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We numerically and experimentally report the acoustic analogue of the super-Klein tunneling in a heterojunction of phononic crystals formed with Willis scatterers that exhibit pseudospin-1 Dirac cones. By comparing with the pseudospin-1/2 Dirac cones, pseudospin-1 ones require in the band structure an additional flatband across the Dirac points. The conventional Klein tunneling, which is predicted in pseudospin-1/2 systems like graphene, consists of perfect transmission only under normal incidence through a potential barrier of any width. However, the super-Klein tunneling that we evidence here is defined for pseudospin-1 systems as a perfect transmission for all incidence angles at one single frequency within the energy barrier. This direct observation may have important implications in the exploration of the rich physics of pseudospin-1 quasiparticles.

Skyrmions with topologically stable configuration have shown a promising route toward high-density magnetic and photonic information processing due to their defect-immune and low-driven energy. Here, we experimentally report and observe the existence of phononic skyrmions as new topological structures formed by the three-dimensional hybrid spin of elastic waves. We demonstrate that the frequency-independent spin configuration leads to ultra-broadband feature of phononic skyrmions, which can be produced in any solid structure, including chip-scale ones. We further experimentally show the excellent robustness of the flexibly movable phononic skyrmion lattices against local defects of disorder, sharp corners, and even rectangular holes. Our research opens a vibrant horizon toward an unprecedented way for elastic wave manipulation and structuration by spin configuration and offers a promising lever for alternative phononic technologies, including information processing, biomedical testing, and wave engineering.

Classical designs of acoustic meta-absorber usually have a trade-off between bandwidth, efficiency and thickness. Here, we introduce the concept of nonlocal acoustic metasurface absorber by using a bridge structure connecting resonating unit cells to improve the performances of the meta-absorber. By utilizing the coupling effect between the adjacent unit cells, ultra-broadband sound absorption is achieved with deep-wavelength thickness. The physical mechanism of the nonlocal acoustic metasurface absorber is investigated by developing analytical models. We theoretically and numerically study the nonlocal metasurface with connecting bridge and the traditional metasurface without bridge. The nonlocality can introduce three specific effects: 1. Optimizing of effective acoustic impedances. 2. Shift of Fabry-Perot resonant frequencies. 3. Strengthening of the coupling effects between adjacent unit cells. These effects help to improve the bandwidth and the efficiency of the acoustic meta-absorber. We numerically and experimentally achieve an averaged absorption coefficient larger than 0.9 within the ultra-broadband bandwidth from about 600 Hz to 2600 Hz, with a sample thickness of 6.8 cm, , /9 for the lowest frequency. Our finding demonstrates the advantage of non-local acoustic metasurface to conceive subwavelength sound meta-absorber.

Seismic metamaterials (SMs) are expected to assist or replace traditional isolation systems owing to their strong attenuation of seismic waves. In this paper, a one-dimensional inverted T-shaped SM (1D ITSM) with an ultra-wide first bandgap (FBG) is proposed. The complex band structures are calculated to analyze the wave characteristics of the surface waves in the SMs. We find that the FBG of the 1D ITSM is composed of two parts; part 1 with surface evanescent waves and part 2 with no surface modes. Similar results are found in the complex band structure of the FBG of the SM consisting of periodically arranged pillars and substrate. The propagation of seismic surface waves in the 1D ITSM is different in these two frequency ranges of the FBG. In part 1, the seismic surface waves are significantly attenuated in the 1D ITSM, while in part 2, the surface waves are converted into bulk waves. Finally, the ultra-wide FBG is verified by using a kind of the two-dimensional ITSM in large-scale field experiments., 30 pages,15 figures

International audience

In last two decades, it has been theoretically and experimentally demonstrated that seismic metamaterials are capable of isolating seismic surface waves. Inertial amplification mechanisms with small mass have been proposed to design metamaterials to isolate elastic waves in rods, beams, and plates at low frequencies. In this Letter, we propose an alternative type of seismic metamaterial providing an ultra-low-frequency bandgap induced by inertial amplification. A unique kind of inertially amplified metamaterial is first conceived and designed. Its bandgap characteristics for flexural waves are then numerically and experimentally demonstrated. Finally, the embedded inertial amplification mechanism is introduced on a soil substrate to design a seismic metamaterial capable of strongly attenuating seismic surface waves around a frequency of 4 Hz. This work provides a promising alternative way to conceive seismic metamaterials to steer and control surface waves.

