Harnessing Multiple Internal Reflections to Design Highly Absorptive Acoustic Metasurfaces

The rapid development of metasurfaces has enabled numerous intriguing applications with acoustically thin sheets. Here we report the theory and experimental realization of a nonresonant sound-absorbing strategy using metasurfaces by harnessing multiple internal reflections. We theoretically and numerically show that the higher-order diffraction of thin gradient-index metasurfaces is tied to multiple internal reflections inside the unit cells. Highly absorbing acoustic metasurfaces can be realized by enforcing multiple internal reflections together with a small amount of loss. A reflective gradient-index acoustic metasurface is designed based on the theory, and we further experimentally verify the performance using a three-dimensional printed prototype. Measurements show over 99% energy absorption at the peak frequency and a 95% energy absorption bandwidth of around 600 Hz. The proposed mechanism provides an alternative route for sound absorption without the necessity of high absorption of the individual unit cells.