  Over the last couple of years, the digital coding acoustic metasurfaces have
been developed rapidly as a highly active research area for their unique and
flexible manipulation of acoustic wavefronts. Nevertheless, all recent
attentions in the acoustic community have been mainly concentrated on
space-encoded architectures, leaving the room free for benefiting from the
unique features of spatiotemporally modulated metasurfaces. By entering the
world of time, here, we propose a space-time-coding acoustic digital
metasurface with exotic ability to dynamically transfer the energy of the
carrier acoustic signal to a series of harmonic components, with equivalent
magnitudes and phases that can be precisely and independently engineered. The
contributing elements are composed of a straight pipe, and four shunted
Helmholtz cavities (HCs) with transmission phases dynamically controlled by
exploiting a high-speed electromechanical actuation system. Several
illustrative examples have been presented to demonstrate that by distributing
the coding sequences in both space and time dimensions, diverse scattering
functionalities can be elaborately acquired for one or multiple harmonic
frequencies in a programmable way. Numerical and theoretical results are in an
excellent agreement, thereby elucidating that this next generation of
programmable acoustic metasurfaces, without restoring to high-cost nonlinear
components, opens up unprecedented potential for efficient harmonic control
used in adaptive beamforming and acoustic imaging systems.