Nonreciprocal mechanical devices are of great interest for directional elastic wave manipulation. In this Letter, we introduce a design of a compact low-frequency nonreciprocal metamaterial for flexural waves, whose dimension is less than [Formula: see text] of the operating wavelength. This structure is made of two well-placed coil-cantilever-magnet resonators, where the electromagnetic forces can be temporally modulated, which enables time varying of the effective stiffness of the resonators. A phase shift is introduced between the stiffness modulations of these two resonators, which breaks the time-reversal symmetry and enables nonreciprocal wave propagation at the resonance frequency of the structure. A semi-analytical method based on harmonic wave decomposition is developed to describe the system, leading to results that match well with numerical predictions from a finite element method. We also experimentally demonstrate nonreciprocal flexural wave propagation with good agreement with the predictions made. Our system could inspire the design of compact nonreciprocal devices for flexural waves.

Classical meta-absorber designs usually have a tradeoff between bandwidth, efficiency, and thickness. Here, we introduce the concept of a nonlocal acoustic metasurface absorber by using a bridge structure connecting resonating unit cells to improve the performances of the meta-absorber. By utilizing the coupling effect between the adjacent unit cells, ultrabroadband sound absorption is achieved with deep-wavelength thickness. The physical mechanism of the nonlocal acoustic metasurface absorber is investigated and uncovered by developing analytical models. We theoretically and numerically study the nonlocal metasurface with a connecting bridge and the traditional metasurface without a bridge. The nonlocality can introduce three specific effects: the optimization of effective acoustic impedances, the shift of Fabry-Perot resonant frequencies, and the enhancement of the coupling effects between adjacent unit cells. These effects contribute to improve the bandwidth and the efficiency of the acoustic meta-absorber. We numerically and experimentally achieve an average absorption coefficient larger than 0.9 within the ultrabroadband bandwidth extending from about 600 to 2600 Hz, with a metasurface of 6.8 cm, viz., \ensuremath{\lambda}/9 for the lowest frequency. Our finding demonstrates the advantage of nonlocal acoustic metasurface to conceive a subwavelength sound meta-absorber.

In engineering, plate structures are one type of the main load-bearing structures. However, most of the designed plate-like metamaterials/metasurfaces need to be heavily grooved or drilled on the original plate surface, which will inevitably cause some destruction to the strength and stiffness of the host plate structures. To overcome this weakness, we propose a new conceptual design of pillared elastic metasurface (~0.495λ) to manipulate flexural waves in plates by considering constructive interference. The interference, which manifests itself as the coupling through out-of-plane vibration of the plate for the two adjacent subunits, is analyzed to reveal the physical mechanisms. In addition, we reveal the mechanism of the phase shift of transmitted waves across the sub-wavelength subunits and establish an analytical model for the multi-resonator subunits to accurately predict the phase shift and amplitude of the transmitted wave. We theoretically design and experimentally demonstrate the deflecting and focusing functionalities of the proposed elastic metasurface. Our design can provide a new route to broad applications of the constructive interference in elastic metamaterials/metasurfaces, which can be used to efficiently engineer arbitrary wave profiles.

