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Underwater acoustic cloaking device could make objects invisible to sonar

A team of researchers at Pennsylvania State University, led by Amanda D. Hanford, has developed a “cloaking device” that works underwater. The new metamaterial shield is able to intercept and bend underwater sound waves, like sonar, around it without scattering them – making it appear as if the cloak and anything within it isn’t there at all.

Metamaterials are materials designed to exhibit properties not found in nature. They are generally constructed from composite materials, like metals, plastics, or ceramics, and engineered into repeating, microscopic structures.

In the case of the underwater acoustic cloak, the goal was to make a metamaterial that could do for submarines or undersea installations what science fiction cloaking devices do for Klingon warbirds. Acoustic cloaking has already been achieved in the lab, but only in the air. Water is much more of a challenge because it’s denser and much less compressible than air, which limits cloak engineering options.

To create their cloak, Haford and her team had to form a metamaterial with cells that are smaller than the acoustic wavelength of the sound. After trying various options, they eventually came up with a 3-ft-tall (91-cm) pyramid made out of perforated steel plates. This was placed in a large water-filled laboratory tank equipped with a hydrophone generating sound waves between 7,000 and 12,000 Hz, and a series of receiver hydrophones to monitor for echoes of the acoustic waves.

The researchers found that sound waves hitting the metamaterial were reflected, but as they did so, their phase matched those being reflected from the surface and the amplitude decreased slightly. The upshot was that the metamaterial could make itself and objects under it in two triangular cloaked areas invisible to instruments like sonar.

According to the team, the new technology could have a wide range of applications, from dampening sound underwater to opening the way for new stealth techniques.

The research will be presented at the 175th Meeting of the Acoustical Society of America in Minneapolis, which runs from May 7 to 18.

Source: Acoustical Society of America

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