3D Printed Porous Material Optimally Graded for Broadband Absorption of Sound
Résumé
Homogeneous open-cell materials have been extensively studied for broadband sound absorption applications. However, their absorption is not perfect over the frequency space and their performances in the low and medium frequencies is limited by their intrinsic loss and the treatment?s thickness. Introducing and optimizing a gradient of properties through a porous material?s thickness is a powerful way to increase their absorption over a large frequency range or to shift their low frequency limit of effectiveness. This work introduces a numerical method for the optimization of the properties of any type of porous material to obtain perfect absorption under normal incidence. This design tool is applied, as an example, to a fibrous medium composed of a periodic arrangement of ordered unit cells, that can be 3D printed. The link between the geometry of the porous medium and its effective acoustic properties is made numerically by means of multiscale asymptotic method and experimentally thought an inverse characterization process. A non linear conjugate gradient algorithm creates the best continuous micro-geometric gradient for maximal absorption coefficient over a targeted frequency range. Numerical and experimental results show that optimally graded material has a near perfect absorption over a large frequency range, or that its perfect absorption peak is shifted to lower frequency. The acoustic performances of the optimally graded material are compared to that of homogeneous and monotonically graded material.