Frequency-selective modulation of reflected wave fronts using a four-mode coding acoustic metasurface

Highlights: • A new method of coding is proposed based on a four-mode coding acoustic metasurface of simple structure. • The frequency-selective modulation of reflected waves is realized. • The equivalent circuit principle and decoupling effect of the meta-atom are theoretically and numerically analyzed. • We also filter and suppress the redundant diffraction orders of the reflected waves to eliminate side lobes. • The frequency degree of freedom plays a key role in the metasurface performs. Designing a multiplexing metasurface to exhibit diverse functionalities has been a trend in electromagnetics, acoustics, and mechanics. Here, we report a simple method to design the 2-bit coding acoustic metasurface by combining the Helmholtz resonator-like structures. Each meta-atom has four coding modes to independently modulate the wave fronts at two different frequencies. The equivalent circuit principle and decoupling effect are theoretically and numerically analyzed to clarify the underlying physical mechanism of our design. We filter and suppress the redundant diffraction beams to realize perfect negative reflection by controlling the number of outgoing diffraction modes. Considering the impact of loss, we simulate the scattered pressure and directivity of far field radiation for oblique illumination at two predesigned frequencies, the results of which show good agreement with the predicted ones. Our work may have potential in the design of multiplexing, frequency-selective, and programmable acoustic devices.