Noise reduction in an open structure by tailoring complex impedances on acoustic metasurfaces

Abstract

In this study, we design acoustic metasurfaces to the inner surfaces of the open structure to mitigate noise emanating through the opening by manipulating complex impedances along unit cells called impedance tiles. The impedance tile is comprised of subwavelength Helmholtz resonators, which have a theoretical model that accurately predicts the effective impedance, considering visco-thermal losses in narrow orifices. By adjusting thegeometrical parameters of these resonators, the surface impedance of the impedance tiles can be finely tuned to minimize noise escaping from the open structure at any desired target frequency. To evaluate the noise reduction performance, the designed metasurfaces were fabricated via 3D printing, and the sound pressure levels (SPLs) were measured before and after applying the metasurfaces inside the open structure. The experimental results show that the λ/31-thick metasurfaces achieve SPL reduction exceeding 12.6 dB at the target frequency of 360 Hz.