Low reflection effect by 3D printed functionally graded acoustic black holes

The acoustic black hole (ABH) effect produces drastic wave compression and energy trapping in thin-walled structures. Recent research has shown that the reflected wave in laboratory ABH structures can be reduced by attaching a thin absorbing layer in specific wave concentration areas. In this paper, a novel design and implementation of one-dimensional (1D) functionally graded acoustic black hole (FG-ABH) is presented, as an alternative to achieve low reflection effects. Two configurations of the FG-ABHs are demonstrated, i.e., an axially graded ABH and a thickness graded ABH. The FG-ABH beams are manufactured by 3D printing technology using an Objet Connex 500 printer. Materials with varying levels of viscoelastic characteristics are produced using two base materials called TangoPlus and VeroWhitePlus. The FG-ABH beams have both diminishing thickness and elastic modulus in the tip of the ABH wedge. Numerical investigation of wave propagation, attenuation and reflection are conducted utilizing a viscoelastic code: University of Michigan's Local Interaction Simulation Approach (UM/LISA). The damping effect of the materials is included based on Kelvin-Voigt viscoelastic theory. Finally, the process of wave propagation in FG-ABHs, traditional ABH and uniform beam are experimentally investigated using a Scanning Laser Doppler Vibrometer (SLDV) system. The reflection coefficients, i.e. the ratios between the amplitude of reflected wave and incident wave, are computed. Results indicate that the FG-ABHs enhance the ABH effect, and lower reflection is observed when compared to a traditional ABH structure. (C) 2019 Elsevier Ltd. All rights reserved.