Dr Bart Van Damme, Empa, Duebendorf (CHE)
HOST: Camille Perrot, Université Paris-Est Marne-la-Vallée (FRA)
STSM Period: 2018-11-05 – 2018-11-23
Prediction of sound absorption of microperforated rigid foams
Closed-cell rigid foams have poor sound absorption efficiency due to the lack of pore interconnections. Only the microporosity of the matrix leads to viscous losses, but this effect is small due to the very high flow resistivity. Larger interconnections can be introduced by perforating the foams using needles or drills with a diameter smaller than the pore size. Impedance tube measurements have shown that perfect absorption can be reached for a good pore size-perforation ratio.
In rigid foams, sound absorption is purely an effect of the viscosity and thermal conduction of air, since the matrix is too stiff to absorb any energy. Several semi-phenomenological models have been proposed in order to predict absorption and reflection properties based on considering the material as an equivalent fluid with a complex, frequency dependent density and bulk modulus. The input parameters of these models can be derived from finite element models of an idealized unit cell of the perforated foams. This approach allows to define an ideal combination of pore size, perforation diameter and perforation rate to achieve a desired absorption profile
Dr. Camille Perrot, Université Paris-Est, MSME UMR 8208 CNRS (FR)
HOST: Pr. Peter Göransson, MWL, KTH Royal Institute of Technology, Stockholm (SWE)
STSM period: 2018-10-15 – 2018-10-26 (12 days)
Identification of representative volume elements of glass wool panels: Coupling deterministic and stochastic approaches
The scientific project of this Short Term Scientific Mission (STSM) was to link multi-scale deterministic approaches for the modeling of elasto-acoustic coupling of fibrous media to stochastic approaches. The originality lies in this coupling of deterministic / stochastic approaches and in the contribution of stochastic approaches where there is no separation of scale. The key steps proposed during this STSM to address the issue of linking bottom-up with top-down approaches within the framework of stochastic modeling and MaxEnt principle were the following : (1) Measurements of local displacement fields for the identification of frequency-dependent viscoelastic properties; collaboration with L. Manzari and Pr. P. Göransson (MWL). (2) Identification of a mesoscopic stochastic model of apparent fields of random properties; collaboration with Pr. C. Desceliers and Pr. C. Soize (MSME).
Dr. Ll. M. Garcia-Raffi, IUMPA-Universitat Politècnica de València (ESP)
Host name: Vicent Romero Garcia, Laboratoire d’Acoustique de l’Université du Mans (LAUM), France
STSM Period: 2018-09-19 – 2018-09-29 (11 days)
2D Hyperuniform Acoustic Metamaterials
Disordered media are also good alternatives to control wave scattering in a different manner as ordered ones. The theory of hyperuniform acoustic media was defined by professor S. Torquato and co-authors in the context of material science and has been used to control the overall scattering strength of the material. Disordered hyperuniform systems are like perfect crystals in the sense that they suppress large-scale density fluctuations but they are statistically isotropic with no Bragg peaks. Hyperuniform materials belong to a subset of a bigger category known as stealth materials. The key point is the use of the structure factor, defined in terms of the positions of the scatterers and vectors in the reciprocal space. In this Short-Term Scientific mission has been obtained the theoretical formulae to extend the notion of hyperuniform medium from 1D to 2D. We have designed all the numerical tests, including the optimization process necessary for obtaining a hyperuniform structure and also, analyse the possible experimental studies for the extension of the results obtained from 1-D to 2-D heterogeneous acoustic systems. This particular case of 2D is especially important because hyperuniformity can allow us to design disordered highly compact isotropic medium that present broadband transparency.
Obioma U. Uche, Frank H. Stillinger, and Salvatore Torquato. “Constraints on collective density variables: Two dimensions”. Phys Rev E 70, 046122 (2004).
Cecilia Casarini, University of Strathclyde (UK)
HOST: Vicente Romero García, LAUM, Le Mans (FRA)
STSM Period: 2018-02-25 – 2018-03-03 (7 days)
Characterization of 3D printed membranes for noise control applications
This STSM aimed at characterizing thin viscoelastic membranes through impedance tube measurements. A novel 3D printing fabrication technique was used to manufacture the membranes, that allowed us to better control thickness, density and Young’s Modulus of the materials used. The results are promising, as resonance frequencies as low as 300 Hz could be achieved in membranes 70 μm thick and having a diameter of 15 mm. The outcomes of this work could be used to build acoustic metamaterials for low-frequency sound absorption in small-scale electroacoustic devices.
