Sound absorption of space-coiled metamaterials with soft walls

Integrating soft components into acoustic metamaterials is an extremely promising way to enhance their adjustability and reconfigurability. Herein, we report a type of space-coiled metamaterials using soft walls to construct internal channels, denoted as soft-wall space-coiled metamaterials (SSM), exhibiting superior sound absorption performance at low frequencies. The physical mechanism of the SSM is revealed through finite element simulations and sound impedance tube-based experiments. It is demonstrated that the exceptional sound absorption of the SSM originates from the strong coupling effect between the friction-like dissipation along the internal channel and the deformation-like dissipation in the soft wall. The experimental results show that the fabricated SSM sample with a subwavelength thickness (<1/25 wavelength) features near-perfect sound absorption (reaching 0.97 at 615 Hz) and a broad half-power bandwidth (1.6 times that of the rigid-wall space-coiled metamaterials sample with the same geometric dimensions). Moreover, the SSM has excellent sound absorption performance under oblique incidence even when the incident elevation angle reaches 75°. This work may bring inspiration for developing soft-components-integrated acoustic metamaterials with compact, adjustable, and reconfigurable characteristics.