International audience; The present work deals with the synthesis of crystalline carbon nitride thin films by microwave plasma assisted chemical vapour deposition in N2/CH4 gas mixture. The discharge analysis by optical emission spectroscopy shows that the increase in the CH4/N2 ratio involves an important production of the CN and C2 radicals. In the films X-ray energy dispersion spectroscopy shows that the N/C ratio decreases when the CH4 percentage in N2 increases. Xray diffraction and electron diffraction are used to study the carbon nitride films nature. Scanning electron microscopy shows that the films consisted of nano-crystalline grains. Carbon balls are also present on the film surface for CH4 percentage higher than 4%. The transmission electron microscopy confirms the nano-structure of the film and shows the isotropic etching of the substrates, during the film growth.

International audience; Semi-infinite elastic metamaterials and metasurfaces on homogeneous elastic half-spaces have attracted significant attention in the past two decades as efficient artificial structures to control and mitigate surface waves. In this research, the first two attenuation zones of an elastic metasurface composed of different arrangements of pillars on a substrate are investigated. First, the lowest frequency attenuation zone (LFAZ) is numerically investigated. Then, the rainbow trapping effect of the second attenuation zone (SAZ) is demonstrated by numerical simulations and experiments. The different characteristics of these two attenuation zones are brought together to connect the lower bound of the SAZ and the upper bound of the LFAZ, allowing to design a seismic metasurface that only uses one kind of artificial structure on a half-space to attenuate Rayleigh waves in the frequency range extending from 5 to 22 Hz.

The partial or complete confinement of waves in an open system is omnipresent in nature and in wave-based materials and technology. Here, we theoretically analyze and experimentally observe the formation of a trapped mode with perfect mode conversion (TMPC) between flexural waves and longitudinal waves, by achieving a quasi-bound state in the continuum (BIC) in an open elastic wave system. The latter allows a quasi-BIC in a semi-infinite background plate when Fano resonance hybridizes flexural and longitudinal waves and balances their radiative decay rates. We demonstrate that when the Fabry-P\'erot resonance of the longitudinal wave is realized simultaneously, the TMPC formed by the elastic BIC approaches infinite quality factor. Furthermore, we show that quasi-BIC can be tuned continuously to BIC through the critical frequency of mode conversion, which offers the possibility of TMPC with an arbitrarily high quality factor. Our reported concept and physical mechanism open new routes to achieve perfect mode conversion with tunable high quality factor in elastic systems., Comment: 6 figures, 25 pages

International audience; In this work, a tunable fish-bone elastic metasurface is used to manipulate the in-plane waves which are more complex than flexural or out-of-plane waves because of the coupling of the P-and SV-waves. First, the refracted generalized Snell's law (GSL) for mixed in-plane wave is deduced which is also testified by the numerical results. Then according to the GSL, the elastic metasurfaces are designed to realize multifunctions such as directional refraction and wave focusing for different working frequencies. The critical refraction is discussed for the directional refraction and the behavior of SV-wave is predicted when the P-wave is designed to focus at any point by the metasurface.

A broadband elastic wave absorption by a sub-wavelength and lightweight structure is of considerable significance in vibration suppression, especially for low frequencies in plate-like structure. However, it has always been a great challenge. In this research, we systematically study the flexural wave diffraction in a thin plate. Based on the diffraction mechanism, we propose the concept of sub-wavelength lossy gradient elastic metasurface for flexural wave absorption. We theoretically reveal high-efficiency and quasi-omnidirectional absorption behavior, which stem from maximum multireflection-enhanced absorption of the 0th order diffraction. We experimentally demonstrate a robust high-efficiency absorption in the frequency range from 343 to 1000 Hz (larger than 1.5 octaves). In addition, we propose a general approach which involves new physics of adjusting an arrangement sequence of subunits to suppress the first-order diffraction mode. This allows to further reduce the sub-wavelength thickness of the metasurface while maintaining its high-efficiency absorption. Our designs could provide new routes to broadband vibration suppression and cancelation in low frequency by lossy elastic metamaterials and metasurfaces.