Stefan Dimitrijevic, University Of Belgrade, School Of Electrical Engineering Belgrade (SRB)
HOST: José Sánchez-Dehesa, professor at Universitat Politècnica de València, València (ESP)
STSM Period: 2018-03-03 – 2018-03-10 (7 days)
Analysis of in situ acoustical performance of sonic crystal barrier by using EN 1793-5 and EN 1793-6
The aim of this STSM was to characterize the sonic crystal barrier implemented in front of Universitat Politècnica de València which had been previously optimized to be efficient against traffic noise. Both EN 1793-6 sound insulation and EN 1793-5 sound reflection standardized measurement procedures were implemented in different scenarios – by using distances suggested by the standards and the distances in which both loudspeaker and microphone grid were equally distant from the barrier. Additionally, the influence of the slit under the barrier was investigated. All measurements are evaluated by using different windowing techniques to investigate influence of diffraction and ground reflection transmitted through the barrier.
Dr. Anastasiia O. Krushynska, Visiting researcher at Department of Civil, Environmental and Mechanical Engineering, University of Trento, Italy
Host: Dr. Jean-Philippe Groby, LAUM-UMR CNRS 6613, Le Mans (FRA)
STSM period: 2018-03-01 – 2018-03-23
Labyrinths of Helmholtz resonators for deep-subwavelength sound control
This project was aimed at theoretical, numerical, and experimental investigation of sound propagation in labyrinthine acoustic metamaterials with incorporated Helmholtz resonators (HRs). First, we have analyzed and understood how the channel coiling can be exploited to improve absorption of acoustic waves as compared to a straight channel of an equivalent length. Then, we have designed a sample geometry of a coiled metamaterial capable of perfectly absorbing sound waves at several frequencies in sub-wavelength frequency range. As follow-up, experimental tests of the designed samples are planned.
Matthieu Malléjac, LAUM – UMR CNRS 6613, Le Mans (FRA)
HOST: Dr. Johan Christensen, Universidad Carlos III de Madrid (ESP)
STSM Period: 2018-02-19 – 2018-03-15
Zero-effective mass density through a 1D periodic structure of thin clamped elastic plates
This STSM aimed at investigating the near-zero dynamic mass density regime and the acoustic super-squeezing effect, by analytically and numerically (COMSOL Multiphysics) modelling a so-called plate-metamaterials composed of a periodic network of thin elastic plates clamped in a circular waveguide. The effect of the arrangement properties (cavity length, plates mechanical properties, losses, number of unit cell,…) have been investigated so has to design a plate-metamaterial permitting to achieve super-squeezing effect with high transmittance.
Tao Yang, Technical University of Liberec (CZE)
HOST: Le Mans University (FRA)
STSM Period: 2018-02-15 – 2018-03-15
4 microphones impedance tube measurement and sound absorption prediction for perpendicular-laid polyester nonwoven materials
The main purpose of this STSM was to investigate the acoustic performance and non-acoustic parameters of perpendicular-laid polyester nonwovens. The non-acoustic parameters were obtained using inverse method based on the results of impedance tube measurement (4 microphone technique). The deduced porosities and airflow resistivities of polyester materials were compared with previously measured values. Also, the dependence of the acoustical performance on the three principal directions of polyester nonwovens has been investigated. The findings indicated that the different interface conditions at the two sides have a significant effect on the determination of thermal characteristic length. It was found that denser polyester materials are more isotropic than perpendicular-laid polyester nonwoven materials.
Dr. Jacques Cuenca, Research Engineer, Siemens Industry Software, Leuven (BEL)
HOST: Prof. Peter Göransson, KTH Royal Institute of Technology, Stockholm (SWE)
STSM Period: 2018-04-02 – 2018-04-20
Alternative methods for inverse estimation of porous material properties in complex structures
Direct measurement of porous material properties often requires the use of a dedicated setup for each of the unknown parameters. Model inversion frameworks are an attractive alternative, as they rely on operational observations and provide a consistent set of material properties within the chosen model. Nevertheless, several challenges arise in the model inversion process. Of particular interest is the non-uniqueness of the solution, which can arise as a consequence of structural complexity due to anisotropy, geometrical symmetries or inhomogeneity. An additional difficulty is the quantification of model and observation uncertainties. During my visit at KTH, we investigated ways of dealing with non-uniqueness and uncertainty in the case of coupled resonant problems involving sound-absorbing media. Both deterministic and probabilistic approaches to the problem were examined, in close collaboration with Dr. Timo Lähivaara from the University of Eastern Finland.
