Scientists have developed a cloaking device that cloak submarines operating under the ocean surface called an underwater acoustic ground cloak. Scientists on the project from Pennsylvania State University have described the physics behind the cloaking device. The team set out to design a metamaterial that allows sound waves to bend around an object as if it’s not there.
The unit cell, which is described as the smallest component of the metamaterial, must be smaller than the acoustic wavelength. Lead researcher Amanda D. Hanford says that the math shows the team that the properties are possible. Prior to the new research, most acoustic metamaterials were designed to deflect sound waves in air.
This team took that a step further by trying to do the same thing underwater. Acoustic cloaking underwater is more complex because water is denser and less compressible than air. After multiple attempts to create an underwater acoustic cloak, the team was able to develop a 3-foot tall pyramid that uses perforated steel plates.
When placed on the bottom of an underwater research tank with a hydrophone that makes acoustic waves between 7,000 and 12,000 Hz along with several receivers to monitor the acoustic waves. The wave reflected by the pyramid device matched the wave reflected from the surface. The result was that the material could make an object appear invisible to underwater sonar.
The team says that the results show potential for real-world applications. Those applications include acoustic materials to dampen sound and make objects appear invisible underwater.