Reflection and refraction characteristics of a spherical acoustic black hole

Abstract

Spherical Acoustic Black Hole (ABH), a kind of acoustic metamaterial, has been made in the new configuration to control three dimensional propagation of acoustic wave and its characteristics have been investigated by experiments and numerical simulations. ABH is new sound absorber composed of shell and absorptive core. Shell guides the incident acoustic wave into the core and ABH traps acoustic wave. Moreover, shell matches the characteristic impedance to the medium around ABH at the surface, which can enlarge absorption area without reflection. The experimental model of spherical ABH is constructed by stacking urchin-like shaped layers made with a 3-D printer on the assumption that the sonic crystal theory (Torrent and Sánchez-Dehesa, 2006, 2007) is valid in new structure. To evaluate reflection and transmission of ABH quantitatively, sound pressure around ABH was measured and angle averaged reflection coefficient and angle averaged insertion loss (Elliott et al., 2014) were calculated. Also, Glass Wool Sphere made of the same material as core of ABH and Rigid Sphere made of the same material as shell of ABH were measured in the same way and ABH was compared with them. The results of experiment and finite element method analysis show spherical acoustic black hole can decrease reflection due to impedance matching. Moreover, it can change the propagation direction of acoustic wave due to refractive index distribution and a large acoustic ratio region caused by the small refraction was observed.