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Periodic structures composed of quarter-wavelength or Helmholtz resonators have been widely used in the design of acoustic metamaterials. An interesting phenomenon achievable through hybridization in such structures is the slow sound, which results from the strong dispersion produced by the local resonances. It gives rise to many applications such as deep subwavelength sound absorbers or metadiffusers. All the applications proposed so far have been analyzed only in the frequency domain (steady state). In this work, we propose a passive treatment that can be used in room acoustics, which requires considering the time domain and all multiple reflections. We analytically design a delay line from a metasurface made of Helmholtz resonators, using slow-sound propagation. We prove numerically and experimentally that such structures can delay a pulse and thus reproduce the sound perception of a propagation over a given distance, larger than the actual size of the treatment. The limitations of real-time pulse propagation, dispersion, and losses on audio fidelity are discussed. © 2022 American Physical Society.

Periodic structures composed of quarter-wavelength or Helmholtz resonators have been widely used in the design of acoustic metamaterials. An interesting phenomenon achievable through hybridization in such structures is the slow sound, which results from the strong dispersion produced by the local resonances. It gives rise to many applications such as deep subwavelength sound absorbers or metadiffusers. All the applications proposed so far have been analyzed only in the frequency domain (steady state). In this work, we propose a passive treatment that can be used in room acoustics, which requires considering the time domain and all multiple reflections. We analytically design a delay line from a metasurface made of Helmholtz resonators, using slow-sound propagation. We prove numerically and experimentally that such structures can delay a pulse and thus reproduce the sound perception of a propagation over a given distance, larger than the actual size of the treatment. The limitations of real-time pulse propagation, dispersion, and losses on audio fidelity are discussed. © 2022 American Physical Society.

[EN] This paper examines the feasibility of cloaking an obstacle using Plate-type Acoustic Metamaterials (PAMs). We present two distinct strategies to cloak this obstacle, using either the near-zero-density regime of a periodic arrangement of plates or the acoustic doping phenomenon to achieve simultaneous zero-phase propagation and impedance matching. The strong limitations induced by viscothermal and viscoelastic losses that cannot be avoided in such a system are studied. A hiding zone is reported analytically, numerically, and experimentally. In contrast to cloaking, where zero-phase propagation must be accompanied by total transmission and zero reflection, the hiding configuration requires that the scattering properties of the hiding device must not be affected by the presence of the obstacle embedded in it. Contrary to cloaking, the hiding phenomenon is achievable even with a realistic PAM possessing unavoidable losses.

[EN] This paper examines the feasibility of cloaking an obstacle using Plate-type Acoustic Metamaterials (PAMs). We present two distinct strategies to cloak this obstacle, using either the near-zero-density regime of a periodic arrangement of plates or the acoustic doping phenomenon to achieve simultaneous zero-phase propagation and impedance matching. The strong limitations induced by viscothermal and viscoelastic losses that cannot be avoided in such a system are studied. A hiding zone is reported analytically, numerically, and experimentally. In contrast to cloaking, where zero-phase propagation must be accompanied by total transmission and zero reflection, the hiding configuration requires that the scattering properties of the hiding device must not be affected by the presence of the obstacle embedded in it. Contrary to cloaking, the hiding phenomenon is achievable even with a realistic PAM possessing unavoidable losses.

[EN] Sound diffusion refers to the ability of a surface to evenly scatter sound energy in both time and space. However, omni-directional radiation of sound, or perfect diffusion, can be impractical or difficult to reach under traditional means. This is due to the considerable size required, and the lack of tunability, of typical quarter-wavelength scattering strategies necessary for producing the required complexity of the surface acoustic impedance. As such, it can be a challenge to design sound diffusing structures that can display near perfect diffusion performance within slim dimensions. In this work, we propose a method for obtaining quasi-perfect and broadband sound diffusion coefficients using deep-subwavelength acoustic diffusers, i.e., metadiffusers. The relation among the geometry of the metasurface, the bandwidth, and the diffusion performance is analytically and numerically studied. For moderate bandwidths, around 1/3 of an octave, the method results in nearly perfect sound diffusion, while for a bandwidth of 2.5 octaves, a normalized diffusion coefficient of 0.8 was obtained using panels 1/30th thinner than traditional phase-grating designs. The ratio between the wavelength and the size of the unit cell was identified as a limitation of the performance. This work demonstrates the versatility and effectiveness of metadiffusers to generate diffuse reflections outperforming those of classical sound diffusers.