Mathieu Gaborit, MWL, KTH Royal Institute of Technology, Stockholm (SWE) and
LAUM UMR CNRS 6613, Le Mans Université, Le Mans (FRA)
HOST: Dr. Timo Lähivaara, Department of Applied Physics, University of Eastern Finland, Kuopio (FIN)
STSM Period: 2018-02-12 – 2018-03-11
Investigating noise modelling and extraction for the development of inverse characterisation methods
This STSM was about noise model extraction i.e. trying to use inversion or identification method to characterise the properties of random processes. Test rigs always taint the data they gather and being able to identify the actual effect of the process on the data is useful when developping new characterisation techniques (particularly inversion-based ones). During the STSM, several tools (neural nets & bayesian inversion) were investigated to mine simulated and mesured datasets and extract information about the generation/measurement setup.
Ms Ferina Saati Khosroshahi
HOST: Prof Kirill Horoshenkov, Department of Mechanical Engineering, University of Sheffield
STSM Period: 2018-01-21 – 2018-02-02
Comparison of inversion models for characterisation and uncertainty modeling of rigid frame porous materials
The aim of this STSM was to study and learn about the models regarding porous materials, computationally and empirically through semi-phenomenological models and the Padé approximation-based models (or pore size distribution models) using the more recent literature results. The acoustic impedance of several different stacks of several different glass beads were measured and the measurements were repeated, in some cases more than a hundred times. The measurement results were used as input to several inversion codes to evaluate the performance of the methods on samples. Other samples with various pore types such as foam and nonwoven fibrous were also chosen for the sake of comparison. The results suggest that the value of the impedance inverted with NUPSD model is more accurate than that predicted using JCA equations for the given samples.
Mr Matheus Pereira, University of Coimbra and Institute for Sustainability and Innovation in Structural Engineering (PRT)
HOST: Dr Jaime Ramis-Soriano, Department of Physics, systems engineering and theory, University of Alicante (ESP)
STSM Period: 2018-02-09 – 2018-03-09
Experimental evaluation and incorporation in equivalent fluid models of macroscopic parameters characterizing granular materials for noise control.
The aims of this STSM were the development of a study on the acoustical and non-acoustical characterization of consolidated porous concrete made of expanded clay granulates, and the incorporation of those properties in equivalent fluid models for metaporous concrete acoustical prediction. Metaporous concrete is a new concept of sound absorption material, being composed by the coupling between a porous material and an acoustic resonator. To validate the proposed methodology, we used experimental techniques to characterize the surface impedance, characteristic impedance and wave number of the studied materials. In addition, an inverse method based on Horoshenkov-Swfit model was used to obtain the macroscopic parameters. The influence of the particle grain size, quantity of cement and material thickness on sound absorption was also studied.
Dr Florian Allein, LAUM – UMR CNRS 6613, Le Mans (FRA)
HOST: Nicholas Boechler, Department of Mechanical Engineering, University of Washington (USA)
STSM Period: 2017-10-30 – 2017-11-14 (16 days)
Granular structures coupled to elastic substrates: nonlinear wave dynamics and attenuation of surface waves
This STSM aimed at improving the understanding of nonlinear wave propagation in granular-based structures taking into account different coupling parameter between the granular structure and a substrate. Theoretical and numerical analyses have been performed in order to investigate new wave phenomena such as nonlinear coupled transverse-rotational modes, and nonlinear zero-group velocity modes.
Dr Logan Schwan, LAUM – UMR CNRS 6613, Le Mans (FRA)
HOST: Andrew Norris, Rutgers University, Piscataway (USA)
STSM Period: 2017-08-10 – 2017-08-30 (21 days)
Asymptotic homogenisation of elastic metaporous materials with embedded resonant inclusions
This STSM has been concerned with the development of a homogenised model for the propagation of long-waves in elastic metaporous materials, that is poro-elastic media with periodically-embedded resonant inclusions. In particular, the case of periodically-embedded Helmholtz resonators with an extended neck and with elastic walls has been studied. To tackle the problem, the theory of two-scale asymptotic homogenisation has been applied. It gives access to a generalised poroelastic Biot model which encapsulates in a closed-form the complex multiple interactions between the inclusions themselves and between the inclusion array and the wavefield.
