducted by Daniel Torrent and José Sánchez-Dehesa, can be found in the June 13 New Journal of Physics, in an article titled 'Acoustic cloaking in two dimensions: a feasible approach'.
One of the first uses of the material is likely to be warships, hoping to avoid sonar radars which pick up on the noise that ships emit, but if developments continue apace it could be used in concert halls to direct noise away from problem spots or even as a way to deal with noisy neighbors.