Researchers from Pennsylvania State University have discovered a new ‘cloaking’ material that could make an object appear invisible to underwater instruments like sonar.

The team is working to develop acoustic ground cloaks, which can redirect approaching waves around an object underwater without scattering the wave energy, concealing the object from the sound waves.

To accomplish this, the researchers worked to engineer a metamaterial that would allow the sound waves to bend around the object.  Metamaterials commonly exhibit extraordinary properties not found in nature, like negative density. However in the study, they needed the unit cell—the smallest component of the material—to be smaller than the acoustic wavelength.

“These materials sound like a totally abstract concept, but the math is showing us that these properties are possible,” Amanda Hanford, a research associate and assistant professor of acoustics in the Applied Research Laboratory at Penn State, said in a statement. “So, we are working to open the floodgates to see what we can create with these materials.”

The majority of acoustic metamaterials have been designed to deflect sound waves in air, but not water. However, the researchers opted to take that one step further and try to deflect sound waves underwater. This is a challenge because water is denser and less compressible than air, limiting the engineering options.

To achieve this, the team designed a three-foot-tall pyramid comprised of perforated steel plates, which was placed 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 were placed around the tank to monitor reflected acoustic waves.

The wave reflected from the metamaterial equaled the phase of the reflected wave from the surface, and the amplitude of the reflected wave from the cloaked object also decreased slightly..

The researchers were able to map the flat surfaces of the bottom of the tank using linear coordinate transformation to determine that space was compressed into two triangular cloaking regions consisting of the engineered metamaterial.

Hanford described the physics behind the underwater acoustic shield designed in her lab during the 175th Meeting of the Acoustical Society of America from May 7 to 11 in Minneapolis.