Using the concepts of slow sound and of critical coupling, an ultra-thin acoustic metamaterial panel for perfect and omnidirectional absorption is theoretically and experimentally conceived in this work. The system is made of a rigid panel with a periodic distribution of thin closed slits, the upper wall of which is loaded by Helmholtz Resonators (HRs). The presence of resonators produces a slow sound propagation shifting the resonance frequency of the slit to the deep sub-wavelength regime (λ/88). By controlling the geometry of the slit and the HRs, the intrinsic visco-thermal losses can be tuned in order to exactly compensate the energy leakage of the system and fulfill the critical coupling condition to create the perfect absorption of sound in a large range of incidence angles due to the deep subwavelength behavior. * noe.jimenez@univ-lemans.fr 1 arXiv:1606.07776v1 [physics.class-ph] Jun 2016The ability to perfectly absorb an incoming wave field in a sub-wavelength material is advantageous for several applications in wave physics as energy conversion [1], time reversal technology [2], coherent perfect absorbers [3] or soundproofing [4] among others. The solution of this challenge requires to solve a complex problem: reducing the geometric dimensions of the structure while increasing the density of states at low frequencies and finding the good conditions to match the impedance to the background medium.A successful approach for increasing the density of states at low frequencies with reduced dimensions is the use of metamaterials. Recently, several possibilities based on these systems have been proposed to design sound absorbing structures which can present simultaneously sub-wavelength dimensions and strong acoustic absorption. One strategy to design these sub-wavelength systems consists of using space-coiling structures [5,6]. Another way is to use sub-wavelength resonators as membranes [4,7] or Helmholtz resonators (HRs) [8,9].Recently, a new type of sub-wavelength metamaterials based on the concept of slow sound propagation have been used to the same purpose. This last type of metamaterials [10][11][12] makes use of its strong dispersion for generating slow-sound conditions inside the material and, therefore, drastically decreasing frequency of the absorption peaks. Hence, the structure thickness becomes deeply sub-wavelength. All of these structures, however, while they bring potentially solutions to reduce the geometric dimensions, face the challenge of impedance mismatch to the background medium.The interaction of an incoming wave with a lossy resonant structure, in particular the impedance matching with the background field, is one of the most studied process in the field of wave physics [1][2][3]. These open systems, at the resonant frequency, are characterized by both the leakage rate of energy (i.e., the coupling of the resonant elements with the propagating medium), and the intrinsic losses of the resonator. The balance between the leakage and the losses activates the condition of critic...

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Sound absorption is important for room acoustics and remediation of noise. Acoustic metamaterials have recently emerged as one of the most promising platforms for sound absorption. However, the working bandwidth is severely limited because of the strong dispersion in the spectrum caused by local resonance. Utilizing the coupling effect among resonators can improve the absorbers' performance, but the requirement of collecting coupling effects among all resonators, not only the nearest-neighbor coupling, makes the system too complex to explore analytically. This Letter describes deep learning based acoustic metamaterials for achieving broadband sound absorption with no visible oscillation in a targeted frequency band. We numerically and experimentally achieve an average absorption coefficient larger than 97% within the ultra-broadband extending from 860 to 8000 Hz, proving the validity of the deep learning based acoustic metamaterials. The excellent ultra-broadband and near-perfect absorption performance allows the absorber for versatile applications in noise-control engineering and room acoustics. Our work also reveals the significance of modulating coupling effects among resonators, and the deep learning approach may blaze a trail in the design strategy of acoustic functional devices.