[EN] We present a 3-dimensional fully natural sonic crystal composed of spherical aggregates of fibers (called Aegagropilae) resulting from the decomposition of Posidonia Oceanica. The fiber network is first acoustically characterized, providing insights on this natural fiber entanglement due to turbulent flow. The Aegagropilae are then arranged on a principal cubic lattice. The band diagram and topology of this structure are analyzed, notably via Argand representation of its scattering elements. This fully natural sonic crystal exhibits excellent sound absorbing properties and thus represents a sustainable alternative that could outperform conventional acoustic materials.

[EN] Mirror-symmetric acoustic metascreens producing perfect absorption independently of the incidence side are theoretically and experimentally reported in this work. The mirror-symmetric resonant building blocks of the metascreen support symmetric and antisymmetric resonances that can be tuned to be at the same frequency (degenerate resonances). The geometry of the building blocks is optimized to critically couple both the symmetric and the antisymmetric resonances at the same frequency, allowing perfect absorption of sound from both sides of the metascreen. A hybrid analytical model based on the transfer-matrix method and the modal decomposition of the exterior acoustic field is developed to analyze the scattering properties of the metascreen. The resulting geometry is three-dimensionally printed and experimentally tested in an impedance tube. The experimental results agree well with the theoretical predictions, proving the efficiency of these metascreens for the perfect absorption of sound in ventilation problems.

[Otros] Dans cette revue, nous présentons des résultats sur l'absorption acoustique parfaite sub-longueur d'onde faisant appel à des métamatériaux acoustiques avec des résonateurs Helmholtz pour différentes configurations. L'absorption parfaite à basse fréquence nécessite une augmentation du nombre d'états aux basses fréquences ainsi que de trouver les bonnes conditions pour une adaptation d'impédance avec le milieu environnant. Si en outre, on souhaite réduire les dimensions géométriques des structures proposées pour des questions pratiques, on peut utiliser des résonateurs locaux judicieusement conçus afin d'attendre une absorption parfaite sub-longueur d'onde. Les résonateurs de Helmholtz se sont révélés de bons candidats en raison de leur accordabilité aisée de la géométrie, donc de la fréquence de résonance, de la fuite d'énergie et des pertes intrinsèques. Lorsqu'ils sont branchés à un guide d'ondes ou à un milieu environnant, ils se comportent comme des systèmes ouverts, avec pertes et résonances caractérisés par leur fuite d'énergie et leurs pertes intrinsèques. L'équilibre entre ces deux aspects représente la condition de couplage critique et donne lieu à un maximum d'absorption d'énergie. Le mécanisme de couplage critique est ici représenté dans le plan de fréquence complexe afin d'interpréter la condition d'adaptation d'impédance. Dans cette revue, nous discutons en détail la possibilité d'obtenir une absorption parfaite par ces conditions de couplage critiques dans différents systèmes tels que la réflexion (à un port), la transmission (à deux ports) ou les systèmes à trois ports., [EN] In this review, we present the results on sub-wavelength perfect acoustic absorption using acoustic metamaterials made of Helmholtz resonators with different setups. Low frequency perfect absorption requires to increase the number of states at low frequencies and finding the good conditions for impedance matching with the background medium. If, in addition, one wishes to reduce the geometric dimensions of the proposed structures for practical issues, one can use properly designed local resonators and achieve subwavelength perfect absorption. Helmholtz resonators have been shown good candidates due to their easy tunability of the geometry, so of the resonance frequency, the energy leakage and the intrinsic losses. When plugged to a waveguide or a surrounding medium they behave as open, lossy and resonant systems characterized by their energy leakage and intrinsic losses. The balance between these two represents the critical coupling condition and gives rise to maximum energy absorption. The critical coupling mechanism is represented here in the complex frequency plane in order to interpret the impedance matching condition. In this review we discuss in detail the possibility to obtain perfect absorption by these critical coupling conditions in different systems such as reflection (one-port), transmission (two-ports) or three-ports systems.

