Ventilated metasurface absorber constructed by synthesized acoustic multipoles

Acoustic metasurfaces have paved the way for the ongoing development of compact sound-absorbing devices that possess excellent ventilation capabilities. However, existing absorption mechanisms primarily focus on plane-wave fronts while neglecting omnidirectionally radiated cylindrical waves. In this study, drawing inspiration from wave-interference theory, we propose an approach to absorb cylindrical sound waves by combining artificially decorated passive monopoles and dipoles, which are synthesized by a coiled space resonator and a double-channel resonator, respectively. To demonstrate the effectiveness of our approach, we construct an ultrasparse annular metasurface absorber with an air-channel-area ratio of 46% and a deep-subwavelength thickness. Remarkably, this absorber achieves an absorption performance of 96% (91% in the experiment) for omnidirectionally radiated cylindrical sound waves. This achievement is realized by utilizing a circular array composed of 36 metamolecules arranged in a ringlike grating structure, with each metamolecule supporting synthesized multipoles. The near-unity absorption is accomplished through destructive interference between the composite monopole and dipole, characterized using a one-dimensional impedance tube. This high-efficiency absorption, combined with excellent ventilation and compactness, introduces alternative possibilities for designing functional sound-absorbing devices that can handle low-frequency sound waves with complex wave fronts.