Fractal acoustic metamaterials with near-zero index and negative properties

A subwavelength two-dimensional fractal acoustic metamaterial was designed based on the Hilbert fractal curve and spatial curl concept. The equivalent acoustic parameters of the fractal acoustic metamaterial were extracted by the equivalent parameter method, and the acoustic metamaterial exhibited near-zero density and negative density properties at specific frequency points. The analysis based on the finite element method shows that the acoustic prism constructed by using the near-zero density property of this acoustic metamaterial can realize the single-channel acoustic unidirectional transmission and multichannel acoustic unidirectional transmission effect, the bidirectional acoustic conduction and multidirectional acoustic conduction effect, and the bidirectional sound insulation effect. We also investigated the effect of embedding rigid scatterers inside the prism on the acoustic one-way transmission performance. Based on the acoustic direction selection mechanism of the nearzero refractive index acoustic metamaterial, arbitrary modulation of the plane acoustic wave emission direction can be achieved by changing the angle of the acoustic prism emission interface. In addition, the negative density property of the acoustic metamaterial is used to design an acoustic triangular prism to realize the acoustic negative refraction phenomenon. next, we discuss the trend of the influence of the geometrical parameters of the metamaterial on its effective properties, where the modulating wave frequency gradually moves to lower frequencies as the geometrical parameters of the metamaterial structural units increase. Finally, we fabricated the corresponding experimental samples of acoustic prisms using 3d printing technology and conducted experimental tests. The results show that the measured acoustic pressure field distribution matches well with the analytical results of the finite element method, which proves the accuracy of the results and the reasonableness of the structure, indicating that acoustic prisms composed of this acoustic super-material are expected to be used in ultrasonic imaging, acoustic energy rectification and other fields.