The ability to hide yourself or an object with an invisibility cloak has been dreamt of for thousands of years, but only recently with metamaterials have such cloaks become a possibility. As impressive as it is to hide the sight of something though, light is not the only phenomenon that can reveal something. Sound waves can also expose things, but now researchers at Duke University have created the first 3D acoustic cloak.
Metamaterials are specially engineered materials that have been given properties that cannot be found in Nature. In an invisibility cloak these properties include negative indices of refraction, causing light to bend backwards. For this acoustic cloak, sound waves are made to reflect off it such that they will seem to have bounced off of a flat surface. As the waves are also travelling a shorter distance, the cloak slows the waves to compensate. Visually the cloak looks like a simple pyramid of perforated plastic, but as the researchers stress, creating it was a very difficult task. The design process required a great many calculations to determine how sound waves will interact with it.
One obvious, if distant possibility for this research is hiding from sonar, but as sound travels differently in water than air, more work will be needed. On land though it could see use to improve the acoustics of buildings, such as auditoriums and concert halls.
Source: Duke University