Modelling the sound transmission properties of a side branch acoustic metamaterial using rigid frame porous model

his paper presents a model to predict the sound transmission properties of a side branch acoustic metamaterial composed of coupled subwavelength Helmholtz resonators.A finite element model (FEM) is proposed to describe the grazing incidence wave propagation in Helmholtz resonators. The Viscothermal losses due to the solid-fluid interaction on the boundary layer interface in the neck of resonators are considered by employing the classical fluid equivalent Johnson-Champoux-Allard (JCA) rigid frame approach. Next, the in-parallel coupling of multiple resonators is conducted to enlarge the frequency range.To validate the results, an experimental campaign was developed based on the four-microphone measurement method (ASTM E2611-09). As expected, the preliminary numerical results show good agreement when compared with experimental values. In turn, the approach presented here enables the coupling of multiple resonances of individual resonators under grazing incidence, which can be effectively used in designing compact attenuators with a wide frequency range, with potential application in different engineering contexts, especially in ventilated systems.