Conversion Of Surface Waves Research Articles

Acoustic metasurface embedded with thin-walled plate based on phase modulation for multi-angle broadband sound absorption

The design of low-frequency broadband acoustic absorbers with thin thicknesses is a longstanding and challenging problem. This work proposes an acoustic metasurface consisting of macro-perforated porous, thin-walled perforated plate, and meta-porous layers embedded by thin-walled plate (MPM) to combine the advantages of porous material and resonator. MPM is formed by four subunits with a phase gradient. The acoustic properties of an MPM unit cell are investigated by a theoretical model based on homogenization theory and transfer matrix methods, and the theoretical model is verified by a numerical model. Then, the sound absorption mechanism of MPM unit cell is explored, and it is revealed that the mid-frequency and low-frequency sound waves are mainly dissipated by thin-walled perforated plate and meta-porous layers, and the macro-perforated porous structure target high-frequency absorption. In addition, the dependence of geometric parameters of MPM unit cell on acoustic performance and reflected phase is investigated. Furthermore, the geometric parameters of each subunit in acoustic metasurface are carefully designed, and excellent absorption in target frequency is obtained by the hybrid resonance of a single unit cell and the surface wave conversion of MPM with reflected phase gradient. Also, the appearance of perfect sound absorption comes from the far-field interference between reflected waves and the wave-trapping phenomenon near the periodic structure. A thin (λ/20) absorption panel MPMs is designed to achieve multi-angle low-frequency broadband absorption and an effective normal sound absorption (>50 %) band over 344–3000 Hz. This work proposes a novel design idea and opens possibilities for acoustic metasurface practical applications in noise mitigation of various scenarios.

Periodic WIB-trench wave barriers for seismic surface waves

Seismic surface waves possess detrimental characteristics of low frequency and significant amplitude, posing a threat to buildings and structures. This paper proposes a novel combined wave barrier, referred to as the periodic WIB-trench barrier, which is designed specifically for the purpose of isolating seismic surface waves. The periodic WIB-trench barrier consists of wave impeding blocks (WIB) and infilled trenches. The surface wave bandgaps of the periodic WIB, infilled trench, and the combined wave barrier have been compared, while the vibration modes of these three barriers have also been acquired to reveal the mechanism of the bandgaps of surface waves. Subsequently, a finite barrier transmission model has been established to analyze the vibration isolation performance of the periodic WIB-trench barrier in both the frequency domain and time domain. The results demonstrate that the proposed combined wave barrier exhibits larger bandgap widths and lower boundary frequencies when compared to the periodic WIB and periodic infilled trenches. Specifically, the bandgaps of the periodic WIB-trench barrier ranges from 4.5 to 6 Hz, 9.5 to 12.8 Hz, and 15 to 20 Hz. Through the confinement of surface wave energy within the barrier unit cells or the conversion of surface waves into body waves, an effective vibration mitigation approach proposed in this paper can be achieved. This research offers a novel perspective on the design of periodic barriers for seismic surface waves isolation purpose.

Reflected wave manipulation by aeroacoustic metasurfaces in sheared grazing flows

Acoustic metasurfaces (AMs) are artificial structures possessing exotic acoustic properties, such as negative refraction and perfect absorption, etc., in sub-wavelength thickness scales. Most previous studies on AMs were conducted in laboratory conditions with a static medium, limiting their applications in practical conditions, e.g., the aircraft engines where an aerodynamic background flow is present. To fill this gap, we study the effects of a grazing shear flow on the reflection performance of periodic AMs in this work. An AMs design method is proposed to realize effective wavefront manipulation with the consideration of the shear flow. The strategy is to form a linear reflected phase-shifting covering 0 to 2π above the shear layer. The reflection characteristics of two acoustic porous metasurfaces (APMs), i.e., APM1 and APM2 designed for the uniform and the sheared flows, respectively, are numerically investigated. Results show that the presence of the shear layer reduces the effectiveness of APM1 under oblique incident waves due to the considerable refraction effect. In contrast, under the same flow conditions, APM2 realizes desired wave manipulation behaviors, such as anomalous reflection and surface wave conversion. This study provides guidelines for AMs design in flow conditions, which offers potential for noise control in aeroacoustic problems.

