Acoustic metasurfaces for reducing noise radiated from structures with openings

Abstract

In various industrial fields, openings or apertures in mechanical systems are essential for heat exchange or fluid flow to maintain the mechanical performance of systems such as home appliances, transformers, and cooling towers. In the mechanical systems, there are various noise sources such as motors and fans. However, since these noise sources cannot be completely enclosed by the structures, noise is emanated to the outside through the openings. In order to reduce this emanated noise, acoustic metasurfaces that can perfectly absorb sound through impedance matching have been proposed. However, even if perfect sound absorption is achieved within the structures, noise can still escape through openings. In this study, to reduce emanated noise through openings, complex impedances are controlled instead of satisfying the impedance matching condition. A design procedure of an acoustic metasurface using complex impedances is proposed as follows. First, when the noise source is inside a structure with an opening, distributions of effective impedances on the interior surfaces of the structure are derived through optimization to minimize the radiated sound power. Next, the effective impedances are implemented with the aid of the inverse design of acoustic metasurfaces consisting of subwavelength Helmholtz resonators. The resulting designed metasurfaces were found to reduce the radiated noise when a single monopole source of 1000 Hz is located inside a structure with an opening. The designed metasurfaces were fabricated through 3D printing, and a noise reduction performance of 13 dB was verified through experiments in an anechoic chamber. In addition, by designing the metasurfaces for various sound source conditions and geometric conditions of the structure, a possibility of radiated noise reduction in a situation closer to the actual conditions is confirmed.