We experimentally report perfect acoustic absorption through the interplay of the inherent losses and transparent modes with high Q factor. These modes are generated in a two-port, one-dimensional waveguide, which is side-loaded by isolated resonators of moderate Q factor. In symmetric structures, we show that in the presence of small inherent losses, these modes lead to coherent perfect absorption associated with one-sided absorption slightly larger than 0.5. In asymmetric structures, near perfect one-sided absorption is possible (96%) with a deep sub-wavelength sample ($\lambda /28$ , where λ is the wavelength of the sound wave in the air). The control of strong absorption by the proper tuning of the radiation leakage of few resonators with weak losses will open possibilities in various wave-control devices.

We present experiments on acoustic carpet cloaking by using a metasurface made of graded Helmholtz resonators. The thin metasurface, placed over the object to hide, is designed such that the reflection phase shifts of the resonators at the resonance frequency are tuned to compensate the shape of the object to cloak. Experimental as well as numerical results show the efficiency of the cloak at the resonance frequency. The reflection of a short pulse is also reported to inspect the broadband character of the cloak.

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.

We study the two-dimensional extension of the Su-Schrieffer-Heeger model in its higher-order topological insulator phase, which is known to host corner states. Using the separability of the model into a product of one-dimensional Su-Schrieffer-Heeger chains, we analytically describe the eigenmodes, and specifically the zero-energy level, which includes states localized in corners. We then consider networks with disordered hopping coefficients that preserve the chiral (sublattice) symmetry of the model. We show that the corner mode and its localization properties are robust against disorder if the hopping coefficients have a vanishing flux on appropriately defined superplaquettes. We then show how this model with disorder can be realized using an acoustic network of air channels, and confirm the presence and robustness of corner modes.

In this work, coherent perfect absorption of sound waves induced by the nonlinear response of a Helmholtz Resonator side loaded to a waveguide, is reported. It is shown that this two-port system can perfectly absorb two high amplitude symmetric incident waves under a certain condition. For the one-sided incidence configuration, this condition leads to an absorption equal to 0.5. Experiments verify these results and are in agreement with an analytical nonlinear impedance model for the resonator. The nonlinear control of perfect absorption opens new possibilities in the design of high amplitude sound attenuators for aero-engine applications.

In this work, we investigate some aspects of an acoustic analog of the two-dimensional Su–Schrieffer–Heeger model. The system is composed of alternating cross-sectional tubes connected in a square network, which in the limit of narrow tubes is described by a discrete model coinciding with the two-dimensional Su–Schrieffer–Heeger model. This model is known to host topological edge waves, and we develop a scattering theory to analyze how these waves scatter on edge structure changes. We show that these edge waves undergo a perfect reflection when scattering on a corner, incidentally leading to a new way of constructing corner modes. It is shown that reflection is high for a broad class of edge changes such as steps or defects. We then study the consequences of this high reflectivity on finite networks. Globally, it appears that each straight part of the edges, separated by corners or defects, hosts localized edge modes isolated from their neighborhood.