Camouflage techniques have been employed by both humans and animals trying to become invisible to their predators or to their prey. Human fascination with invisibility can be traced from ancient times to modern literature. Materials that have the property of making objects or bodies invisible to the eye or undetectable by the ears or other sensing devices have been a dream of military engineers and scientists for centuries. While camouflage has been the solution adopted to achieve optical invisibility, for acoustic invisibility the reduction of noise generated when the object or body moves was formerly the more feasible solution.