Omni-directional and broadband acoustic anti-reflection and universal acoustic impedance matching.

The mechanism of the perfect anti-reflection of acoustic waves, regardless of frequency and incident angle, is presented. We show that reflections at a planar interface between two different acoustic media can be removed by adding a nonlocal metamaterial that compensates for the impedance mismatch. The properties required of a nonlocal metamaterial are explicitly specified through spatio-temporally dispersive mass density and bulk modulus. We analyze the characteristics of spatio-temporal dispersion according to the thickness of the matching layer. We discuss the issue of the total internal reflection caused by conventional matching layers and explain how our nonlocal matching layer avoids this. The practical design of our nonlocal layer using metamaterials is explained. The omni-directional frequency-independent behavior of the proposed anti-reflection matching layer is confirmed through explicit numerical calculation using the finite element method, and comparisons made to the conventional quarter-wave matching layer approach.