Folded metaporous material for sub-wavelength absorption of sound

Over the past decade, open-cell acoustic materials have been widely investigated for broadband noise absorption applications. However, sub-wavelength absorption remains a major challenge. A slab of porous material presents low absorption below its so called "quarter wavelength resonance frequency", which is related to the thickness of the slab. Thus, absorbing lower frequencies usually requires increasing the thickness of the porous treatments, making them too bulky and too heavy for many applications. The main purpose of this work is to introduce a folded porous material having an acoustic effective thickness greater than the thickness of the treatment itself. The folded porous material consists in cavities folded in helical shape and filled with a porous material. Designing the pitch of the cavities and the pore size of the filling porous material is done separately, allows a separate control of the effective thickness and of the intrinsic losses of the material. Its optimization (best pitch and best pore size) enables to achieve perfect absorption at frequencies much lower than the quarter wavelength frequency of the slab. Finally, a parallel assembly of several folded porous materials, each one tuned at a distinct frequency, generates a metaporous material allowing to achieve sub-wavelength and large band absorption simultaneously. The normal incidence acoustic behavior of the metaporous material is predicted analytically by a mode matching model. The optimized metaporous material is 3D printed and tested experimentally. The mean value of the measured absorption coefficient is 0.98 for frequencies corresponding to a wavelength in air λ such that 5.3L < λ < 9.9L, where L is the thickness of the material.