Ultra-thin metamaterial for perfect and quasi-omnidirectional sound absorption

Abstract

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...

Iridescent Perfect Absorption in Critically-Coupled Acoustic Metamaterials Using the Transfer Matrix Method

“…As shown in Reference [15], by tuning the geometrical parameters, the structure described in this work can be impedance matched with the exterior medium, and perfect acoustic absorption can be obtained. For locally reacting materials, this occurs when Equation (4) vanishes, leading to the impedance matching conditions:…”

“…Its geometry is composed of a neck of length l c . Note that the geometry is similar to the one presented in Reference [15], but here we consider an array of different HRs. …”

“…Note that the TMM using a similar metamaterial has been validated using finite element simulations, modal expansion, effective parameters, and experiments in References [15,16].…”

“…These specific materials are artificial structures composed of an arrangement of resonant unit cells-smaller than the characteristic wavelength-that present effective properties not observed in the materials that compose the structure. Examples of such absorbers are metaporous materials [6][7][8][9], metamaterials composed of membrane-type resonators [10][11][12][13], Helmholtz resonators (HRs) [13][14][15][16], and quarter-wavelength resonators (QWRs) [4,[17][18][19]. These last types of metamaterials [4,[15][16][17][18]] make use of strong dispersion, giving rise to slow-sound propagation inside the material.…”

Broadband quasi perfect absorption using chirped multi-layer porous materials

“…Perfect absorption (PA) is recently receiving an increasing interest. [1][2][3][4][5][6] Chirped or graded materials are widely used in the wave physics community due to their possibilities to manipulate the wave propagation. These artificial materials are emerging as promising tools for potential applications in several branches of research and technology.…”