Theoretical And Experimental Study On Sound Wave Manipulation Via Acoustic Metamaterials

Acoustics metamaterials is a new type of artificial structures with unprecedented ability to engineer the sound properties that are not found in nature.Acoustic metamaterials with zero,or negative refractive index offer new possibilities for applications like acoustic imaging,abnormal transmission/refraction and enable the control of sound at subwavelength scales by use of various elements such as labyrinth structures,thin membranes and etc.In this thesis,several acoustic metamaterials are carefully designed,with the related applications analyzed in detail,and the physical predications verified by experiments.The main content and innovation points are summarized as follows:1)The theory of fractal metamaterials and the related parameter retrieval method are discussed.A new kind of acoustic metamaterial is designed based on the Hilbert fractal geometry.Different functionalities are experimentally demonstrated including wideband focusing,wavefront conversion,negative refraction and sound absorption at low frequencies,showing advantages of wide bandwidth,matched impedance and design flexibility.2)The omnidirectional radiation based on anisotropic zero refractive index metamaterial ring is designed and experimentally verified.It shows that the designed metamaterial can excite the fundamental omnidirectional mode no matter how many sources are included,making it possible for spatial power combination in the future.3)A hyperbolic anisotropic metamaterial is designed with two components of the mass density tensor opposite in sign.Both the negative refraction and sub-wavelength imaging phenomena are obtained from numerical simulations.4)An immersed acoustic magnifier based on transform acoustics is proposed.The measured results show that the magnifying lens can realize super-resolution imaging of two sound sources with the 0.375 ? distance from 5650 to 6350 Hz.5)A planar acoustic retroreflector is proposed,which is constituted by a cascade of the transmissive metasurface and the reflective metasurface.The designed acoustic retroreflector is fabricated and experimentally verified.It can work in the angular range of ±70 ° and the operating efficiency above 40%.6)An ultra-thin acoustic coding metasurface is designed to achieve multi-beam backward acoustic scattering.Compared to the hard wall,the ultra-thin random coding metasurface can redistribute the incident sound into different directions with significant reduced sound pressure level.7)A double membrane metasurface is proposed to realize sound and microwave manipulation simultaneously.The upper layer of the metasurface is a pet film coated with a square ring array of ITO films,and the lower layer is composed of the PET-ITO film as a metal ground.The composite metasurface can achieve wide band sound insulation.Both the numerical and measured results demonstrate that wideband sound insulation,electromagnetic absorption and light transparency are achieved at the same time.