[EN] Mirror-symmetric acoustic metascreens producing perfect absorption independently of the incidence side are theoretically and experimentally reported in this work. The mirror-symmetric resonant building blocks of the metascreen support symmetric and antisymmetric resonances that can be tuned to be at the same frequency (degenerate resonances). The geometry of the building blocks is optimized to critically couple both the symmetric and the antisymmetric resonances at the same frequency, allowing perfect absorption of sound from both sides of the metascreen. A hybrid analytical model based on the transfer-matrix method and the modal decomposition of the exterior acoustic field is developed to analyze the scattering properties of the metascreen. The resulting geometry is three-dimensionally printed and experimentally tested in an impedance tube. The experimental results agree well with the theoretical predictions, proving the efficiency of these metascreens for the perfect absorption of sound in ventilation problems.

[EN] We report the experimental design of a one-dimensional stealth acoustic material, namely a material that suppresses the acoustic scattering for a given set of incident wave vectors. The material consists of multiple scatterers, rigid diaphragms, located in an air-filled acoustic waveguide. The position of the scatterers has been chosen such that in the Born approximation a suppression of the scattering for a broad range of frequencies is achieved and thus a broadband transparency. Experimental results are found in excellent agreement with the theory despite the presence of losses and the finite size of the material, features that are not captured in the theory. This robustness as well as the generality of the results motivates realistic potential applications for the design of transparent materials in acoustics and other fields of wave physics.

We present the design of a structured material supporting complete absorption of sound with a broadband response and functional for any direction of incident radiation. The structure which is fabricated out of porous lamellas is arranged into a low-density crystal and backed by a reflecting support. Experimental measurements show that strong all-angle sound absorption with almost zero reflectance takes place for a frequency range exceeding two octaves. We demonstrate that lowering the crystal filling fraction increases the wave interaction time and is responsible for the enhancement of intrinsic material dissipation, making the system more absorptive with less material.

We predict and show the existence of unlocked evanescent waves in two dimensional periodic structures. In a one dimensional periodic structure an evanescent mode are located either at the center or at the boundary of the Brillouin Zone, and the real part of wave-vectors is locked either to zero or to the modulation vector of the periodicity. However, we report an unexpected observation that this is not the case in 2D or 3D PhCs where "unlocked" complex modes can be predicted. As an interesting consequence, due to their “complex” character, or, equivalently, due to their phase freedom, beams constructed from such evanescent waves can show focalization effects. Indeed, we numerically simulate the propagation of such beams in the bandgap of PhC slabs, and we find significant focalization., Postprint (published version)

A theoretical and experimental study of the propagation of sound beams in- and behind three-dimensional sonic crystals at frequencies close to the band edges is presented. An efficient collimation of the beam behind the crystal is predicted and experimentally demonstrated. This effect could allow the design of sources of high spatial quality sound beams., Postprint (published version)

[EN] Within the framework of the new curriculum, where active learning is the student, we propose a practice of modelling in science education to work with basics of mathematics and physics. The practice involves the modelling of wave reflection by rough surfaces. In a first stage, the reflection is analysed by a completely flat surface using geometric and physical concepts. The wave reflected from the surface is calculated from the waves emitted by a virtual source placed at the position symmetrical to the source position with respect to the mirror surface. To calculate the reflected wave, probability concepts are introduced. The wave reflection by a rough surface is obtained from the waves emitted by different sources for virtually every point of reflection. Courses in architecture and basic engineering, or even a master of acoustics are exceptional framework for the implementation of this practice., [ES] Dentro del marco de los nuevos planes de estudio, donde el activo del aprendizaje es el alumno, se propone una práctica de modelización para trabajar conceptos básicos de matemáticas y física en los primeros cursos Universitarios. La práctica consiste en la modelización de la reflexión de ondas por superficies rugosas. En un primer estadio de la práctica se analiza la reflexión por una superficie totalmente plana usando conceptos geométricos y físicos. La onda reflejada por la superficie plana se calcula a partir de las ondas emitidas por una fuente virtual colocada en la posición totalmente simétrica a la posición de la fuente respecto a la superficie especular. Para el cálculo de la onda reflejada se introducen conceptos de probabilidad. La reflexión de la onda por una superficie rugosa se obtiene a partir de las ondas emitidas por diferentes fuentes virtuales para cada punto de reflexión. Cursos de arquitectura y de ingeniería básica, o incluso algún máster de acústica, son marcos excepcionales para la aplicación de esta práctica.

