• Scientists have developed a material that makes an underwater object invisible to sonar detectors, an advance that brings us a step closer to Star Trek-style invisibility cloaks.
  • A cloaking device is a fictional stealth technology that can cause objects, such as spaceships or individuals, to be partially or wholly invisible.
  • Now, researchers at Pennsylvania State University in the US are taking the introductory steps to make acoustic ground cloaks.
  • Metamaterials commonly exhibit extraordinary properties not found in nature, like negative density
  • These materials redirect approaching waves around an object without scattering the wave energy, concealing the object from the sound waves.

What about Metamaterials?

  • To date, most acoustic metamaterials have been designed to deflect sound waves in air.
  • Acoustic cloaking underwater is more complicated because water is denser and less compressible than air.
  • These factors limit engineering options, researchers said. After multiple attempts, the team designed a three-foot-tall pyramid out of perforated steel plates.
  • They then placed the structure on the floor of a large underwater research tank.
  • Inside the tank, a source hydrophone produced acoustic waves between 7,000 Hz and 12,000 Hz, and several receiver hydrophones around the tank monitored reflected acoustic waves.
  • The wave reflected from the metamaterial matched the phase of the reflected wave from the surface.
  • Additionally, the amplitude of the reflected wave from the cloaked object decreased slightly, researchers said.
  • These results demonstrate that this material could make an object appear invisible to underwater instruments like sonar.
  • The results show potential to contribute to real-world applications, such as acoustic materials to dampen sound and appear invisible underwater.