Studies Of Manipulation Of Group Velocities And Perfect Sound Absorption Based On Acoustic Metamaterials

Acoustic metamaterials is a type of artificial composite structures,which can be used to manipulate sound waves.Especially in recent years,acoustic metamaterials develop rapidly and exhibit great application potentials in different fields.In this work,we use acoustic metamaterials to realize the manipulation of the group velocities of acoustic waves,simultaneously achieving negative group velocities,fast and slow waves.Additionally,we design a tunable structure for perfect sound absorption based on a membrane-type metamaterial.The thesis covers five parts as follows:In chapter 1,the origin and developing of acoustic metamaterials are firstly introduced,then the research background and progresses of the studies are presented.In chapter 2,an acoustic metamaterial is designed based on a simple and compact structure of one string of side pipes arranged along a waveguide,in which the group velocities of acoustic waves are manipulated.Theoretical and experimental studies show that negative group velocities,fast and slow waves can be simultaneously obtained in this structure.Due to Fabry-Perot resonance induced by the side pipes,which resembles the Lorentz resonance in electromagnetic fields,an extraordinary dispersion arises at the resonant frequency,results in spectral rephasing.Then,a abrupt jump can be observed in the phase spectrum,which can be used to manipulate the group velocities of acoustic waves.In chapter 3,a type of phononic crystal is presented based on a LiNbO3 slab,in which a line defect is created within a honeycomb lattice array of cylindrical holes etched in the LiNbO3 slab.Finite element simulations in frequency domain and time domain demonstrate that a highly localized slow mode is obtained in the defect.Due to the piezoelectricity of LiNbO3,coupled slow waves,slow acoustic waves and electromagnetic waves,are simultaneously achieved in the line defect based on the piezoelectric slab.In chapter 4,a tunable membrane-type acoustic metamaterial is designed to achieve perfect sound absorption.Due to the multiple scattering in the resonant cavity behind the membrane,a hybridized resonance is obtained between two eigenfrequencies of the membrane,which results in perfect sound absorption without reflection of acoustic waves.Then,based on the mechanism,we create a tunable metamaterial to realize perfect sound absorption.Based on the nonlinearities,by changing the intensities of the input acoustic fields,the stiffness of the membrane can also be changed,and thus the frequency for the perfect sound absorption is adjusted.Finally,the summary of the thesis and the plan for future work are presented in chapter 5.