Lotus Metasurface for Wide-Angle Intermediate-Frequency Water–Air Acoustic Transmission

Abstract

Only 0.1% of the acoustic energy can transmit across the water–air interface because of the huge acoustic impedance mismatch. Enhancing acoustic transmission across the water–air interface is of great significance for sonar communications and sensing. However, due to the interface instability and subwavelength characteristics of acoustic metamaterials, wide-angle intermediate-frequency (10 kHz-100 kHz) water–air acoustic transmission remains a great challenge. Here, we demonstrate that the lotus leaf is a natural low-cost acoustic transmission metasurface, namely, the lotus acoustic metasurface (LAM). Experiments demonstrate the LAM can enhance the acoustic transmission across the water–air interface, with an energy transmission coefficient of about 40% at 28 kHz. Furthermore, by fabricating artificial LAMs, the operating frequencies can be flexibly adjusted. Also, the LAM allows a wide-angle water-to-air acoustic transmission. It will enable various promising applications, such as detecting and imaging underwater objects from the air, communicating between ocean and atmosphere, reducing ocean noises, etc.

Remote Water‐to‐Air Eavesdropping with a Phase‐Engineered Impedance Matching Metasurface

“…The proposed unit cell has subwavelength dimensions and a rational design, built by grooving an acoustically rigid cube with a labyrinthine‐like channel filled with air. Such an acoustic architecture overcomes the inherent limitations of existing antireflection designs (based, for instance, on a single layer of bubbles, [ 7 ] hydrophobic materials, [ 12 ] or membranes [ 11 ] ) in terms of low transmission efficiency, short interaction distance, and weak mechanical rigidity, as demonstrated in the following. Despite some structural designs of airborne acoustic metasurfaces that may share a similar geometry, [ 20 ] their underlying phenomena here are totally different.…”

“…[ 1–6 ] Due to the large ratio in acoustic impedance between water and air (around 3600 times), only 0.1% of the acoustic energy can naturally be transmitted through such a nearly perfect reflective boundary (namely, 30 dB loss), equivalent to the loss experienced in underwater acoustic propagation accumulated over 30 km at 10 kHz. [ 7 ] In comparison to conventional approaches to impedance matching, which feature bulky and impractical devices, [ 8–10 ] recently emerging acoustic metamaterials, such as membrane‐type [ 11 ] and bubble‐based [ 7,12–15 ] artificial materials, among others, [ 16–18 ] may enable enhanced transmission between water and air in more efficient and compact platforms. However, their application in real‐world systems, requiring mechanical stiffness, high endurance, and large transmission efficiency, still remains poorly explored.…”

Hybrid Metasurfaces for Perfect Transmission and Customized Manipulation of Sound Across Water–Air Interface

“…The required acoustic impedance is expected to be dynamically satisfied by fully taking advantage of the fluid–solid interactions. Different from previous works, [ 21,22,42 ] we can avoid strong resonances of tiny air bubbles or narrow air cavities, which may introduce thermoviscous losses to seriously weaken the transmitted sound intensity. [ 21,22 ] Topology optimization method based on the genetic algorithm [ 50,51 ] is adopted to find appropriate topological configurations for $$\rm MS_1$$.…”

“…[ 21 ] For the enhanced transmission at intermediate frequencies around 10 kHz, an artificial lotus metasurface composed of a superhydrophobic aluminum plate is proposed based on a similar principle. [ 42 ] The immersing part of the superhydrophobic aluminum plate in water can form µm‐scale air layers to make up the mass‐spring resonance system. [ 42 ] But the necessary hydrophobic treatment on metal materials inevitably brings some tedious operations to large‐scale fabrication and preparation of samples.…”

“…[ 42 ] The immersing part of the superhydrophobic aluminum plate in water can form µm‐scale air layers to make up the mass‐spring resonance system. [ 42 ] But the necessary hydrophobic treatment on metal materials inevitably brings some tedious operations to large‐scale fabrication and preparation of samples. [ 42 ] In general, alternative simple and practical approaches with the aid of new physical mechanisms remain to be explored.…”