[EN] Within the framework of the new curriculum, where active learning is the student, we propose a practice of modelling in science education to work with basics of mathematics and physics. The practice involves the modelling of wave reflection by rough surfaces. In a first stage, the reflection is analysed by a completely flat surface using geometric and physical concepts. The wave reflected from the surface is calculated from the waves emitted by a virtual source placed at the position symmetrical to the source position with respect to the mirror surface. To calculate the reflected wave, probability concepts are introduced. The wave reflection by a rough surface is obtained from the waves emitted by different sources for virtually every point of reflection. Courses in architecture and basic engineering, or even a master of acoustics are exceptional framework for the implementation of this practice., [ES] Dentro del marco de los nuevos planes de estudio, donde el activo del aprendizaje es el alumno, se propone una práctica de modelización para trabajar conceptos básicos de matemáticas y física en los primeros cursos Universitarios. La práctica consiste en la modelización de la reflexión de ondas por superficies rugosas. En un primer estadio de la práctica se analiza la reflexión por una superficie totalmente plana usando conceptos geométricos y físicos. La onda reflejada por la superficie plana se calcula a partir de las ondas emitidas por una fuente virtual colocada en la posición totalmente simétrica a la posición de la fuente respecto a la superficie especular. Para el cálculo de la onda reflejada se introducen conceptos de probabilidad. La reflexión de la onda por una superficie rugosa se obtiene a partir de las ondas emitidas por diferentes fuentes virtuales para cada punto de reflexión. Cursos de arquitectura y de ingeniería básica, o incluso algún máster de acústica, son marcos excepcionales para la aplicación de esta práctica.

[EN] Within the framework of the new curriculum, where active learning is the student, we propose a practice of modelling in science education to work with basics of mathematics and physics. The practice involves the modelling of wave reflection by rough surfaces. In a first stage, the reflection is analysed by a completely flat surface using geometric and physical concepts. The wave reflected from the surface is calculated from the waves emitted by a virtual source placed at the position symmetrical to the source position with respect to the mirror surface. To calculate the reflected wave, probability concepts are introduced. The wave reflection by a rough surface is obtained from the waves emitted by different sources for virtually every point of reflection. Courses in architecture and basic engineering, or even a master of acoustics are exceptional framework for the implementation of this practice., [ES] Dentro del marco de los nuevos planes de estudio, donde el activo del aprendizaje es el alumno, se propone una práctica de modelización para trabajar conceptos básicos de matemáticas y física en los primeros cursos Universitarios. La práctica consiste en la modelización de la reflexión de ondas por superficies rugosas. En un primer estadio de la práctica se analiza la reflexión por una superficie totalmente plana usando conceptos geométricos y físicos. La onda reflejada por la superficie plana se calcula a partir de las ondas emitidas por una fuente virtual colocada en la posición totalmente simétrica a la posición de la fuente respecto a la superficie especular. Para el cálculo de la onda reflejada se introducen conceptos de probabilidad. La reflexión de la onda por una superficie rugosa se obtiene a partir de las ondas emitidas por diferentes fuentes virtuales para cada punto de reflexión. Cursos de arquitectura y de ingeniería básica, o incluso algún máster de acústica, son marcos excepcionales para la aplicación de esta práctica.

