A compact low-frequency sound-absorbing metasurface constructed by resonator with embedded spiral neck

This Letter reports an acoustic metasurface for low-frequency sound absorption with a compact size in both vertical and lateral directions, which is constructed by a series of Helmholtz resonators with spiral extended necks. Analytical, numerical, and experimental results show that the metasurface with a thickness of 13.5 mm possesses total sound absorption at 320 Hz under normal incidence. The thickness of the prototype is only about 1/80th of the operating wavelength (λ). What is more, the side length of the basic unit (25 mm) is also on the deep-subwavelength scale (λ/43). To widen the narrow effective absorption bandwidth of the uniform absorber, the strategy of parallel arrangement of different elements is employed. A wideband absorber consisting of four inhomogeneous units is optimally designed to maximize the averaged absorption coefficient in a prescribed frequency range of 360, 410 Hz. The experimental results show that the absorber with a thickness of 13.5 mm exhibits high absorption (the averaged absorption coefficient is about 0.9) in the desired frequency range. The features of effective low-frequency sound absorption, compact dimension, and high absorption with a tunable bandwidth make the proposed acoustic metasurface promising for various applications in noise control engineering.