Absorption of a poroelastic material with lateral air gaps

In order to reduce noise in automotive or aircraft cabins, sound absorbing materials such as polymer foams or fibrous materials are widely used. To be efficient, their thickness must be in relation with the acoustical wavelength, making them not suitable in the low frequency range. To overcome this limitation while reducing weight, multilayer architectures and inclusions involving air gap or impervious structures, may be used. Up to now, most of the proposed models dealing with inclusions are based on rigid frame modeling. This assumption is not valid in general applications where a coupling with the frame may be observed. The aim of this paper is to investigate both the effect of air inclusions on the sound absorption of a poroelastic material and oblique incidence. The studied configuration is based on a square based hexahedre foam sample surrounded by air gaps of constant thickness. Measurements, performed in an impedance tube, are compared with finite element predictions (Biot-Allard model). The influence of the incident angle on the absorption coefficient is addressed assuming a periodic architecture. It is shown that lateral air gaps may increase the absorption by acting on the coupled air/skeleton resonances of the sample.