Theoretical requirements and inverse design for broadband perfect absorption of low-frequency waterborne sound by ultrathin metasurface

Abstract

Effective absorption of low-frequency waterborne sound with subwavelength absorbers has always been a challenging work. In this paper, we derive two theoretical requirements for broadband perfect absorption of low-frequency waterborne sound by ultrathin acoustic metasurface under a finite-thickness steel plate followed by semi-infinite air. Based on the theoretical requirements, an acoustic metasurface, a rubber layer embedded periodically with cavities, is inversely designed to achieve perfect absorption at 500 Hz. The metasurface is as thin as 1% of the working wavelength and maintains a substantially high absorptance over a relatively broad bandwidth. The perfect absorption peak is attributed to the overall resonance mode of the metasurface/steel plate system. Besides, high absorption can still be achieved even if the loss factor of the given rubber material cannot meet the ideal requirement. Finally, a strategy to utilize the inherent frequency-dependent characteristics of dynamic parameters of rubber material is suggested to achieve an ultra-broadband perfect absorption. When the frequency-dependent characteristics of the given rubber matrix cannot meet the theoretical requirements, a broadband super-absorption can still be realized by properly designing the frequency position of perfect absorption of the cavity-based metasurface.

A Review of Acoustic Metamaterials and Phononic Crystals

“…In 2019, Zhong et al [100] put forward the two theoretical requirements for the ultra-thin acoustic surface to fully absorb low-frequency underwater acoustic broadband under the action of semi-infinite air, and based on this, designed a rubber periodically embedded in the cavity (see Figure 33). The layer is acoustically super-surface and achieves perfect sound absorption at a frequency of 500 Hz.…”

“…(a) A schematic view of an underwater absorptive metasurface with a finite-thickness steel plate followed by air; (b) A physical realization of the proposed metasurface for perfect absorption[100].…”

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

“…The concept of acoustic metamaterials is not limited only to locally resonant materials or periodic structures. Several other types of acoustic metamaterials have been reported, such as coiling-up space type AMs [59,60], split-ring type AMs, topological acoustics [61], fractal acoustic metamaterials [62][63][64][65][66], helical-structured metamaterials [67,68], and acoustic metasurfaces [69,70]. For coiling-up space type acoustic metamaterials, the incident acoustic waves are confined in coiled subwavelength cross-section channels, thereby resulting in extraordinary acoustic properties such as double negativity and near-zero index to a high effective refractive index of unit cells [71,72].…”

“…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]. metasurfaces [69,70]. For coiling-up space type acoustic metamaterials, the incident acoustic waves are confined in coiled subwavelength cross-section channels, thereby resulting in extraordinary acoustic properties such as double negativity and near-zero index to a high effective refractive index of unit cells [71,72].…”

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

“…The concept of acoustic metamaterials is not limited only to locally resonant materials or periodic structures. Several other types of acoustic metamaterials have been reported, such as coiling-up space type AMs [58,59], split-ring type AMs, topological acoustics [60], fractal acoustic metamaterials [61][62][63][64][65], helical-structured metamaterials [66,67], and acoustic metasurfaces [68,69]. For coiling-up space type acoustic metamaterials, the incident acoustic waves are confined in coiled subwavelength cross-section channels, thereby resulting in extraordinary acoustic properties such as double negativity and near-zero index to a high effective refractive index of unit cells [70,71].…”