Cloaking devices -- it's not just 'Star Trek' anymore
Researchers designed an underwater acoustic ground cloak by
engineering material with properties not typically found in nature,
rendering an object invisible
May 11, 2018
devices play a pivotal role in many sci-fi television programs.
Scientists are now working to take this technology from the dramatic
realm of science fiction and make it real. Amanda D. Hanford, at
Pennsylvania State University, is taking the introductory steps to make
acoustic ground cloaks. These materials redirect approaching waves
around an object without scattering the wave energy, concealing the
object from the sound waves.
During the 175th Meeting of the Acoustical Society of America, being
held May 7-11, 2018, in Minneapolis, Minnesota, Hanford will describe
the physics behind an underwater acoustic shield designed in her lab.
Hanford and her team set out to engineer a metamaterial that can allow
the sound waves to bend around the object as if it were not there.
Metamaterials commonly exhibit extraordinary properties not found in
nature, like negative density. To work, the unit cell -- the smallest
component of the metamaterial -- must be smaller than the acoustic
wavelength in the study.
Scientists are now working to take
cloaking devices from the dramatic realm of science fiction and make
them real. Amanda D. Hanford, at Pennsylvania State University, is
taking the introductory steps to make acoustic ground cloaks. These
materials redirect approaching waves around an object without scattering
the wave energy, concealing the object from the sound waves. During the
175th ASA Meeting, Hanford will describe the physics behind an
underwater acoustic shield designed in her lab. This image is a Profile
picture of the assembled perforated plate ground cloak.
"These materials sound like a totally abstract concept, but the math is
showing us that these properties are possible," Hanford said. "So, we
are working to open the floodgates to see what we can create with these
To date, most acoustic metamaterials have been designed to deflect sound
waves in air. Hanford decided to take this work one step further and
accept the scientific challenge of trying the same feat underwater.
Acoustic cloaking underwater is more complicated because water is denser
and less compressible than air. These factors limit engineering options.
After multiple attempts, the team designed a 3-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.
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. These results demonstrate
that this material could make an object appear invisible to underwater
instruments like sonar.
Using linear coordinate transformation, the researchers were able to map
the flat surface of the bottom of the tank and determined that space was
compressed into two triangular cloaking regions consisting of the
These results show potential to contribute to real-world applications,
such as acoustic materials to dampen sound and appear invisible