Acoustic metamaterials, or materials engineered to have acoustic properties not found in nature, have promising applications like acoustic cloaking, subwavelength imaging and acoustic wave manipulation. Most experiments have focused on 2-D acoustic metamaterials for manipulating in-plane sound waves, or 3-D designs that can only control waves coming in from certain directions.

In Applied Physics Letters, researchers report on the design of a 3-D acoustic metamaterial ball with isotropic negative characteristics. A larger metamaterial system using these balls as building blocks demonstrated both sound cloaking and super-tunneling within a finite space. The system blocks the low frequency sounds between the monopolar and dipolar resonances in both numerical and experimental investigations.

The ball’s design evolved from a regular icosahedron consisting of 20 identical tetrahedrons. Each tetrahedron, rounded off to form a cone, contained a labyrinthine structure to artificially extend the propagation path of sound waves. The ball, with a radius of 35 millimeters, was then 3-D printed in a photosensitive resin.

The authors performed eigenstate analysis via computer simulation, finding that the metamaterial system had two significant Mie-type eigenmodes: a monopole resonance at 1135 hertz and a dipole resonance at 1722 hertz. After confirming the negative properties, where the metamaterial has near-zero density near the dipole resonance, the experiment investigated the ball hanging from a thread in a standing wave tube. Here, too, they observed blocking of sound waves in the low-frequency range between monopole and dipole resonances.

In a two-scale computational model, a structure created with the balls as building blocks was able to cloak a rigid obstacle from being detected by an acoustic wave. The wave shape remained unchanged without any distortion of the wavefront in a finite 3-D space.

According to co-author Xiaodong Huang, future research in this area includes the systematic design of acoustic metamaterials utilizing a topology optimization technique.

Source: “A 3-D space coiling metamaterial with isotropic negative acoustic properties,” by X. F. Fu, G. Y. Li, M. H. Lu, G. Lu, and X. Huang, Applied Physics Letters (2017). The article can be accessed at https://doi.org/10.1063/1.5005553.