Abstract
Subwavelength metasurfaces for quasi-omnidirectional broadband high absorption are designed to attenuate the low-frequency noise of around 100 Hz produced in industrial environments. They are made of resonant structures denominated as serial Helmholtz resonator (SHR) consisting of an inserted-neck resonator embedded into the cavity of an extended-neck Helmholtz resonator (EHR). The transfer matrix method and equivalent medium theory are combined to develop a quasi-analytical model to investigate the absorption mechanisms and characteristics of the SHR. The theoretical analysis and simulations show that the metasurface containing a single SHR exhibits three peaks with almost perfect absorption. The quality factor in the one-third octave band (89¿112 Hz) centered at 100 Hz is above 80 % for the incidence angle range of 0~80°, indicating the quasi-omnidirectional broadband high absorption. Experiments under normal incidence have supported the predicted performance. The subwavelength thickness of the SHR optimized by sequential quadratic programming algorithm is 11.1 cm; 1/31 of the wavelength at 100 Hz. Using the impedance coupling effect and optimization algorithm, quasi-omnidirectional broadband high absorption of another metasurface consisting of three SHRs can be realized within two consecutive one-octave bands (100¿400 Hz). The quality factor reaches 100 % when the incidence angle is between 57° and 67°. The subwavelength thickness of the metasurface is 18.2 cm, which is 1/19 of the wavelength at 100 Hz. Compared with other sound-absorbing structures, the proposed metasurfaces have subwavelength thickness, quasi-omnidirectional broadband high absorption, and easy processing, which provide promising solutions for low-frequency noise attenuation
