(A) sound-absorbing metasurface using Helmholtz resonators having cavities partitioned non-uniformly by membranes

Abstract

Sound-absorbing metasurfaces are proposed, which consist of subwavelength Helmholtz resonators having cavities partitioned non-uniformly by membranes. Perfect sound absorption with sound absorption coefficients greater than 0.99 is achieved using hybrid resonance between the Helmholtz resonators with the membranes embedded at different locations. To design the metasurfaces, the effective acoustic impedance of the Helmholtz resonators with the membranes is theoretically derived by considering visco-thermal losses in Helmholtz resonators and end corrections due to radiation impedances at the ends of the neck of the Helmholtz resonator. Higher modes of the membrane and the damping effect in them are also taken into account to enhance the accuracy of the theoretical model. Two approaches are presented to design the metasurface for perfect absorption at a given target frequency using the theoretical model. The first involves adjusting tension in the membrane in each resonator at a fixed location, and the second approach involves adjusting the location of the membrane with a fixed tension value. Metasurfaces thus are designed so as to achieve perfect sound absorption via impedance matching with the air at the target frequency. Furthermore, the proposed metasurface can perfectly absorb the sound energy with a thinner thickness of λ/26.8 than the metasurface composed of the Helmholtz resonator without membranes. Perfect absorptions are theoretically achieved at dual frequencies with only two types of resonators. To verify their performance, a unit cell and resonators were fabricated by end milling. Perfect sound absorptions ware experimentally validated at frequencies of 632 Hz and 500 Hz using the fabricated metasurfaces with thicknesses of λ/13.6 and λ/17.2, respectively, in impedance tube.