A METHOD TO DESIGN THE PORE SHAPE FOR ACOUSTIC MATERIALS

Acoustic behaviour modelling of porous materials can be achieved by means of several approaches each based on a different number of required parameters. More are the parameters, greater is the ability of the model to describe the acoustic properties of many typologies of porous materials. For example the so-called model of Johnson-Lafarge-Champoux-Allard requires the knowledge of six parameters (tortuosity, porosity, air-flow resistivity, thermal and viscous characteristic length and the static thermal permeability) allowing a more detailed description of the material structure while the simple model of Delany and Bazley requires only one parameter (the resistivity) but is just valid for fibrous materials. In this preliminary study, the model of Champoux-Johnson-Allard (JCA) is used to obtain the sound absorption coefficient in a chosen frequency range. Then, given a shape of single pore boundary surface, numerical simulations are used to solve the fluid-dynamic field inside the pore. Starting by their mathematical definitions, the tortuosity, the air-flow resistivity, the viscous characteristic length and the thermal characteristic length are calculated taking into account the geometric size of the pore. The effects of different surface boundary shapes on JCA model parameters are investigated. Finally, by using an iterative procedure for the considered pore shape, a set of the involved parameters values improving the sound absorption coefficient is found. This study may be applied in situation in which small dimensions of the material constituted by the optimized pore shape can be obtained by using 3D print or by using stereolitografy technology.