Research On Non-reciprocal Unidirectional Acoustic Transmission Effects Of Space-time Modulated Acoustic Metamaterials

Acoustic metamaterial is a branch of metamaterials in the acoustic field,whose core concept is to break some apparent natural laws through structure design.These artificial acoustic structures can achieve supernormal acoustic properties that are unavailable from nature,as well as some extraordinary function that provides a much wider range of possibilities for acoustic devices.As a newly proposed functional acoustic device,acoustic diode has a great significance and value in both theoretical and applied acoustics.However,due to their inherent mechanisms,all those proposed acoustic diodes based on nonlinearity or external flow fields have some intrinsic drawbacks,including system bulkiness,low transmission efficiency,highly acoustic signal distortion,and so on.These disadvantages greatly hinder their practical application in the industry production and real life.The emergence of space-time modulation offers a brand-new idea to solve those problems.As an additional design freedom,temporal modulation can easily break the time-reversal symmetry of a system with spatial bias,and in principle realize non-reciprocal transmission of acoustic waves,electromagnetic waves or even electricity,namely the diode effect.So far,the application of space-time modulation in acoustic metamaterials is still at an early stage.In this dissertation,systematic studies are carried out on non-reciprocal unidirectional acoustic transmission based on space-time modulated acoustic metamaterials.Firstly,to solve the computational problem of space-time modulated systems with multiple load nodes and complex boundary condition,a generalized model for space-time modulated one-dimensional acoustic metamaterial/system is proposed,which is based on transfer matrix method.This method is expected to play a significant role in the following research of space-time modulated acoustic metamaterials and functional acoustic devices.Then,an effective medium model of acoustic metamaterial with space-time modulated density or elastic modulus is established.Studies on extraordinary acoustic effects of space-time modulated acoustic metamaterials under two classic modulation profiles are carried out,which is expected to provide the principle and guideline for the following non-reciprocal acoustic devices’ design.Afterwards,acoustic transmission characteristics of space-time modulated membranes are studied.Based on the extraordinary acoustic effects of space-time modulated acoustic metamaterials,two kinds of non-reciprocal acoustic devices based on membrane structure are theoretically demonstrated.Lastly,the acoustic transmission characteristics of space-time modulated Helmholtz resonators is also studied.An acoustic diode based on Helmholtz resonator structure is designed and an acoustic experiment is set up to test its non-reciprocal transmission performance.Here come the main results of this dissertation.A transfer matrix method for space-time modulated one-dimensional system is developed.This method can solve the computational problem of space-time modulated systems with multiple load nodes and complex boundary condition,which also improves the accuracy and efficiency of computation.An effective medium model with space-time modulated density or elastic modulus is proposed.Studies on extraordinary acoustic effects of acoustic metamaterials under subtractive and additive modulation profiles are carried out,which demonstrate the frequency conversion and parametric amplification effects in acoustics respectively.A theoretical model of space-time modulated membrane system is established.The mechanism of non-reciprocal acoustic transmission in space-time modulated membrane system is revealed.Combining the temporal modulation effect together with a spatial bias from modulation phase difference between membranes,an acoustic diode and a tunable unidirectional acoustic amplifier are theoretically realized based on a two-membrane structure.The acoustic transmission characteristics of space-time modulated Helmholtz resonators is studied.An inherent contradiction between the transmission efficiency and modulation effect of cascaded Helmholtz resonator structure is revealed.Thus,an acoustic diode based on an interconnected two-Helmholtz resonator structure is proposed,whose isolation factor and working band are successfully demonstrated in experiment.