[EN] Within the framework of the new curriculum, where active learning is the student, we propose a practice of modelling in science education to work with basics of mathematics and physics. The practice involves the modelling of wave reflection by rough surfaces. In a first stage, the reflection is analysed by a completely flat surface using geometric and physical concepts. The wave reflected from the surface is calculated from the waves emitted by a virtual source placed at the position symmetrical to the source position with respect to the mirror surface. To calculate the reflected wave, probability concepts are introduced. The wave reflection by a rough surface is obtained from the waves emitted by different sources for virtually every point of reflection. Courses in architecture and basic engineering, or even a master of acoustics are exceptional framework for the implementation of this practice., [ES] Dentro del marco de los nuevos planes de estudio, donde el activo del aprendizaje es el alumno, se propone una práctica de modelización para trabajar conceptos básicos de matemáticas y física en los primeros cursos Universitarios. La práctica consiste en la modelización de la reflexión de ondas por superficies rugosas. En un primer estadio de la práctica se analiza la reflexión por una superficie totalmente plana usando conceptos geométricos y físicos. La onda reflejada por la superficie plana se calcula a partir de las ondas emitidas por una fuente virtual colocada en la posición totalmente simétrica a la posición de la fuente respecto a la superficie especular. Para el cálculo de la onda reflejada se introducen conceptos de probabilidad. La reflexión de la onda por una superficie rugosa se obtiene a partir de las ondas emitidas por diferentes fuentes virtuales para cada punto de reflexión. Cursos de arquitectura y de ingeniería básica, o incluso algún máster de acústica, son marcos excepcionales para la aplicación de esta práctica.

[EN] Within the framework of the new curriculum, where active learning is the student, we propose a practice of modelling in science education to work with basics of mathematics and physics. The practice involves the modelling of wave reflection by rough surfaces. In a first stage, the reflection is analysed by a completely flat surface using geometric and physical concepts. The wave reflected from the surface is calculated from the waves emitted by a virtual source placed at the position symmetrical to the source position with respect to the mirror surface. To calculate the reflected wave, probability concepts are introduced. The wave reflection by a rough surface is obtained from the waves emitted by different sources for virtually every point of reflection. Courses in architecture and basic engineering, or even a master of acoustics are exceptional framework for the implementation of this practice., [ES] Dentro del marco de los nuevos planes de estudio, donde el activo del aprendizaje es el alumno, se propone una práctica de modelización para trabajar conceptos básicos de matemáticas y física en los primeros cursos Universitarios. La práctica consiste en la modelización de la reflexión de ondas por superficies rugosas. En un primer estadio de la práctica se analiza la reflexión por una superficie totalmente plana usando conceptos geométricos y físicos. La onda reflejada por la superficie plana se calcula a partir de las ondas emitidas por una fuente virtual colocada en la posición totalmente simétrica a la posición de la fuente respecto a la superficie especular. Para el cálculo de la onda reflejada se introducen conceptos de probabilidad. La reflexión de la onda por una superficie rugosa se obtiene a partir de las ondas emitidas por diferentes fuentes virtuales para cada punto de reflexión. Cursos de arquitectura y de ingeniería básica, o incluso algún máster de acústica, son marcos excepcionales para la aplicación de esta práctica.

[EN] Within the framework of the new curriculum, where active learning is the student, we propose a practice of modelling in science education to work with basics of mathematics and physics. The practice involves the modelling of wave reflection by rough surfaces. In a first stage, the reflection is analysed by a completely flat surface using geometric and physical concepts. The wave reflected from the surface is calculated from the waves emitted by a virtual source placed at the position symmetrical to the source position with respect to the mirror surface. To calculate the reflected wave, probability concepts are introduced. The wave reflection by a rough surface is obtained from the waves emitted by different sources for virtually every point of reflection. Courses in architecture and basic engineering, or even a master of acoustics are exceptional framework for the implementation of this practice., [ES] Dentro del marco de los nuevos planes de estudio, donde el activo del aprendizaje es el alumno, se propone una práctica de modelización para trabajar conceptos básicos de matemáticas y física en los primeros cursos Universitarios. La práctica consiste en la modelización de la reflexión de ondas por superficies rugosas. En un primer estadio de la práctica se analiza la reflexión por una superficie totalmente plana usando conceptos geométricos y físicos. La onda reflejada por la superficie plana se calcula a partir de las ondas emitidas por una fuente virtual colocada en la posición totalmente simétrica a la posición de la fuente respecto a la superficie especular. Para el cálculo de la onda reflejada se introducen conceptos de probabilidad. La reflexión de la onda por una superficie rugosa se obtiene a partir de las ondas emitidas por diferentes fuentes virtuales para cada punto de reflexión. Cursos de arquitectura y de ingeniería básica, o incluso algún máster de acústica, son marcos excepcionales para la aplicación de esta práctica.