Labyrinthine acoustic metamaterials with space-coiling channels for low-frequency sound control

We numerically analyze the performance of labyrinthine acoustic metamaterials with internal channels folded along a Wunderlich space-filling curve to control low-frequency sound in air. In contrast to previous studies, we perform direct modeling of wave propagation through folded channels without introducing effective theory assumptions. We reveal that metastructures with channels that allow wave propagation in the opposite direction to incident waves, have different dynamics as compared to those for straight slits of equivalent length. These differences are attributed to tortuosity effects and result in 100% wave reflection at band gap frequencies. This total reflection phenomenon is found to be insensitive to thermo-viscous dissipation in air. For labyrinthine channels generated by recursive iteration levels, one can achieve broadband total sound reflection by using a metamaterial monolayer, and efficiently control the amount of absorbed wave energy by tuning the channel width. Thus, the work contributes to a better understanding of labyrinthine metamaterials with potential applications for reflection and filtering of low-frequency airborne sound.