Acoustic Conjugate Metamaterials

Nature materials inherently have limits in manipulation of acoustic waves,while acoustic metamaterials provide a new way to control acoustic waves due to their subwavelength size,flexible design freedom and novel performance.In this work,the acoustic conjugate metamaterial(ACM),a type of acoustic metamaterial possessing conjugate complex mass density and bulk compressibility,is presented,which provides a new way for the modulation of material parameters.This metamaterial simultaneously contains gain and loss and can produce unique acoustic laser mode and coherent perfect absorption at special incident angles.The article is mainly divided into the following four parts:In chapter 1,the origin of acoustic metamaterials is introduced.Then the progress in the manipulation of the real parts,imaginary parts and complex parameters of acoustic metamaterial is presented.In chapter 2,on the analogy of electromagnetic conjugate metamaterials,the conjugate characteristics of ACMs is derived by the method of field transformation acoustics,and the chiral properties are classified according to the phase factors.The reflection and transmission coefficients of a semi-infinite ACM and an ACM slab are calculated and the distribution of acoustic field inside the ACMs are studied.Theoretical analysis and finite element simulations show that acoustic laser mode,perfect absorption or coherent perfect absorption can be achieved at critical incident angles.Moreover,it is observed from the dispersion equations that a variation of the incident angle or the phase factor can produce a mode shift between even modes and odd modes,acoustic laser mode and coherent perfect absorption.In chapter 3,the method to realize an ACM is studied.The effective parameters of a core-shell structure,which is composed of a fluid matrix and a solid inclusion,or a solid matrix and a solid inclusion,are deduced by using the multiple scattering theory.Based on the analogy between electromagnetic and acoustic waves,we explore the combination of real and complex parameters in core-shell structures and extract dynamic effective parameters,which provides a method to design an ACM.In chapter 4,the summary of the thesis and the plans for the future work are presented.