Towards acoustic metafoams: the enhanced performance of a poroelastic material with local resonators

Acoustic foams are commonly used for sound attenuation purposes. Due to their porous microstructure, they efficiently dissipate energy through the air flowing in and out of the pores at high frequencies. However, the low frequency performance is still challenging for foams, even after optimisation of their microstructural design. A new, innovative, approach is therefore needed to further improve the acoustic behaviour of poroelastic materials. The expanding field of locally resonant acoustic metamaterials shows some promising examples where resonating masses incorporated within the microstructure lead to a significant enhancement of low frequency wave attenuation. In this paper, a combination of traditional poroelastic materials with locally resonant units embedded inside the pores is proposed, showing the pathway towards designing acoustic metafoams: poroelastic materials with properties beyond standard foams. The conceptual microstructural design of an idealised unit cell presented in this work consists of a cubic pore representing a foam unit cell with an embedded micro-resonator and filled with a viscothermal fluid (air). Analysis of complex dispersion diagrams and numerical transmission simulations demonstrate a clear improvement in wave attenuation achieved by such a microstructure. It is believed that this demonstrates the concept, which serves the future development of novel poroelastic materials