International audience; Acoustic equalization is the process of adjusting the frequency response of a broadband sound signal, which is widely used in communication system and audio acoustics. Here, we introduce the unique concept of acoustic meta-equalizer, viz., a passive acoustic metamaterial-based filter that has tunable frequency response within ultra-broadband (200Hz-20000Hz), and capable to generate a frequency equalization on the input sound signal. By exploiting two kinds of acoustic resonant elements, viz., Helmholtz and Fabry-Perot resonators, we realize and demonstrate ultra-broadband and tunable filters, within an integrated meta-structure. In analogy with the conventional equalizer concept in signal processing, we numerically, analytically, and experimentally demonstrate functional filters, signal reproductions, and sound effect controls by the conceived acoustic meta-equalizer.

International audience; A severe limitation of current acoustic metasurfaces remains in their modest tunability to meet multi-frequency requirements and alterable functionalities on demand. Here, a reconfigurable curved acoustic metasurface for acoustic cloaking and illusion is reported. The structure is composed of an array of tunable helical units to break this limitation and realize continuously versatile sound manipulations. We theoretically, numerically and experimentally investigate the channel length represented by the helical depth, which is used to achieve full 2π phase shift continuously over the frequency range from 2 to 7 kHz. As pragmatic examples, we present by full-wave numerical simulations the concept of a curved metasurface for the continuously tunable acoustic multifunction, including broadband carpet cloaking and ground illusion at a wide working band. Then, we experimentally demonstrate these functionalities by showing an excellent effect to restore the disturbed reflective field from a cloaked object or to mimic an arbitrary shaped ground.

International audience; The discovery of the disorder effect in traditional metamaterials has opened the possibilities in the search for the disordered metasurfaces. Photonic, dielectric and elastic metamaterials exhibiting the added value of the disorder effect on wave propagation physics, have been reported. Despite this extensive attention and progress in disordered metamaterials, the elastic metasurfaces however, involving disorder have not yet been reported. Here, we introduce the concept of disordered elastic metasurface composed of identical pillared resonators with a random arrangement in the subwavelength range. Based on theoretical formalism and direct acoustic measurement, we observe anomalous deflection and focusing effects of flexural waves in a plate, and elucidate in details the related physics. This research extends the disorder effect to metasurfaces and may lead to innovative acoustoelastic devices.

Conventional vibration absorber is challenging to use in the extreme environment of high (low) temperature, due to the low tolerance of its additional damping material to temperature. To adapt to the extreme environment, here we propose an elastic meta-absorber (EMA) based on the quasi-bound states of the continuum (BICs) physical approach. The proposed absorber does not have any additional damping material, depends only on the extreme-low structural loss, and is capable of achieving perfect absorption of the flexural waves propagating in an elastic beam. The proof-of-concept we provide consists of three parallel subwavelength sub-beams sharing a single-port radiating channel in a host beam, and supports elastic Friedrich–Wintgen quasi-BICs. Based on the latter, the subwavelength EMA can achieve a high quality factor in the elastic wave system to enhance, in an unprecedented way, the wave energy dissipation. Using this physical mechanism, we experimentally and numerically validate and demonstrate the perfect absorption of flexural waves at subwavelength regime. Our work opens a new route to deal with vibration absorption with subwavelength structures in specific and extreme environments.

International audience; We theoretically and experimentally propose two designs of broadband low-frequency acoustic metasurface coiled absorbers (Sample I/Sample II) for the frequency ranges of 460 Hz–972 Hz and 232 Hz–494 Hz (larger than 1 octave), with absorption larger than 0.8 (average is 0.87), and having the ultra-thin thickness of 4.5 cm and 9 cm respectively (λ/17 for the lowest working frequency and λ/8 for the highest frequency). The designed supercell consists of 16 different unit cells corresponding to 16 eigen frequencies for resonant absorptions. The coupling between multiple resonances leads to broadband absorption effect in the full range of the targeted frequency spectrum. In addition, the coiling structure lead to the ultra-thin thickness of the metasurface absorbers. Our conceived ultra-thin low-frequency broadband absorbers may lead to pragmatic implementations and applications in noise control field.

