A modular sound-attenuating metasurface with subwavelength thickness and hexagonal unit cells

A sound attenuating subwavelength metamaterial efficient in the low audible frequency range is proposed. The metasurface unit cell has a hexagonal shape with an axial ventilation channel inside. The side walls of the hexagonal cell are sub-divided into six hollow chambers connected to the central air path via the six thinner channels of different diameters. The hollow chambers act as Helmholtz resonators providing six different resonant frequencies for each cell. The negative effective bulk modulus property of the metasurface allows near-full sound block at the resonant frequencies and partial sound adsorption inside the intermediate frequency bands. By carefully designing the size of the connecting side holes we can adjust the desired sound attenuation frequency range. The thickness of the metasurface is in the range of 0.05–0.15 wavelengths for the sound frequency range between 300 and 1000 Hz. The hexagonal shape of the unit cell allows full honeycomb tessellation of the metasurface volume. By designing a hybrid metasurface containing individual cells with different resonators, the absorption peaks can be multiplied or merged together thus further broadening the frequency range with the transmission coefficient lower than the desired threshold value.