Research On Acoustic Functional Devices Based On Surface Acoustic Impedance Modulation

This research is about acoustic meta-surface based on surface acoustic impedance modulation and related new-principle-based acoustic functional devices.Firstly,an artificial acoustic impedance meta-surface composed of sub-wavelength air gap structure is introduced,and its acoustic characteristics of conducting and modulating evanescent sound waves are discussed.Secondly,the principle of in-plane modulation for the meta-surface is proposed.Two new evanescent wave lenses on which evanescent sound waves can be allowed to propagate are designed through theoretically designing the surface refractive index distribution of the lens and making use of the artificial acoustic impedance meta-surface.After that,combined with the holographic principle,we have realized the meta-surface out-plane modulation of evanescent waves.Three new kinds of functional directional acoustic antenna are imparted by using the artificial acoustic impedance meta-surface to realize the conversion of the surface evanescent wave to a directional far-field radiating antenna,with the introduction of acoustic orbital angular momentum and multiplexing concepts.In the first chapter,the basic concepts,development history and research status of acoustic meta-surface are briefly reviewed,and the main contents of this research work are summarized.In the second chapter,mechanism for in-plane modulation of evanescent waves and the method of designing a new evanescent wave lens using acoustic artificial meta-surface are discussed.Firstly,the sub-wavelength air gap structure is introduced.After that,how to extract the surface refractive index corresponding to this surface cell and then how to use the proposed cell structure based on the surface refractive index to design the two evanescent wave lenses(Bessel evanescent wave lens and Fourier transforming evanescent wave lens)are illustrated.The corresponding simulation results effectively verify the correctness of the theoretical design.Bessel evanescent wave lens and Fourier transform evanescent wave lens have great application advantages in the field of non-diffracting surface energy transmission system and regulating sound source directivity respectively.In the third chapter,the concept of artificial holographic acoustic impedance surface,and the mechanism for out-plane modulation are discussed.The method of designing low profile holographic directional acoustic antenna by using the holographic interferogram characterized by acoustic impedance unit to modulate impedance are also included,and its equivalence with leaky wave antennas and characteristics of spatial beam frequency scanning are verified.The acoustic surface impedance is the key physical quantity that connects the near-field surface evanescent wave with the far-field radiated acoustic wave,whose size is controlled by the sizes of the surface acoustic impedance unit.The corresponding simulation results effectively verify the feasibility of the concept,and the realized holographic leaky wave antenna is simple in structure and flexible in design,which can meet most of the design requirements of beam radiation and creates a foundation for the subsequent design of functional devices for antenna radiation.In the fourth chapter,based on the introduction of the relevant modeling theory of artificial acoustic impedance meta-surface and the corresponding antenna design method in the previous chapters,the method of designing multi-functional low-profile holographic leakage wave directional acoustic antenna using acoustic artificial holographic impedance surface is discussed.By introducing a new degree of freedom OAM into the hologram or combining the multiplexed concept to the hologram to design a multi-beam multiplexed and a multi-wavelength multiplexed hologram,holographic leaky wave acoustic vortex antenna,multi-beam holography leaky wave acoustic antenna,and dual-frequency holographic leaky wave acoustic antenna are realized respectively.The corresponding simulation results verify the effectivity of the design method and the ability of the holographic impedance surface to be freely regulated.The design of these holographic leakage wave directional acoustic antennas is beneficial to improve communication capacity and space utilization,which has great application prospects in antenna communication systems.The fifth chapter summarizes the work and looks into the future.In summary,this thesis explored the acoustic characteristics of artificial acoustic impedance meta-surface conduction and modulated evanescent sound waves composed of sub-wavelength air gap structures.Based on these technologies,a variety of new-principle acoustic functional devices are designed.This design method based on impedance modulation has deep prospects from the perspective of theoretical exploration and practical application.