Underwater acoustic metamaterials

Abstract

Acoustic metamaterials have been widely investigated over the past few decades and have realized acoustic parameters that are not achievable using conventional materials. After demonstrating that locally resonant acoustic metamaterials are capable of acting as subwavelength unit cells, researchers have evaluated the possibility of breaking the classical limitations of the material mass density and bulk modulus. Combined with theoretical analysis, additive manufacturing, and engineering applications, acoustic metamaterials have demonstrated extraordinary capabilities including negative refraction, cloaking, beam formation, and super-resolution imaging. Owing to the complexity of impedance boundaries and mode transitions, there are still challenges in freely manipulating acoustic propagation in an underwater environment. This review summarizes the developments in underwater acoustic metamaterials over the past 20 years, which include underwater acoustic invisibility cloaking, underwater beam formation, underwater metasurfaces and phase engineering, underwater topological acoustics, and underwater acoustic metamaterial absorbers. With the evolution of underwater metamaterials and the timeline of scientific advances, underwater acoustic metamaterials have demonstrated exciting applications in underwater resource development, target recognition, imaging, noise reduction, navigation, and communication.

Enhanced Ultrasound Transmission through Skull Using Flexible Matching Layer with Gradual Acoustic Impedance

“…17 Dong et al reported a kind of bioinspired metagel with gradient acoustic impedance, which was capable of establishing broadband acoustic transmission between two mismatching media. 18 However, the above work focused on the transmission properties at relatively low frequencies (<0.3 MHz), which is unsuitable for transcranial ultrasound that typically employs a higher frequency of 0.6−3 MHz. 19,20 In this study, we propose a novel approach to overcome the limitations of existing acoustic metamaterials and matching layers for transcranial ultrasound, which often suffer from rigid structures or low operational frequencies.…”

“…To achieve suitable impedance matching over a wide frequency band, Pedersen et al conducted theoretical research and found that a matching layer with an exponential impedance exhibited the most stable high sound energy transmission efficiency over a wide frequency range . Dong et al reported a kind of bioinspired metagel with gradient acoustic impedance, which was capable of establishing broadband acoustic transmission between two mismatching media . However, the above work focused on the transmission properties at relatively low frequencies (<0.3 MHz), which is unsuitable for transcranial ultrasound that typically employs a higher frequency of 0.6–3 MHz. , …”

Liquid Metal‐MXene‐Based Hierarchical Aerogel with Radar‐Infrared Compatible Camouflage

“…[ 2 ] Nevertheless, given the complexity and variability of the surrounding environment, single‐function stealth technologies such as visible light stealth, infrared stealth, radar stealth, etc., face challenges in adequately addressing modern detection system requirements. [ 3–6 ] Therefore, there is an urgent need to develop advanced multispectral‐compatible stealth materials and technologies.…”

“…[2] Nevertheless, given the complexity and variability of the surrounding environment, single-function stealth technologies such as visible light stealth, infrared stealth, radar stealth, etc., face challenges in adequately addressing modern detection system requirements. [3][4][5][6] Therefore, there is an urgent need to develop advanced multispectral-compatible stealth materials and technologies. The term multifunctional stealth technology commonly encompasses radar-infrared compatible stealth, infrared and laser compatible stealth, visible light-near infrared-far infrared, and other multi-band stealth.…”

Acoustic Bilayer Gradient Metasurfaces for Perfect and Asymmetric Beam Splitting

“…[1,2] As the counterpart of electromagnetic metasurfaces, [3][4][5] acoustic metasurfaces have provided a novel approach for designing flat and compact devices that can manipulate sound waves in a two-dimensional (2D) planar situation. [6][7][8] In particular, acoustic gradient metasurfaces (AGMs) usually consist of periodic arrays of structurally graded subwavelength blocks that are designed as Helmholtz resonators [9,10] or coiled-up space structures. [11,12] AGMs introduce an in-plane phase gradient, which is equivalent to an additional wave vector, thus altering the fundamental law of reflection and refraction of acoustic waves at the interface.…”