International audience; A meta-slab composed of subunits with graded thicknesses is proposed for spatially separating the zeroth- and first-order acoustic waveguide modes according to the generalized Snell's law. The phase velocity for the first-order mode depends on the waveguide thickness, whereas that for the zeroth-order mode does not. This enables the required phase-shift gradients of the meta-slab to be obtained for spatially separating the two waveguide modes. To design the meta-slab, analytic solutions are derived for the transmission coefficients and phase shifts of the incident acoustic waves with the two modes propagating through the subunits. Numerical simulations with transient finite-element analyses are implemented to demonstrate the propagation of the two waveguide modes. Sinusoidal signal and tone-burst excitations are applied in the analyses. The latter case shows that the two waveguide modes can be successfully separated in space by the designed meta-slab.

International audience; Creating complex spatial objects from a flat sheet of material using origami folding techniques has attracted attention in science and engineering. We design, model, and fabricate reconfigurable origami-inspired metamaterials whose unit cells can be easily tailored into numerous specific shapes. Via combinations of different shapes of the unit cell in the metamaterial panel, we numerically and experimentally demonstrate various specific sound wave manipulations, including acoustic focusing, beam splitting, sound localization, and one-way transmission. Our design features simple configuration, tunable structure, multiple functionalities, and is light weight. This finding provides an alternative route for controllable sound manipulation, paving the way toward a reconfigurable functional metadevice based on origami-inspired metamaterials.

Acoustic metasurfaces, generally comprising several types of localized meta-atoms, draw great interest for their flexibility in acoustic field manipulation. However, the unavoidable $n\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l$ coupling between meta-atoms leads to lackluster performance for large-angle refraction. By embracing rather than ignoring nonlocality and periodicity, this research shows that acoustic metagratings can almost perfectly bend sound for very large angles (g80\ifmmode^\circ\else\textdegree\fi{}), with up to 95% transmission. Besides efficiency, the suitably treated nonlocality provides another degree of freedom to manipulate sound propagation, extending the realm of acoustic metasurfaces and promoting innovative techniques.

International audience; We demonstrate multifunctional acoustic metasurfaces that can simultaneously realize the same functionality or multiple different functionalities at multiple tunable frequencies. The fundamental physical mechanism is based on designing a supercell of Helmholtz resonators with multiple resonances operating at different frequencies. We theoretically, numerically, and experimentally demonstrate the achievement of multiple functionalities by the proposed designed metastructure, which produces extraordinary reflection at different angles and different acoustic focusing localizations under the same normal incidence. Our finding paves the way towards multifunctional compact acoustic devices and can lead to pragmatic contemporary applications such as multiple beam shaping, or functional devices with special dispersion properties.

International audience; We propose a multiplexed-acoustic-metasurface hologram that can project multiple predesigned images at different frequencies. We demonstrate the generation of fourfold acoustic holographic images by one metasurface at 3500, 4500, 5500, and 6500 Hz, respectively, and discuss their fundamental physics. We uncover the design of this acoustic hologram which simultaneously realizes multiplexing and modulations of amplitude and phase. The proposed metasurface unit cell is composed of multiple acoustic resonant cavities leading to the multiplexing effect. We consider here both leaky and intrinsic losses, which provide simultaneous controllable manipulations of amplitude and phase. These proposed strategies significantly increase the degree of freedom of the classical metasurfaces, which may open up additional avenues for multifunctional compact acoustic devices.

International audience; We theoretically and experimentally investigate a helical acoustic metasurface capable of providing a modulated sound-reflected wavefront and a continuously tunable broadband feature. Compared to existing metasurface designs, which mostly manipulate acoustic waves in a fixed or narrow frequency range, the matched screw-and-nut mechanism has added value and unusual properties, opening a realistic solution to achieve broadband tunability of acoustic metasurfaces with multifunctions. The conceived tunable metasurface is composed of identical units, which consist of matched screws rotated into helical channels from bottom to top to modulate the sound-reflected wavefront pattern. As a pragmatic example, we present the design details and functionalities of a flat metasurface operating at a wide working band. Then a sample of the metasurface is manufactured with the three-dimensional (3D)-printed cylindrical units to experimentally demonstrate the continuously tunable multifunction, including anomalous reflection, arbitrary focusing, and self-bending beams. All results are in good agreement. The proposed spiral matching concept could open up an alternative avenue for broadband tunable metasurfaces of reflected wavefront modulations and associated applications.

