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

Compared to traditional natural materials,acoustic metamaterials have numerous specific properties and exhibit great application potentials in various fields,as manipulation of acoustic waves,sound absorption and insulation,and etc.In this work,we study the manipulation of the group velocities of acoustic waves and design an adjustable acoustic metamaterial on the basis of membranes.The article is divided into four parts as follows:In chapter 1,the origin and global development of acoustic metamaterials are introduced.Additionally,the research background concerning the work of this thesis is presented.In chapter 2,an acoustic metamaterial based on a fractal structure,the Koch curve,is presented,which realizes the manipulation of the group velocities of acoustic waves.Theoretical and experimental results show that slow and fast acoustic waves,even with negative group velocities,are simultaneously obtained in the structure.Due to the multiple transmitting paths in the structure,the acoustic waves travelling along diverse paths interfere with each other.Therefore,sound energy is trapped in the Koch-curve-shaped loop and results in slow waves.On the other hand,the fast waves originate from the destructive interference of two acoustic waves with opposite phase,which even leads to negative group velocities.Moreover,simulated results exhibit that we can achieve fast and slow waves at more frequencies by designing a metamaterial based on a high-order Koch curve,which can be applied in the fields requiring the manipulation of group velocity.In chapter 3,we study the nonlinear effects and active controlling in a type of metamaterial for perfect sound absorption based on a membrane and cavity.The structure is created on the basis of a cavity with hard walls and sealed by a mass-attached membrane,which can realize perfect sound absorption at given frequencies.We study the influences of the nonlinearities of the membrane on the sound absorption of the structure.It is observed that the frequency for perfect sound absorption blueshifts if the intensity of the input acoustic wave is increased,which results in the increase of the vibration displacement of the membrane.Additionally,we can actively control the stiffness of membrane with a system based on electromagnets,which realizes the adjustable perfect sound absorption.In chapter 4,the summary of the thesis and the plan for future work are presented.