Broadband near-perfect absorption of low-frequency sound by subwavelength metasurface

Abstract

The emerging absorptive metasurface relies on arrays of structured meta-atoms with various geometries for customized sound localization, which can significantly enhance the energy dissipation. However, most of the existing absorbers are for given frequencies at an optimal incident angle. This limitation on the working frequency and incident angle remains a challenging obstacle for their practical applications, in addition to the perfect absorptance demand. Guided by the causality principle, a physical model is established in which the absorptive properties of such systems can be fully controlled by two simple parameters (i.e., leakage factor and loss factor) which are dictated by the geometrical properties of the underlying structures. We demonstrate a subwavelength metasurface absorber which shows near-perfect absorptance (at 95%) in a broad frequency regime from 228 Hz to 319 Hz (wavelength λ from 12.6 to 9.0 times thickness) and even allows 93% reduction with a large incident angle of 60°. We prove that this broadband near-perfect absorption behavior stems from the tunable damping conditions, which can be achieved by coupling an ordinary ultrathin surface sponge coating with an artificial underdamped multiband absorptive system. From the view of the causality principle, the subwavelength near-perfect absorptions originate from the finite working bandwidth. As the research premise, we also demonstrate a λ/21.7-thick, 16.7%-filling ratio ultrasparse absorber with unity absorptance by modulating the displacements between uniformly sized coiled space resonators. The paradigm may pave the way for versatile devices in noise remediation engineering.

Recent Advances in Acoustic Metamaterials for Simultaneous Sound Attenuation and Air Ventilation Performances

“…The phase delay is calculated as ∆∅ = , where is the acoustic wave number and is the effective channel length [13]. Recently, several types of space coiling structures have been demonstrated for low-frequency sound attenuation, such as co-planar spiral tubes [60], axially coupled circular tubes [73], coiled air chambers [74], spiral metasurfaces [75], and labyrinthine structures [76][77][78]. [13], © 2016 Author(s) under a Creative Commons Attribution Non-Commercial License 4.0 (CC BY-NC), and Yang et al [44], © 2015 Académie des sciences, respectively.…”

Subwavelength broadband sound absorber based on a composite metasurface

“…In order to significantly enhance the coupling with the ultrathin sponge coating, the backing metasurface is integrated by multiple coiled space resonators (CSRs), which show a perfectly-matched impedance at 12 discrete frequencies and near-perfectly-matched impedances in the entire intervening frequency ranges. The absorption efficiency is largely enhanced in comparison with the case of rigid wall backing, whereas the absorptive bandwidth is extended when compared with the case of soft boundary backing 22 . In this work, the theoretical complex frequency planes calculated with the admittance-sum method and transfer-matrix method have been further employed to analyze the absorptive performances.…”

“…Due to the highly concentrated sound energy at resonance, very small dissipation coefficients, i.e., viscosity from surface tensor in membrane and fractional viscosity between air and framework in fluid-solid system, can achieve high-efficiency or even PA. However, the highly concentrated sound energy in resonant system also results in a little sound energy radiated 22 , which induces the sharp absorptive peak associated with a small radiation or leakage factor. Hence, most of these absorbers generally work at a single or multiple discrete narrow bands.…”

“…Unfortunately, achieving broader bandwidth and thinner device are contradict. Regarding coupled-mode theory (CMT), to achieve larger radiation loss (indicating broader bandwidth), the total thickness should be increased 22 . Moreover, due to the sharp absorptive peak of individual unit cell, the absorptance of the combined system is bound to emerge low-efficiency absorption valleys at some extent.…”

“…www.nature.com/scientificreports/ a wavelength being from 12.6 to 9.0 times of the thickness at absorptance > 95% ) has been devised 22 . However, the bandwidth of absorption is still limited, i.e., less than one octave.…”