Research On Acoustic Modulation And Application Based On Metamaterials

Metamaterials have become a hot spot in the past decades because of their extraordinary physical properties which natural materials do not possess.The combination of transformation acoustics and highly anisotropic acoustic metamaterials can precisely control the deformation of field,resulting in acoustic cloaking.Recently,a kind of ultrathin metamaterial,known as metasurface,has emerged.Metasurfaces greatly reduce the thickness and produce many unique physical phenomena and functions,such as abnormal reflection/refraction,plane focusing,holography,absorption,and vortices.Moreover,active metamaterials use external control to create effective gain property that is not possible with passive structures,and have led to the development of dynamically reconfigurable,loss-compensating and parity-time-(PT-)symmetric materials for sound manipulation.This thesis conducts theoretical and experimental researches on the propagation of acoustic waves in metamaterials and metasurfaces,such as the transmission of zero-index metamaterials(ZIMs)with defects,novel physical effects in periodic PT-symmetric ZIMs,and acoustic metasurface carpet cloaks based on groove structures.The first chapter mainly reviews the research background of metamaterials,metasurfaces and PT-symmetric acoustics,and summarizes the main research contents.In chapter two,we theoretically and numerically investigate the acoustic transmission in a ZIMs waveguide with a rectangular defect.The effective mass density and the reciprocal of the bulk modulus of the labyrinthine metamaterials approach zero at the same time within a certain frequency range,so it can be regarded as a kind of ZIMs.Numerical simulations show that by adjusting the acoustic parameters of the defect,the transmission amplitude of the system can cover the entire [0,1] range,thus obtaining the effects of total transmission,total reflection,and cloaking.This chapter studies the manipulation mechanism of ZIMs on acoustic transmission,expands the manipulation methods of metamaterials on acoustic field.It has rich academic significances and important application prospects.In chapter three,we propose an acoustic waveguide consisting of periodic PT-symmetric ZIMs.Consistent analytical derivations and numerical simulations show that a rich variety of extraordinary scattering effects can be achieved in this waveguide system.For example,unidirectional transparency is found to occur at an exceptional point.At singular points,the system works in the acoustic coherent perfect absorption-laser mode,which can perfectly absorb specific coherent incident waves and simultaneously emit outgoing coherent waves for other incidences.In addition,bidirectional transparency,induced by the Fabry-Pérot resonance or balanced dissipation/amplification,is also observed.These extraordinary effects are achieved by tailoring the geometric parameters instead of the loss/gain,which may facilitate experimental realizations.This work provides an alternative way to demonstrate the physics of PT-symmetry,and may offer a new approach to the design of acoustic switches,absorbers,amplifiers,and functional devices with directional responses.In chapter four,we propose and investigate an acoustic metasurface carpet cloak(AMCC)based on groove structure unit.The carpet cloak consists of a series of grooved structural units closely arranged.The depth of grooves in each structural element is carefully designed,so that the phase delay introduced by the whole metasurface exactly compensates for the additional phase delay introduced by the object,thereby achieving the acoustic cloaking effect.The key idea is the phase compensation via local phase modulation,and thus the cloak is only with a thickness of a half wavelength.We first design a conical invisibility cloak,and prove that the proposed carpet cloak can achieve excellent cloaking effect for a conical object through numerical simulations and experiments.Its working bandwidth is 6200-7500 Hz,and it can work well under both normal incidence and small-angled incidence.Besides,we design a semispherical invisibility cloak and verify its excellent cloaking effect.Compared with the acoustic cloak based on transformation acoustics,this kind of metasurface carpet cloak has the characteristics of ultra-thin thickness,simple geometric structure,and easy to implement.Therefore,it is beneficial to future practical applications.Finally,we give the conclusions of this thesis and the prospects for future work.