Broadband high sound absorption from labyrinthine metasurfaces

Metamaterials are artificial structures which exhibit fascinating properties unreachable by traditional materials. Here, wereport on the design, fabrication, and characterization of acoustic metasurfaces consisting of dead-end channels coiled in a 2D plane. It is found that when the area of the channel's cross section is about 1/10 of the area (4.3 cm x 4.3 cm) of the upper surface of the building block, the sound loss in channels approaches to a critical value, resulting in near- perfect absorption (A > 99%) at resonant frequency. When the building block contains ten channels with specially designed lengths, sound waves can be highly absorbed above a cutoff frequency fc (A > 90% for f(c) < f < 3f(c)). The wavelength at the cutoff frequency can be 7.1 times of the thickness of the metasurface. Our results could find applications in noise reduction and sound detection. (C) 2018 Author(s).