Acoustic metasurfaces have been widely studied due to their thin thickness in low frequency and extraordinary manipulation of sound waves in the past few years. Full-space wavefront control can enrich the functionality of metasurfaces, however, most metasurfaces are constrained by unidirectional incidence, which is not conducive to their spatial adaptability. To fulfill this gap, we proposed a bioinspired metasurface unit consisting of coiling slits and Helmholtz transmission tunnels, which can independently modulate reflected and transmitted sound waves, and the transmitted and reflected phases can be combined arbitrarily within a range of structural parameters. Moreover, benefiting from its symmetrical configuration, this metasurface can work under forward or backward incidence conditions. To illustrate the presented acoustic metasurface, the acoustic impedance model of this unit was established theoretically, and a variety of acoustic functions are realized simultaneously in reflection and transmission modes, such as generating Bessel beams, focusing sound waves, and converting surface waves. Both numerical and experimental results demonstrate the extraordinary wave-manipulation performance of the metasurfaces. This finding provides new ideas for expanding the acoustic function of metasurfaces.
Xiao et al. (Wed,) studied this question.