Acoustic energy harvesting metasurface based on surface wave conversion

In this work, we propose and demonstrate an acoustic gradient metasurface with phase modulation that can convert the propagating waves into surface bounded waves for energy concentration. Due to the periodic design, multiple surface waves can form the standing wave in the case of normal incidence. Then, all piezoelectric composite sheets (PCSs) are placed in the antinode regions for acoustic energy harvesting and the output voltage of different PCS keeps the same phase. Numerical simulation and experimental results confirm that the sound pressure is 5 times larger than the incident waves, and the output power is increased by 8 times for single PCS. Our work provides an alternative way to reduce the incident noise in an enclosed space like machine room and work as a power source with a simplified design.

Broadband acoustic meta-porous layer for reflected wave manipulation and absorption

Acoustic metasurfaces (AMs) used for reflected sound wavefront manipulation are generally designed based on the generalized Snell’s law (GSL) at a single frequency, suffering from inferior broadband properties. Herein, an acoustic meta-porous layer (AMPL) with periodic structures is designed to realize reflected wavefront manipulation and effective sound absorption over a wide frequency band. The AMPL is constructed by four periodically arranged units, each consisting of porous elements inserted by acoustically rigid partitions, forming a linear reflected phase-shifting within 0 to 2π maintaining in a target frequency range of [1000,3000]Hz. To predict the reflected response of the element of the AMPL, an analytical model is proposed based on the transfer matrix method and is numerically validated. The sound reflection and absorption performance of the AMPL is investigated numerically and experimentally. The scattering sound pressure fields under normally and obliquely incident sound waves at different frequencies demonstrate the broadband reflected wavefront manipulation capability, including negative reflection and surface wave conversion. Compared to a uniform porous foam, an effective absorption is achieved by the AMPL due to the surface wave excitation at an identical thickness of 40 mm, with an averaged absorption coefficient greater than 0.9 in [800,3000]Hz. The results suggest the proposed structure could be a promising candidate of broadband noise absorption metamaterial for practical applications.

An all-dielectric metasurface absorber based on surface wave conversion effect

The high-absorption ultra-thin metasurface absorber has important applications in EM radiation prevention and stealth design. In this paper, the proposed metasurface absorber composed of one-dimensional phase gradient supercell and uniform resonant supercell can reach perfect absorption at 10 GHz by reducing the scattering of the surface wave. Furthermore, the two-dimensional phase gradient metasurface absorber has better absorption of waves with different polarization angles and a wider range of incident angles. Our work provides a promising method to achieve high absorption, which is simpler and more robust than adjusting the effective EM parameters of metasurface absorber.

Local coupling and conversion of surface waves due to Earth’s rotation. Part 1: theory

SUMMARYEarth’s rotation affects wave propagation to first order in the rotation through the Coriolis force. The imprint of rotation on wave motion has been accounted for in normal mode theory. By extending the theory to propagating surface waves we account for the imprint of rotation as a function of propagation distance. We describe the change in phase velocity and polarization, and the mode conversion of surface waves by Earth’s rotation by extending the formalism of Kennett for surface wave mode conversion due to lateral heterogeneity to include the Coriolis force. The wavenumber of Rayleigh waves is changed by Earth’s rotation and Rayleigh waves acquire a transverse component. The wavenumber of Love waves in not affected by Earth’s rotation, but Love waves acquire a small additional Rayleigh wave polarization. In contrast to different Rayleigh wave modes, different Love wave modes are not coupled by Earth’s rotation. We show that the backscattering of surface waves by Earth’s rotation is weak. The coupling between Rayleigh waves and Love waves is strong when the phase velocities of these modes are close. In that regime of resonant coupling, Earth’s rotation causes the difference between the Rayleigh wave and Love wave phase velocities that are coupled to increase through the process of level-repulsion.

Local Coupling and Conversion of Surface Waves due to Earth’s Rotation. Part 2: Numerical Examples

