| Acoustic metamaterials typically have an artificial periodic structure.By innovatively designing artificial unit structures,acoustic metamaterials can exhibit unusual physical properties such as negative effective mass density,negative effective modulus,and negative refractive index.Because of extraordinary physical properties,metamaterials can achieve acoustic effects such as low-frequency band gap,acoustic waveguide,and acoustic stealth,which cannot be achieved by conventional materials in nature.Research on acoustic metamaterials has improved the ability of metamaterials to control and manipulate the propagation of the elastic waves.Related studies have shown that the introduction of smart materials into the structure of acoustic metamaterials can help achieve active control of its dynamic characteristics and improve its ability to control elastic waves.In this paper,on local resonance acoustic metamaterial,a parameter-modulated acoustic metamaterial with a magnetorheological elastomer(MRE)as the coating layer and miniature electromagnetic coil as the mass unit is proposed.Based on this structure,the active dynamic characteristics of acoustic metamaterials are studied in terms of transmission,waveguide,and acoustic stealth characteristics.The work of this paper is organized as follows:The tunable transmission properties of MRE acoustic metamaterials are investigated.Based on the principle of local resonance,two acoustic metamaterial structures with the micro electromagnetic coil as a mass unit and MRE as single and double cladding layers are proposed for the first time.Based on the propagation theory of elastic wave in periodic structure and the variable magnetic modulus of MRE,the propagation equation of elastic wave in MRE acoustic metamaterial is established,and the relationship between magnetic field strength and elastic wave displacement field in metamaterial is obtained,which provides a theoretical basis for the study of adjustable transmission characteristics.The band gap structure and transmission spectrum characteristic curves of the two kinds of metamaterials are obtained by finite element simulation.The effects of magnetic field strength,material parameters,and filling ratio on the band gap frequency and transmission loss of the two kinds of metamaterials are analyzed.The analysis results show that when the magnetic field strength used for the cladding layer is 0-10 k Oe,the band gap frequency of the single cladding layer metamaterial changes from 488-1114 Hz to 770-1758 Hz,and the transmission loss changes from-30 d B to-51 d B,that is,as the magnetic field strength increases,the band gap frequency increases and the bandwidth increases,and the transmission loss increases.The double-layer metamaterial has the same change trend;in addition,with the increase of the core mass and elastic modulus,the band gap frequency decreases and the bandwidth increases,and the transmission loss decreases.As the filling rate increases,the band gap frequency increases and the bandwidth increases,and the transmission loss increases.The research results provide theoretical guidance for the active control of the elastic wave transmission characteristics of MRE acoustic metamaterials.The tunable waveguide characteristics of MRE acoustic metamaterials are studied.By changing the magnetic field strength and material parameters,the defects at different positions and different path lines are introduced into the supercell structure.Based on the plane wave supercell method with periodic structure and the propagation equation of elastic wave in MRE acoustic metamaterials,the relationship between the magnetic field strength and the propagation frequency of the elastic wave in the defective supercell is obtained,which provides a theoretical basis for the study of tunable waveguide characteristics.Through the finite element simulation,the band gap structure and displacement field of the supercell with point defect and line defect introduced by changing the magnetic field strength and material parameters are obtained.The results show that there are localized states and waveguide states in the point and line defect band structures.The point and line defects can form elastic wave localization and waveguide respectively.When the magnetic field strength changes from 4 to10 k Oe,the local frequency of elastic wave increases from 540 Hz to 750 Hz,and the frequency range of waveguide increases from 528-540 Hz to 692-760 Hz.Also,with the increase of core mass,the local frequency of elastic wave and waveguide frequency decrease.The research results provide theoretical guidance for the active control of elastic waveguide by MRE acoustic metamaterial structure and the research of new filter devices.A cylindrical array acoustic cloak is designed based on the MRE acoustic metamaterial,and its active acoustic stealth characteristics are investigated.Based on the theory of transformation acoustics,the theory of multi-layer homogeneous medium,and the properties of periodic array acoustic meta fluid,the material and geometric parameters needed to realize the cylindrical array acoustic cloak is obtained.Through the finite element simulation,the acoustic characteristics of the cylindrical array acoustic cloak are analyzed,and the sound pressure field distribution under zero-field and the different magnetic field strength are obtained.The simulation results show that under zero-field conditions,a good acoustic stealth effect can be achieved when the incident plane wave frequency is 3900 Hz;acoustic stealth can be realized when the magnetic field strength is 4,6,and 8 k Oe,the incident plane wave frequency is 4460,5030,and 5610 Hz,respectively,that is,the active adjustment of the acoustic stealth frequency is realized by changing the magnetic field strength.The results provide a theoretical reference for the structure design and active control of the acoustic cloak.The experimental research on the tunable transmission characteristics and waveguide characteristics of MRE acoustic metamaterial is completed.The experimental samples of MRE acoustic metamaterial are fabricated,and the experimental test platform is built.The adjustable transmission characteristics and waveguide characteristics are tested respectively.The influence of magnetic field strength,material parameters,and structural parameters on the transmission characteristics and waveguide characteristics is obtained.The experimental test results are compared with the finite element simulation results.The results show that when the magnetic field strength used for cladding is 6 k Oe,the transmission loss of single cladding metamaterial is-35 d B,and the relative error is 7.8%;the first-order transmission loss of double cladding metamaterial is-37 d B,and the second-order transmission loss is-59 d B,and the relative error is 7.5% and 15.7%,respectively;the magnetic field strength of introducing point and line defects is 6 k Oe the experimental test the local frequency of the elastic wave is 635 Hz,the highest frequency of waveguide is 645 Hz,and the relative error is5.8% and 6.6%,respectively.This shows that the experimental results are in good agreement with the simulation results,which verifies the correctness of the simulation analysis. |
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