Manipulation Of Acoustic Waves Based On Acoustic Metamaterials

Recently,the appearance and rapid development of acoustic metamaterials have aroused great interests around the world.It is typical to classify the acoustic metamaterials into two types including photonic crystal and acoustic artificial structure.The feature size of photonic crystal is comparable to the wavelength,however,the feature size of the artificial structure is much smaller than the wavelength and can be described by the effective medium theory.Since acoustic metamaterials can provide many extreme parameters which cannot be easily realized by the traditional materials in nature such as negative mass density,negative bulk modulus and acoustic anisotropic parameters,they are widely used to realize many novel phenomena,for example,anomalous reflection/refraction,one-way transmission,cloak,absorber and etc.The research on the propagation of sound waves in artificial structures and the interaction between sound waves and structures has played a significant role not only in academic research but also in the real applications.Based on this background,this paper studies the manipulation of sound waves by acoustic metamaterial.In Chapter ?,the background of the research and the progresses of the investigation on acoustic wave propagation in acoustic metamaterials and the related theories on the acoustic wave propagation in acoustic metamaterials are introduced respectively.In Chapter ?,we studied the transmission properties of acoustic waves in two-dimensional waveguides and realize sound focusing in multi-focus.In our research,an acoustical meta-structure is proposed to focus reflected wave into multiple focus in a broad band.This meta-structure has a simple configuration and planar shape,consisting of an array of groove-like metamaterial units with subwavelength width.The method of harnessing the convergence of reflected wave by controlling the local phase response is proposed.An analytical expression of the parameter distribution for realizing broadband focusing at given foci is derived,which can be used to precisely design the acoustic meta-structure capable with broadband multi-focus focusing.The performance of the designed meta-structure is demonstrated via numerical simulations,showing that the meta-structure is able to converge the acoustic energy of incident wave into the preset multiple focus.This scheme enables free and precise control of the number and relative locations of foci,and can be designed to yield desired focusing effect even in the presence of obstacles with sizescomparable to the working wavelength.With the merits of simple fabrication andtunable focus,our design(?)important situants such as medical ultrasound and non-destructive tests.In Chapter ?,we report the theoretical design,numerical simulation and experimental demonstration of a flat mirror capable to efficiently focus reflected sound in three-dimensional space within an ultra-broad band.The proposed mirror is implemented with a textured rigid surface,enabling simply design and easyfabrication.We analytically derive the distribution of geometric parameters needed for producing ultra-broadband focusing,and verify the performance of resulting mirror both numerically and experimentally.Furthermore,our scheme allows further extending the working band by resizing individual elements.Our design with simplicity and capability may promote the application of focusing devices generally subject to limited bandwidth.In Chapter ?,we propose to produce efficient three-dimensional sound converging in broadband with binary reflected phases on a planar surface with unit cells consisting of only two kinds of elements.The mechanism is experimentally demonstrated by focusing airborne sound and by forming an "acoustic needle",with handmade arrays of commercial test tubes with/without lids.Both the simulated and measured results show precise control of converging acoustic energy despite misalignment errors obvious even to naked eyes.Our approach with extreme simplicity yet good robustness may apply in various scenarios that conventionally need complicated elements and continuous variation of parameters for focusing sound.Above all,in this paper,we mainly researched the manipulation of sound waves based on acoustic metamaterials.These results could provide some unique thinking in the design of acoustic devices in many fields including medical ultrasound applications and architectural acoustical designs.