Summary Rotation of the Earth affects the propagation of seismic waves. The global coupling of spheroidal and toroidal modes by the Coriolis force over time is described by normal-mode theory. The local action of the Coriolis force on the propagation of surface waves can be described by coefficients for the coupling between propagating Rayleigh and Love waves as derived by (Landau & Lifshitz 1959). Using global wavefield simulations we show how the Coriolis force leads to coupling and conversion between both surface wave types depending on latitude, propagation direction, frequency, and local velocity structure. Surface wave coupling is most efficient for periods where the modes have similar phase velocities, a condition that is equivalent to the selection rules of the angular degree in the normal-mode framework, a phenomenon that we refer to as resonant coupling. In the time-domain, resonant coupling gradually converts energy from one wave type–Rayleigh waves or Love wave–into the other, which then propagates independently. Due to the lateral heterogeneity, the condition of equal phase velocity renders the rotational coupling location-dependent. East-west oriented ray path segments and segments at high latitudes (across the Poles) only weakly couple the fundamental mode Rayleigh and Love waves while coupling is strongest for propagation along the meridians across the equator. At 250 s period, where Love and Rayleigh waves have similar phase velocities, the net energy transfer from Rayleigh to Love wave reaches about 10% for one orbit.

Elastic Wave Energy Entrapment for Reflectionless Metasurface

This paper presents an elastic metasurface that can entrap the elastic wave energy of both longitudinal and shear wave modes in a lossy region, resulting in almost no reflection environment, which we call a reflectionless metasurface. Unlike previous attempts in acoustics, elastic waves are governed by different physics, such as mode conversion, and different behavior of shear waves. Accordingly, achieving a reflectionless elastic metasurface requires a different approach to that of previous studies on acoustic metasurfaces for sound absorption. Here, physics for entrapping the elastic wave energy is proposed by combining two phenomena: the local resonance of metasurface unit cells and the surface wave conversion. As a result, the elastic wave energy originating from an incident longitudinal wave can be highly concentrated in the metasurface, and energy dissipation can be significantly increased if any damping is implemented in the metasurface region. To realize the idea, a reflectionless metasurface, consisting of rod-shaped unit cells with various lengths, is designed. Numerical and experimental investigations show that the proposed reflectionless metasurface, with a small amount of loss, can effectively reduce both the reflected longitudinal and shear waves under longitudinal wave incidence with broad angles. We expect that the proposed idea, based on a thin artificial elastic layer, will provide an alternative route to design ultrasonic absorbers.

A review of exact solutions for conversion of a surface wave into a propagating wave

In this paper, we summarize our recent theoretical and numerical studies of conversion of a surface wave to a propagating wave (and vice versa) with maximized conversion efficiency. We show that the exact solution for the homogeneous plane wave to surface wave conversion in a homogeneous medium by a point-wise lossless metasurface is restricted by Maxwell’s equations. Such performance is possible only with approximations and implies some limitations, including impossibility to fully avoid unwanted scatterings. Nevertheless, an inhomogeneous plane wave can be converted into a surface wave perfectly, and it is described by an exact solution formed by a superposition of only two modes of the Floquet expansion. The results of the studies are expected to be of fundamental importance for leaky-wave antennas efficiency improvement. Besides, they can lead to novel applications in various research areas.

Exact Solution for Conversion of Surface Waves to Space Waves by Periodical Impenetrable Metasurfaces

We show that there exists an exact solution for a lossless and reciprocal periodic surface impedance which ensures full conversion of a single-mode surface wave propagating along the impedance boundary to a single plane wave propagating along a desired direction in free space above the boundary. In contrast to known realizations of leaky-wave antennas, the optimal surface reactance modulation which is found here ensures the absence of evanescent higher-order modes of the Floquet wave expansion of fields near the radiating surface. Thus, all the energy carried by the surface wave is used for launching the single inhomogeneous plane wave into space, without accumulation of reactive energy in higher-order modes. The results of the study are expected to be useful for creation of leaky-wave antennas with the ultimate efficiency, and, moreover, can potentially lead to novel applications, from microwaves to nanophotonics.

Manipulation of acoustic wavefront by transmissive metasurface based on pentamode metamaterials**Project supported by the National Natural Science Foundation of China (Grant Nos. 61571222 and 11474160), the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20161009), and the Six-Talent Peaks Project of Jiangsu Province, China.

An underwater acoustic metasurface with sub-wavelength thickness is designed for acoustic wavefront manipulation. In this paper, a pentamode lattice and a frequency-independent generalized acoustic Snellʼs law are introduced to overcome the limitations of narrow bandwidth and low transmittance. The bulk modulus and effective density of each unit cell can be tuned simultaneously, which are modulated to guarantee the achievement of refractive index profile and high transmission. Here, we actualize anomalous refraction, generation of non-diffracting Bessel beam, sub-wavelength flat focusing, and surface wave conversion by constructing inhomogeneous acoustic metasurface. This design approach has potential applications in medical ultrasound imaging and underwater acoustic communications.