International audience; Acoustic perfect absorption via a structure with deep subwavelength thickness is of great and continuing interest in research and engineering. This study analytically and experimentally investigates acoustic systems based on Helmholtz resonators which have embedded-apertures. The strategy of embedding apertures greatly improves the ability to manipulate the impedance of the systems. Based on the inverted configuration, perfect absorption has been realized (reaching 0.999 in experiments) via a design whose thickness is only ∼1/50th of the operating wavelength. Moreover, a tunable resonant frequency (137–300 Hz) and tunable absorption frequency bandwidth (22%–46%) can be achieved while preserving the perfect absorption performance and constant external shape. In tuning the perfect absorbers having a constant thickness, a conservation factor is revealed experimentally and then verified analytically, which could guide absorbers' design and facilitate the tuning. In addition, the distinct features of the proposed design were evaluated and validated and were compared with those of a related structure, a metasurface with a coiled backing cavity. The results have the potential to help with the design of highly efficient, thin, and tunable acoustic absorbers.

We introduce a multi-coiled acoustic metasurface providing a quasi-perfect absorption (reaching 99.99% in experiments) at extremely low-frequency of 50 Hz, and simultaneously featuring an ultrathin thickness down to {\lambda}/527 (1.3 cm). In contrast to the state of the art, this original conceived multi-coiled metasurface offers additional degrees of freedom capable to tune the acoustic impedance effectively without increasing the total thickness. We provide analytical derivation, numerical simulation and experimental demonstrations for this unique absorber concept, and discuss its physical mechanism which breaks the quarter-wavelength resonator theory. Furthermore, based on the same conceptual approach, we propose a broadband lowfrequency metasurface absorber by coupling unit cells exhibiting different properties.

International audience; In this paper, we report on the use of tantalum and iridium as adhesion layers for platinum electrodes used in high temperature SAW devices based on langasite substrates (LGS). Unlike iridium, tantalum exhibits a great adhesive strength, and a very low mobility through the Pt film, ensuring a device lifetime of at least half an hour at 1000°C. The latter is limited by morphological modifications of platinum, starting by the apparition of crystallites on the surface, and followed by important terracing and breaking of the film continuity. SNMS and XRD measurements allowed us to show that these phenomena are likely intrinsic to platinum film, whatever be the nature of the adhesion layer. Finally, after having outlined a possible scenario leading to this deterioration, we consider some solutions that could replace platinum in order to increase the lifetime of LGS-based SAW devices in high temperatures conditions.

In this work, we analytically and experimentally present perfect acoustic absorbers via spiral metasurfaces composed of coiled channels and embedded apertures. Perfect absorption (reaching 0.999 in experiments) is realized with an ultra-thin thickness down to ∼1/100th of the operating wavelength. Owing to the superior impedance manipulation provided by the embedded apertures, perfect absorption with tunable frequencies is demonstrated. Our results would contribute to paving a way towards designing thin and light absorbers for the low frequency absorption challenge.

International audience; Acoustic metasurfaces derive their characteristics from the interaction between acoustic waves and specifically designed materials. The field is driven by the desire to control acoustic wave propagation using compact devices and is governed by fundamental and physical principles that provide the design rules and the functionality of a wave. Acoustic metasurfaces have added value and unusual functionalities compared with their predecessor in materials science, namely, acoustic metamaterials. These rationally designed 2D materials of subwavelength thickness provide a new route for sound wave manipulation. In this Review, we delineate the fundamental physics of metasurfaces, describe their different concepts and design strategies, and discuss their functionalities for controllable reflection, transmission and extraordinary absorption. In particular, we outline the main designs of acoustic metasurfaces, including those based on coiling-up space, Helmholtz-resonator-like and membrane-type structures, and discuss their applications, such as beam focusing, asymmetrical transmission and self-bending beams. We conclude with an outlook of the future directions in this emerging field.

