Enlarging Sound Attenuation in the Low Frequency Domain by Giving a Poroelastic Material a Lamella Structure

Compared with a uniform porous layer, arranging a poroelastic material following a lamella network structure provides additional sound absorption below the quarter wavelength resonance frequency, i.e. in the low frequency domain. The mechanisms involved in this additional sound absorption are highlighted by a numerical approach that incorporates geometric periodicity. It is shown that structural and viscous dissipation are combined within the lamellas when the shear and bending resonances of the lamellas are excited at oblique or grazing incidence. Since the frequency of the shear resonance is related to the thickness of the material whereas the bending resonance is also a function of the width of the lamellas, the frequencies at which additional sound absorption can be obtained are adjustable. Experiments are finally conducted on samples made of melamine foam lamellas that demonstrate the efficiency of this arrangement in (1) a free field at various oblique incidence angles and (2) in a duct with several orientations of the lamellas structure.