Dr Nicolaas Bernardus (Bert) Roozen, NOVIC Noise and Vibration control, Heeze (NL) & KU Leuven, Leuven (BEL)
STSM Period: 2017-07-02 – 2017-07-21 (20 days)
HOST: Francesco Pompoli , Engineering Department University of Ferrara, Ferrara (ITA)
Characterization of elastic parameters of acoustical porous materials using Lamb-waves
The scientific work during the STSM was mainly focused on the estimation of the Young’s modulus and loss factor from measurement data, which are relevant for the structure-borne transmission of waves through porous materials. New methods were developed that take into account the thickness of the test samples (Mindlin theory, Lamb wave theory), using both propagating and evanescent waves. The complex wavenumber of the propagating wave was thus estimated. Moreover, the type of excitation (a knife) and its reflections from the boundaries were taken into account using the Green’s function of the Mindlin theory and an image source type of model. Numerical experiments were carried out first to test the fitting algorithms and these algorithms were finally applied to experimental data.
Dr Tomasz G. Zielinski, Institute of Fundamental Technological Research of the Polish Academy of Sciences, Warsaw (POL)
HOST: Elke Deckers, KU Leuven, Department of Mechanical Engineering, Leuven (BEL)
STSM Period: 2017-04-09 – 2017-04-29 (21 days)
Application of homogenization techniques to micro-slit materials
Two research topics were investigated during the STSM: (1) micro-slit materials, (2) poroelastic layers with small mass/elastic inclusions. In the first case, the techniques common in microstructure-based modelling of porous media (IPPT PAN) were applied to micros-slit metamaterials (proposed by KU Leuven). In the second case, the effects related to the presence of local mass inclusions inside poroelastic layers were investigated and found to be very promising with respect to the development of novel poroelastic composites for overall or selective attenuation of waves.
Dr Paolo Bonfiglio, Engineering Department University of Ferrara, Ferrara (ITA)
HOST: Christ Glorieux, KU Leuven, Leuven (BEL)
STSM Period: 2017-04-24 – 2017-04-28 (5 days)
Micro-macro characterization and modelling of porous media
The main purpose of this short-term scientific mission was the comparison between two different techniques for the determination of the complex modulus of porous media using a laser Doppler vibrometer approach. The sample was excited at a fixed position (at the outer end of the sample) by mean of a shaker. The response of the sample wes measured by means of a laser Doppler vibrometer, along the full length of the sample. As a preliminary step the team worked extensively to improve the S/N ratio of measured signals introducing an impulse response with exponential sine sweep calculation in order to avoid non-linear (harmonic) distortions from the force transducer. The result was successful and the quality of the data was improved in a larger frequency range of investigation. Results from both measurement methods can be considered reliable although additional tests have been already planned to investigate the wave front of propagating waves.
Mr Iván Herrero-Durá, Universitat Politècnica de València, Grao de Gandia (ESP)
HOST: Vicente Romero García, LAUM – UMR CNRS 6613, Le Mans (FRA)
STSM Period: 2017-03-15 – 2017-04-15 (32 days)
Experimental analysis of sonic crystals made of clamped elastic beams
The purpose of the STSM is the experimental and numerical characterization of a periodic distribution of clamped scatterers embedded in air. The attention has been focused in 1D periodic structures made of clamped cylindrical membranes and beams and, thus, four different systems have been analysed: a complete membrane, a membrane with slits at both sides, a cantilever beam and two cantilever beams. Two main effects are expected, the resonances of the structures and the Bragg reflections due to periodicity.
Dr Timo Lähivaara, University of Eastern Finland, Kuopio (FIN)
HOST: Peter Göransson, KTH Royal Institute of Technology, Stockholm (SWE)
STSM Period: 2017-04-02 – 2017-04-08 (7 days)
Bayesian framework for optimal alignment of a multilayered anisotropic poroelastic acoustic absorber
Simulations of wave propagation in poroelastic media are necessary for solving many problems of physics and engineering. Applications for poroelastic models are wide, including groundwater exploration and noise absorbing materials. The latter one was the main research topic during the STSM. We used a Bayesian inversion framework together with a model based on a Transfer Matrix Method approach to optimizing multilayered anisotropic poroelastic systems in terms of acoustic absorption.
