“The mathematics behind cloaking has been known for several years,” said Professor John Pendry of Imperial College London, UK, an expert in cloaking. “What hasn’t been available for sound is the sort of materials you need to build a cloak out of.”
…
The Spanish team who conducted the new work believe the key to a practical device are so-called “sonic crystals”. These artificial composites – also known as “meta-materials” – can be engineered to produce specific acoustical effects.
…
The research builds on work by scientists from Duke University in North Carolina, US, and the Hong Kong University of Science and Technology. Earlier this year, independent teams from the two institutions demonstrated the mathematics necessary to create an acoustic cloak. Other scientists have shown that objects can be cloaked from electromagnetic radiation, such as microwaves.