Hiding from the sound hunter

One way to make something invisible is to shield it from sound waves so it cannot be detected. This “acoustic cloaking” has many applications, from camouflaging underwater vehicles to preventing buildings from collapsing. Conventional acoustic cloaking devices, however, require incredibly complex geometries to work. Yang et al. developed a new acoustic cloaking device that is simple in structure and easy to fabricate.

The team’s cloak made it seem as if the sound wave passed right through the object. The authors found the cloaked object retained the sound waves’ energy, up to 97.2 percent of its original, and yielded a sound field that was equivalent to one that would result if the object was not present. These two observations would trick viewers into believing the sound wave had not run into anything at all.

“The proposed hybrid acoustic cloaking is composed of binary splitting metasurfaces and near-zero-index metamaterials,” author Gang Wang said, and added that the devices use simple geometries like cylinders and cavities. “The principle of our work is to make the incident waves transmit around the obstacle by metasurfaces.”

To design the cloak, the authors calculated the ideal geometry for their cloaking device using two computational techniques: the half-analytic method to determine the initial geometry of their device, and the multi-objective genetic algorithm to optimize their initial result. To validate their simulation, the authors 3D printed their device and visualized the sound field of a cloaked object with schlieren photography.

The team is now developing a device that can acoustically cloak objects from a broader range of wavelengths.

Source: “A hybrid acoustic cloaking based on binary splitting metasurfaces and near-zero-index metamaterials,” by Rui Yang, Xiaodong Zhang, and Gang Wang, Applied Physics Letters (2022). The article can be accessed at https://doi.org/10.1063/5.0077829 .