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The absorption properties of a metaporous material made of non-resonantsimple shape three-dimensional inclusions (cube, cylinder, sphere, cone andtorus) embedded in a rigidly backed rigid frame porous material is studied. Anearly total absorption can be obtained for a frequency lower than thequarter-wavelength resonance frequency due to the excitation of a trapped mode.To be correctly excited, this mode requires a filling fraction larger in thethree-dimensions than in the two-dimensions for purely convex (cube, cylinder,sphere, and cone) shapes. At low frequencies, a cube is found to be the bestpurely convex inclusion shape to embed in a cubic unit cell, while theembedment of a sphere or a cone cannot lead to an optimal absorption for someporous materials. At fixed position of purely convex shape inclusionbarycentre, the absorption coefficient only depends on and filling fraction anddoes not depend on the shape below the Bragg frequency arising from theinteraction between the inclusion and its image with respect to the rigidbacking. The influence of the angle of incidence is also shown. The results, inparticular the excitation of the trapped mode, are validated experimentally incase of cubic inclusions.

Using simple shape three-dimensional inclusions to enhance porous layer absorption