Sound Field Manipulation Based On Passive Artificial Structrues

Sound field manipulation is one of the most important problems in acoustics,but still suffers from the acoustical properties available in natural materials on which the conventional acoustic theory depends,posing many fundamental limits to the wave-steering capability.Representative examples include the difficulties in realizing asymmetric acoustic transmission and control of low-frequency waves with compact structures.Consequently,it is of remarkable scientific and practical significance to explore the possibility of using artificial structures to break through the fundamental limits in conventional acoustics and to use passive structures to enable unconventional acoustic manipulation,which may find wide applications in many application scenarios ranging from noise control to electroacoustics design.The thesis contains mainly 4 parts,1.We propose to utilize natural material with easy fabrication,low loss,nondispersion,chosen as xenon,to fill a spatial region of asymmetric shape.Due to the asymmetric geometry and contrast between the refraction indices of air and xenon,the structure allows the incident plane wave to pass along one particular direction while gives rise to nearly total reflection as the incident direction is reversed regardless of its frequency.The effectiveness of the proposed design is verified via numerical simulations which show a highly asymmetric transmission of airborne sound persists within an ultra-broad frequency range.The realization of high efficiency and broadband asymmetric acoustic manipulation with a simple,ultra-lightweight and optically transparent structure offers new possibility for the design of acoustic one-way devices and has application potential in a variety of scenarios such as noise control in ducts.2.We investigate the mechanism of designing open structures for producing broadband asymmetric acoustic transmission while allowing other entities such as fluid to pass.Two identical size right triangles filled with xenon are placed on the two sides of the duct at a distance which forms a gap ensuring the continuity of the background media as a passage for fluid.Based on the first design which is capable of realizing asymmetric transmission in the frequency range from 3 kHz to 10 kHz,we present the other structure with improved performance that works in a wider range with higher efficiency and wider open passage for better ventilation3.We explore mechanisms for high-efficiency collimation of airborne sound through an ultra-thin planar structure perforated with a single deep-subwavelength aperture by combining a zigzag-shaped structure with arrays of subwavelength Helmholtz-like resonators.The aperture is designed to have a zigzag-shaped cross section for increasing the propagation distance of incident acoustic wave substantially,mimicking a straight slit filled with a high refractive index medium and enabling effective enhancement of transmission at relatively low frequency region.On the other hand,the surface of the plate is rationally engineered to modify the radiation pattern on the transmitted side to acoustic surface modes.Numerical simulations demonstrate the proposed mechanism enables the coupling between F-P resonance and surface modes,leading to remarkably-enhanced transmission and acoustic radiation with high directivity which propagates for a strikingly-long distance exceeding 125 wavelengths.The capability of the proposed design opens up possibilities for deep subwavelength acoustic-steering devices and may have impacts in diverse applications such as acoustic communications and electroacoustics design.4.We investigate the dependence of transmission efficiency and far-field directivity on the period of the Helmholtz-like array,D,and the number of the grooves,r.The results show that the change of D and r does not affect the resonant frequency,transmission coefficient and directivity significnatly,so the collimation functionality is dependent of the coupling between F-P and Helmholtz-like resonances instead of Bragg scattering.This suggests the possibility to further downscale the collimation device and enable production of a high directivity beam with a compact size for airborne sound at lower frequencies.