Acoustic holograms have promising applications in sound-field reconstruction, particle manipulation, ultrasonic haptics and therapy. This paper reports on the theoretical, numerical, and experimental investigation of multiplexed acoustic holograms at both audio and ultrasonic frequencies via a rationally designed transmission-type acoustic metamaterial. The proposed meta-hologram is composed of two Fabry-Perot resonant channels per unit cell, which enables the simultaneous modulation of the transmitted amplitude and phase at two desired frequencies. In contrast to conventional acoustic metamaterial-based holograms, the design strategy proposed here, provides a new degree of freedom (frequency) that can actively tailor holograms that are otherwise completely passive and hence significantly enhances the information encoded in acoustic metamaterials. To demonstrate the multiplexed acoustic metamaterial, we first show the projection of two different high-quality meta-holograms at 14 kHz and 17 kHz, with the patterns of the letters, N and S. We then demonstrate two-channel ultrasound focusing and annular beams generation for the incident ultrasonic frequencies of 35 kHz and 42.5 kHz. These multiplexed acoustic meta-holograms offer a technical advance to tackle the rising challenges in the fields of acoustic metamaterials, architectural acoustics, and medical ultrasound.

We provide theoretical and numerical analyses of the behavior of a plate-type acoustic metamaterial considered in an air-borne sound environment in view of sound mitigation application. Two configurations of plate are studied, a spring-mass one and a pillar system-based one. The acoustic performances of the considered systems are investigated with different approaches and show that a high sound transmission loss (STL) up to 82 dB is reached with a metamaterial plate with a thickness of 0.5 mm. The physical understanding of the acoustic behavior of the metamaterial partition is discussed based on both air-borne and structure-borne approaches. Confrontation between the STL, the band structure, the displacement fields and the effective mass density of the plate metamaterial is made to have a complete physical understanding of the different mechanisms involved.

Metasurface-based acoustic hologram projectors fabricated with fixed microstructures can only generate the predesigned images at a single or few discrete frequencies. Here, a variety of acoustic holographic applications can be realized in broadband by a matched helical design of the tunable lossy acoustic metasurface (TLAM). The proposed TLAM unit is composed of a grating channel and an adjustable internal absorber to achieve the independent amplitude and phase modulations (APM) in a continuous frequency range. We demonstrate the excellent performance of the scattering-free anomalous refection by the APM method for tuning loss without foam materials. Then, the multi-plane acoustic holograms and the broadband holographic images are demonstrated by the flexible reconfigurations of one designed TLAM. Due to the compact design and the great flexibility, this proposal may be more practical to achieve the high-quality holograms with multi-scale fine manipulation and multiplexed acoustic communication with high information content.

International audience; A subwavelength seismic metamaterial (SMs) consisting of a three-component SM plate (SMP) and a half space is proposed to attenuate ultra-low frequency seismic surface waves. The design concept and models are verified firstly by lab-scale experiments on the SM consisting of a two-component SMP and a half space. Then we calculate the band structures of the one-dimensional and two-dimensional subwavelength SMs, and evaluate their ability to attenuate Rayleigh waves. A wide ultra-low frequency band gap can be found. And the Rayleigh waves are deflected by the subwavelength SM and converted into bulk waves in the frequency range of this band gap. When the number of the unit cells of the subwavelength SM is sufficient, the transmission distance and deflection angle of the Rayleigh waves are constant at the same frequency. This discovery is expected to open up the possibility of pragmatic 2 seismic protection for large nuclear power plants, ancient buildings and metropolitan areas.