Sound Manipulation By Acoustic Metasurfaces And Its Applications

In the past decade,the investigations of acoustic metamaterials have received growing attentions.As a kind of artificial composite structures comprising unit cells with size much smaller than the wavelength(namely sub-wavelength),acoustic metamaterial can be characterized by effective medium theory and have equivalent acoustical properties unattainable in the nature,e.g.,negative mass density,negative elasticity modulus,anisotropic parameters,and so on.Recently,the emergence of acoustic metasurfaces provides a new way to manipulate wavefront freely by introducing abrupt changes of acoustical phases,which breaks the dependence on the propagation effect,and enables molding wavefront into arbitrary shapes with subwavelength resolution.For example,we can realize extraordinary refraction/reflection,planar focusing,non-diffracting beams,sound absorption,or vortex by acoustic metasurface.The researches on acoustic metasurfaces and broadband devices have played a significant role not only in academic research but also in real applications.Based on this background,this paper studied the manipulations of sound wave by acoustic metasurfaces.In Chapter I,we review the background of the current research and recent advances in acoustic metasurfaces,and briefly introduce the related theories on the acoustic wave propagation in acoustic metasurfaces.In Chapter II,we first propose the concept of dispersionless wavefront manipulation and design of a reflective surface capable of controlling the acoustic wavefront arbitrarily without bandwidth limitation.We demonstrate extraordinary acoustic reflection,sound focusing,planar focusing and non-diffractive beam within an ultra-broad band.A physical model is also proposed on the basis of phased array theory to analytically predict the extraordinary phenomena,which may enlighten the design of acoustic broadband device.Then,we present the design of multi-frequency acoustic metasurfaces with simple structure that can work not only at fundamental frequency,but also at their harmonic frequencies,which breaks the single frequency limitation in conventional resonance-based acoustic metasurfaces.Our finding may extend the application of acoustic metasurface to nonlinear sources.In Chapter III,first,we design and fabricate a straight channel capable of realizing unidirectional acoustic transmission within a broad band while leaving a gap much wider than the wavelength that may serve as a passage for other entities such as fluids or objects.Then,we propose an acoustic tunnel completely open for substances like fluids or other energy fluxes to exchange while allowing sound to pass only in one direction.Furthermore,we present the mechanism for breaking the symmetry in sound transmission between any two neighboring ports in a passive multi-port system.The number and the position of the ports can be adjusted freely.At last,we experimentally demonstrate the nonreciprocal sound transmission based on angular-momentum-biased resonator.We propose a new method to provide angular-momentum-bias that does not require very large rotation speed.The proposed four mechanisms above will be helpful in design of new type acoustic one-way devices.In Chapter IV,we show that by tailoring the loss in acoustic metasurface,it is possible to realize simultaneous and decoupled manipulation of amplitude and phase of sound,and realize fine manipulation of sound.We further demonstrate the significance of our approach by producing self-bending beams,multi-focal focusing,single-plane 2-D hologram,and multi-plane 3-D hologram with quality better than the previous phase-controlled approach.Our work provides a paradigm-shift for harnessing sound via a new degree of freedom on engineering the loss,and gives rise to promising device applications on acoustics and related fields.In Chapter V,a new class of ultra-thin and planar Schroeder diffuser is proposed based on the concept of acoustic metasurface.Both numerical and experimental results demonstrate satisfactory sound diffuse reflection produced from the metasurface-based Schroeder diffuser despite it being approximately one order of magnitude thinner than the conventional one.We also propose a hybrid structure to improve the bandwidth of ultra-thin metasurface-based Schroeder diffuser.The proposed design not only offer promising building blocks with great potential to profoundly impact architectural acoustics and related fields,but also constitutes a major step towards real-world applications of acoustic metasurfaces.Finally,the main conclusions of the present study and the prospect for the future work are drawn in Chapter VI.