Figure 3. Characteristics of the pyramidal cone metamaterial matching layer.

(a) Distribution of the equivalent acoustic parameters along the thickness direction. Both the specific acoustic impedance and the longitudinal velocity vary continuously and monotonically. (b) Sound transmittance through this metamaterial matching layer. The simulated and theoretical results are indicated by dashed and solid lines, respectively. The transmission performance of the matching layers with thicknesses of 610 μm (solid yellow and dashed black) and 1000 μm (solid blue and dashed red) show better broadband transmissions than the single matching layer (solid green and dashed pink) (c) The distribution of the displacement field in this matching layer. Plane incident acoustic waves remain intact after travelling through the metamaterial matching layer, as the high diffraction orders of the scattering waves are essentially confined to the surfaces of the cone structures.