Dr Alan Geslain, Université de Bourgogne Franche-Comté, Nevers (FRA)
HOST: José Sánchez-Dehesa, Universitat Politècnica of València, Valencia (ESP)
STSM Period: 2017-03-06 – 2017-03-31 (26 days)
Acoustic characterization of superlattice metamaterials made of aerogel disks for the elastic energy damping
Silica aerogel has been widely studied in the past as bulk material for its extremely low density and low thermal conductivity. Plates or membranes made of this extremely soft materials, exhibits sub wavelength resonances. With a suitable structure made of combination of aerogel plates, possibly exhibits interesting properties as a negative density, as a near zero density and as a perfect absorption (acoustic absorption near 1). The aim of this STSM project is to exploited the aerogel plates in a tunable configurations (1D or 2D) to obtain interesting metamaterials properties. The first step of this project is covered by the STSM and is consist to acoustically characterize the aerogel in order to determine the material parameters. The second step of this project is the perspective of a future collaboration, is to design 1D metamaterials or 2D superlatticemetasurfaces.
Mr Jean-Philippe Groby, LAUM – UMR CNRS 6613, Le Mans (FRA)
HOST: Víctor J. Sánchez Morcillo, Polytechnical University of Valencia, Department of Applied Physics, Gandia (ESP)
STSM Period: 2017-03-06 – 2017-03-31 (26 days)
Design of perfect subwavelength natural sound absorbing materials with poseidonia balls
The aim of this STSM was to investigate the potential of posidonia based material for sound absorption. Several samples were characterized and a toy model was developed to describe the evolution of the Johnson Lafarge model parameters depending on the porosity of the sample. This material was found to have huge potential of applications.
Prof Livia Cveticanin, University of Novi Sad, Novi Sad (SRB)
HOST: Dan Sporea, National Institute for Laser, Plasma and Radiation Physics, Magurele (ROU)
STSM Period: 2017-03-19 – 2017-03-25 (7 days)
Advances in Theoretical and experimental investigation in metamaterials and structures
This STSM Grant was used to establish links between two research entities: one, where the theory of acoustic metamaterial is developed and the other, where the experiments are done. Namely, the theoretically obtained result for nonlinear acoustic metamaterial has to be proved experimentally. We discuss about the preliminary setup for some tests on metamaterials which would be used in sound insulation. We plan future cooperation which will include the work of our PhD students at Laboratories in Romania.
Prof Kirill Horoshenkov, University of Sheffield, Sheffield (GBR)
HOST: Patrizio Fausti, University of Ferrara, Ferrara (ITA)
STSM Period: 2017-03-06 – 2017-03-10 (5 days)
A new laboratory method for acoustic characterisation of nanoporous media
This STSM grant was used to develop further research links with Ferrara University in Italy. The work was focused on acoustic properties of materials with pores which diameter can be as small as a few nano-meters. The properties of these materials are unique and poorly understood. The purpose of this STSM was to focus on new methods to measure the acoustical properties of these materials and predict them with a mathematical model. One outcome of this STSM is a new PhD student at the University of Sheffield who will start her work from September 2017. She will focus on studying of nano-porous membranes for water purification. She will use acoustic methods to measure the size of nano-pores and effects of various catalysts and chemical reactions on the pore size.
Mr Alexandre Lardeau, ISAT, Nevers (FRA)
HOST: Olga Umnova, Acoustics Research Centre, University of Salford, Salford (GBR)
STSM Period: 2017-02-06 – 2017-02-17 (12 days)
Propagation of high amplitude sound waves in structured materials with periodic inner cavities
Metamaterials are an ever growing topic in the acoustic community, and used for sound absorption purposes amongst other applications. Though their behaviour in the linear regime are well known, their nonlinear behaviour is much less studied. This STSM aims at studying experimentally the propagation of high amplitude sound waves in structured materials with periodic inner circular cavities. The results are meant to help us derive a model for these materials.
