Acoustically soft and mechanically robust hierarchical metamaterials in water

Isolating noise in water relies on materials with low acoustic impedance. However, reducing the existing materials' acoustic impedance severely compromises their stiffness and strength, resulting in a longstanding challenge of sound isolation in deep-sea environments with high ambient pressure. To overcome the mutual exclusion of low acoustic impedance and high mechanical properties, we propose a design principle including two steps that regulate the lattice orientation and incorporate a hierarchical morphology in an anisotropic metamaterial. Regulating the lattice orientation leads to low effective acoustic impedance while counterintuitively improving the initial stiffness. By learning from nature, incorporating a hierarchical morphology enables the metamaterial with an unprecedented decoupling characteristic that the mechanical strength can be enhanced independently from the acoustic impedance. A hierarchical metamaterial is constructed as a proof-of-concept demonstration and displays high sound transmission loss over 16 dB in a low and broad frequency range from 400 to 1200 Hz. Of note, the hierarchical metamaterial could maintain stable acoustic performance even under a high ambient pressure of 2 MPa. This work not only opens an alternative avenue for realizing sound isolation in deep-sea environments but also offers a design principle for metamaterials combining antagonistic functional properties.