Complex-valued impedance tiles to reduce noise emanating through openings in mechanical systems

We present acoustic metasurfaces to achieve high noise reduction for an open structure, which refers to a structure with openings, by manipulating complex impedances along unit cells called impedance tiles. These acoustic metasurfaces are applied to inner walls of the open structure to suppress noise emanating through openings. Each impedance tile is made up of subwavelength Helmholtz resonators (HRs), and its effective impedance is tuned to minimize the noise level outside the open structure at an arbitrary target frequency by adjusting the geometrical parameters of those HRs. Rather than absorbing incident waves, the proposed metasurfaces significantly affect the magnitude and phase of scattered waves, causing the sound energy to be trapped near their surfaces and dissipated by visco-thermal losses in narrow orifices of the impedance tiles. As a result, the metasurfaces enable more effective noise reduction for the open structure compared to impedancematching conditions, which absorb sound waves impinging on the surface. Experimental results show that the sound pressure level reduction exceeding 12.6 dB is verified for a single source at a target frequency of 360 Hz with lambda/31-thick metasurfaces fabricated via three-dimensional printing. Furthermore, we demonstrate that the proposed metasurfaces are also useful in reducing noise when the number of target frequencies is more than one or when multiple sources are distributed in different locations, thereby offering a potential solution for noise reduction in a wide range of mechanical systems with open structures.