Multifunctional Metasurfaces Based on the “Merging” Concept and Anisotropic Single-Structure Meta-Atoms

Metasurfaces offer great opportunities to control electromagnetic (EM) waves, attracting intensive attention in science and engineering communities. Recently, many efforts were devoted to multifunctional metasurfaces integrating different functionalities into single flat devices. In this article, we present a concise review on the development of multifunctional metasurfaces, focusing on the design strategies proposed and functional devices realized. We first briefly review the early efforts on designing such systems, which simply combine multiple meta-structures with distinct functionalities to form multifunctional devices. To overcome the low-efficiency and functionality cross-talking issues, a new strategy was proposed, in which the meta-atoms are carefully designed single structures exhibiting polarization-controlled transmission/reflection amplitude/phase responses. Based on this new scheme, various types of multifunctional devices were realized in different frequency domains, which exhibit diversified functionalities (e.g., focusing, deflection, surface wave conversion, multi-beam emissions, etc.), for both pure-reflection and pure-transmission geometries or even in the full EM space. We conclude this review by presenting our perspectives on this fast-developing new sub-field, hoping to stimulate new research outputs that are useful in future applications.

Negative reflection and negative surface wave conversion from obliquely incident electromagnetic waves

Complete control of spatially propagating waves (PWs) and surface waves (SWs) is an ultimate goal that scientists and engineers seek for, in which negative reflection of PW and negative surface wave are two exotic phenomena. Here, we experimentally demonstrate an anisotropic digital coding metasurface capable of controlling both PWs and SWs with a single coding pattern. On the basis of the digital description of coding metasurfaces, a simple coding method is proposed to allow dual functionalities (either PW or SW manipulations) under two orthogonal polarizations at arbitrarily oblique incidences, thus improving the adaptability of digital coding metasurfaces in more practical circumstances. With elaborately designed ellipse-shaped coding particles, we experimentally demonstrate various functions under oblique incidences, including the negative reflection of PW, negative SW, anomalous reflection and their arbitrary combinations, all having good agreements with theoretical and numerical predictions. We believe that the proposed method may enable the digital coding metasurfaces to have broad applications in radar detections, wireless communications and imaging.

Wavefront modulation and subwavelength diffractive acoustics with an acoustic metasurface.

Metasurfaces are a family of novel wavefront-shaping devices with planar profile and subwavelength thickness. Acoustic metasurfaces with ultralow profile yet extraordinary wave manipulating properties would be highly desirable for improving the performance of many acoustic wave-based applications. However, designing acoustic metasurfaces with similar functionality to their electromagnetic counterparts remains challenging with traditional metamaterial design approaches. Here we present a design and realization of an acoustic metasurface based on tapered labyrinthine metamaterials. The demonstrated metasurface can not only steer an acoustic beam as expected from the generalized Snell's law, but also exhibits various unique properties such as conversion from propagating wave to surface mode, extraordinary beam-steering and apparent negative refraction through higher-order diffraction. Such designer acoustic metasurfaces provide a new design methodology for acoustic signal modulation devices and may be useful for applications such as acoustic imaging, beam steering, ultrasound lens design and acoustic surface wave-based applications.

Wideband radar cross section reduction using two-dimensional phase gradient metasurfaces

Phase gradient metasurface (PGMs) are artificial surfaces that can provide pre-defined in-plane wave-vectors to manipulate the directions of refracted/reflected waves. In this Letter, we propose to achieve wideband radar cross section (RCS) reduction using two-dimensional (2D) PGMs. A 2D PGM was designed using a square combination of 49 split-ring sub-unit cells. The PGM can provide additional wave-vectors along the two in-plane directions simultaneously, leading to either surface wave conversion, deflected reflection, or diffuse reflection. Both the simulation and experiment results verified the wide-band, polarization-independent, high-efficiency RCS reduction induced by the 2D PGM.

Nonlocal effect of plasma resonances on the electron energy-distribution function in microwave plasma columns

Spatially resolved trace rare gases optical emission spectroscopy was used to analyze the electron energy-distribution function (EEDF) in low-pressure argon plasma columns sustained by surface waves. At frequencies >1 GHz, in the microwave-sustained region, the EEDF departs from a Maxwellian, characterized by a depletion of low-energy electrons and a high-energy tail, whereas in the field-free zone, the EEDF is Maxwellian. Abnormal behavior of the EEDF results from the acceleration of low-energy electrons due to the conversion of surface waves into volume plasmons at the resonance point where the plasma frequency equals the wave frequency and their absorption by either collisional or Landau damping.

