The rapid development of metasurfaces has enabled numerous intriguing applications with acoustically thin sheets. Here we report the theory and experimental realization of a non-resonant sound absorbing strategy using metasurfaces by harnessing multiple internal reflection. We theoretically and numerically show that the higher order diffraction of thin gradient-index metasurfaces is tied to multiple internal reflection inside the unit cells. Highly absorbing acoustic metasurfaces can be

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... by enforcing multiple internal reflection 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 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.