Dr Vicent Romero Garcia, LAUM – UMR CNRS 6613, Le Mans (FRA)
HOST: Luis Miquel Garcia Raffi, Departamento de Matématica Aplicada, Valencia (ESP)
STSM Period: 2017-01-09 – 2017-02-09 (32 days)
Nonlinear propagation in periodic acoustic media
In this STSM we have developed theoretical and numerical models to study the nonlinear wave propagation in periodic and locally resonant acoustic media. In particular in the framework of this STSM we have develop analytical models for the study of large amplitude waves propagating in 1D multilayered structures. The theoretical tools have been borrowed from optics by using the couple mode theory (CMT), in which the host institution has a deep understanding in the linear regime. Here, we have collaborated to extend this method to the nonlinear regime.
Dr Noé Jiménez González, LAUM – UMR CNRS 6613, Le Mans (FRA)
HOST: Trevor Cox, The University of Salford, Salford, Greater Manchester (GBR)
STSM Period: 2016-10-15 – 2016-12-01 (48 days)
Deep-subwavelength acoustic diffusers based in slow-sound metamaterials
Sound diffusers composed of acoustic metamaterials are presented. These sound diffusers are rigid backed panels composed of periodic array of slits, each one loaded by an array of Helmholtz resonators (HRs). Strong dispersion is produced in each slit generating slow sound conditions. Thus, the length of the slits can be shortened to the sub-wavelength regime to achieve the quarter wavelength resonance. By tuning the geometry of the system, the complex reflection coefficient of each slit can be tailored to obtain either a desired phase or strong absorption. Based of this concept, we present thin diffusers where the geometries of the metamaterials have been tuned to get surfaces with a spatially dependent reflection coefficient having uniform magnitude Fourier transforms. Various designs are shown, where quadratic residue diffusers, primitive root diffusers, and index and ternary sequence diffusers are mimicked by using metadiffusers from 20 to 40 times thinner than the wavelength. Finally, a broadband metadiffuser panel of 3 cm thick was designed using optimization methods presenting a high diffusion coefficient for frequencies ranging from 500Hz to 2 kHz.
Jiménez, N., Cox, T.J., Romero-García, V., Groby, J.-P., 2017. Metadiffusers: Deep-subwavelength sound diffusers. Scientific Reports 7. doi:10.1038/s41598-017-05710-5
Dr Timo Lähivaara, University of Eastern Finland, Kuopio (FIN)
HOST: Jean-Philippe Groby, LAUM – UMR CNRS 6613, Le Mans (FRA)
STSM Period: 2016-10-24 – 2016-11-06 (14 days)
Full-wave inversion techniques for porous material characterization
There is a clear need for accurate methods for modelling wave fields in poroelastic media. Applications for modelling such wave fields are, for example, groundwater exploration, medical ultrasonics, and noise absorbing materials. In many cases, the material properties of the poroelastic media are the primary unknowns. Typically, we have only indirect measurements of the material properties that makes the actual estimation problem often very challenging. During the STSM grant, we took major steps in the code development of estimating poroelastic plates.
Mr Luis J. Salmerón-Contreras, Universitat Politècnica de València, València (ESP)
HOST: Jean-Philippe Groby, LAUM – UMR CNRS 6613, Le Mans (FRA)
STSM Period: 2016-10-01 – 2016-10-31 (31 days)
Vibroacoustic measurements in periodic elastic beam
The aim of this STSM was to acquire skills in vibroacoustic measurements and its corresponding processing. For this purpose the LAUM has provided the equipment required to perform an experimental set-up which basically consists in a 1D phononic crystal (from now called PC). It is made by cylindrical layers of aluminum and poly(methyl methacrylate) (PMMA), also known as by the trade name, Plexiglas or Lucite. The excitation of PC is performed by an ultrasonic transducer and the motion acquirement by a laser vibrometer and an accelerometer (Detailed report).
Dr Logan Schwan, LAUM – UMR CNRS 6613, Le Mans (FRA)
HOST: Olga Umnova, Acoustics Research Centre, University of Salford, SALFORD (GBR)
STSM Period: 2016-08-01 – 2016-08-19 (19 days)
Unconventional mode conversion of acoustic waves by resonant metasurface
This STSM aimed at investigating sound interaction with resonant metasurfaces through experiments performed in anechoic environment on full-scale prototypes with more than 500 resonators arranged at a rigid backing, and under realistic working conditions (finite size array and point-source excitation). Uniform and graded resonant metasurfaces have been tested, to observe the conversion of bulk waves into surface waves and the apparent violation of Descartes’ Laws upon reflection.