Perfect absorption in mirror-symmetric acoustic metascreens

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 3D printed and experimentally tested in an impedance tube. Experimental results agree well with the theoretical predictions proving the efficiency of these metascreens for the perfect absorption of sound in the ventilation problems

Design of acoustic metamaterials made of Helmholtz resonators for perfect absorption by using the complex frequency plane

[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.The authors gratefully acknowledge the ANR-RGC METARoom (ANR-18-CE08-0021) project and the project HYPERMETA funded under the program Étoiles Montantes of the Région Pays de la Loire. NJ acknowledges financial support from the Spanish Ministry of Science, Innovation and Universities (MICINN) through grant ¿Juan de la Cierva-Incorporación¿ (IJC2018-037897- I). This article is based upon work from COST Action DENORMS CA15125, supported by COST (European Cooperation in Science and Technology).Romero-García, V.; Jimenez, N.; Theocharis, G.; Achilleos, V.; Merkel, A.; Richoux, O.; Tournat, V.... (2020). Design of acoustic metamaterials made of Helmholtz resonators for perfect absorption by using the complex frequency plane. 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Physical and biological processes at the Subtropical Convergence in the South-west Indian Ocean

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