Surface wave conversion analysis on a lengthwise soldered circular cylindrical shell

This paper deals with wave conversion phenomena through a study of the acoustic scattering from a stiffened cylindrical shell at normal incidence. The analysis presented follows the experimental study which explored the acoustic wave propagation and scattering processes in the case of air-filled submerged cylindrical shells having internal axial solder [J. Chiumia, N. Touraine, D. Decultot, G. Maze, A. Klauson, and J. Metsaveer, J. Acoust. Soc. Am. 105, 183-193 (1999)]. The significant observed phenomena were generation, reflection, and conversion of circumnavigating waves at the solder. The present work confirms the presence of the three phenomena through a theoretical approach based on the elasticity theory. In particular, resonances with new feature whose frequencies are very close to those of the S(0) wave are highlighted here. The origin of this new resonance feature is identified and can be associated to wave type conversions between A(0) and S(0) Lamb waves, occurring when these propagating waves encounter the solder. The air-filled stainless steel studied tube submerged in water has an internal lengthwise solder, which is, in theoretical computations, considered as an internal axial mass layer. The reduced frequency range (k(1)a) of the study is between 25 and 90, (k(1): wave number in water; a: external radius of the shell).

Scattered Surface Wave Energy in the Seismic Coda

One of the many important contributions that Aki has made to seismology pertains to the origin of coda waves (Aki, 1969; Aki and Chouet, 1975). In this paper, I revisit Aki's original idea of the role of scattered surface waves in the seismic coda. Based on the radiative transfer theory, I developed a new set of scattered wave energy equations by including scattered surface waves and body wave to surface wave scattering conversions. The work is an extended study of Zeng et al. (1991), Zeng (1993) and Sato (1994a) on multiple isotropic-scattering, and may shed new insight into the seismic coda wave interpretation. The scattering equations are solved numerically by first discretizing the model at regular grids and then solving the linear integral equations iteratively. The results show that scattered wave energy can be well approximated by body-wave to body wave scattering at earlier arrival times and short distances. At long distances from the source, scattered surface waves dominate scattered body waves at surface stations. Since surface waves are 2-D propagating waves, their scattered energies should in theory follow a common decay curve. The observed common decay trends on seismic coda of local earthquake recordings particular at long lapse times suggest that perhaps later seismic codas are dominated by scattered surface waves. When efficient body wave to surface wave conversion mechanisms are present in the shallow crustal layers, such as soft sediment layers, the scattered surface waves dominate the seismic coda at even early arrival times for shallow sources and at later arrival times for deeper events.

Empirical and Numerical Modeling of T-phase Propagation from Ocean to Land

T-phase propagation from ocean onto land is investigated by comparing data from hydrophones in the water column with data from the same events recorded on island and coastal seismometers. Several events located on Hawaii and the emerging seamount Loihi generated very large amplitude T phases that were recorded at both the preliminary IMS hydrophone station at Point Sur and land-based stations along the northern California coast. We use data from seismic stations operated by U. C. Berkeley along the coast of California, and from the PG&E coastal California seismic network, to estimate the T-phase transfer functions. The transfer function and predicted signal from the Loihi events are modeled with a composite technique, using normal mode-based numerical propagation codes to calculate the hydroacoustic pressure field and an elastic finite difference code to calculate the seismic propagation to land-based stations. The modal code is used to calculate the acoustic pressure and particle velocity fields in the ocean off the California coast, which is used as input to the finite difference code TRES to model propagation onto land. We find both empirically and in the calculations that T phases observed near the conversion point consist primarily of surface waves, although the T phases propagate as P waves after the surface waves attenuate. Surface wave conversion occurs farther offshore and over a longer region than body wave conversion, which has the effect that surface waves may arrive at coastal stations before body waves. We also look at the nature of T phases after conversion from ocean to land by examining far inland T phases. We find that T phases propagate primarily as P waves once they are well inland from the coast, and can be observed in some cases hundreds of kilometers inland. T-phase conversion attenuates higher frequencies, however we find that high frequency energy from underwater explosion sources can still be observed at T-phase stations.