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The acoustic properties of a porous sheet of medium resistivity backed by arigid plate in which are embedded a periodic set of circular inclusions isinvestigated. Such a structure behaves like a multi-component diffractiongratings. Numerical results show that this structure presents a quasi-totalabsorption peak below the essential spectrum, i.e. below the frequence of thefundamental quarter-wavelength resonance of the porous sheet in absence ofinclusions. This result is explained either by the excitation of a complextrapped mode, or by the increase of viscous loss associated with a largervelocity gradient inside the layer at the modified quarter-wavelength resonancefrequency. When more than one inclusion per spatial period are considered,additional quasi-total frequency peak are observed. The numerical results ascalculated with the help of the mode-matching method described in the paperagree those as calculated with the help of a Finite Element method.

Quasi-total absorption peak by use of a backed rigid frame porous layer with circular periodic inclusions embedded