Acoustic metasurface constructed by Helmholtz-like resonators with embedded necks of gradient cross-section

Abstract This study proposes a compact acoustic metasurface consisting of Helmholtz-like resonators arranged in series/parallel. Unlike the traditional design equipped with uniform necks, the proposed necks possess gradient cross-section in a 2D plane. The geometric parameters of the present structure are designed and optimized by using a particle swarm optimization algorithm (PSO). This structure exhibits excellent sound absorption below 200 Hz with a deep subwavelength thickness. Adjusting the neck length is effective in altering the absorption peak frequency. A structure with resonators in series yields approximately 80%–90% absorption at 129 Hz, while the one with resonators in parallel yields almost the same absorption coefficient, but the absorption peak frequency is increased by 15%. Theoretical and numerical finite-element-method predictions match well with experimental results.