| With the continuous progress of global technology,industry and transportation,low-frequency noise sources have become more and more extensive,mainly including power transmission and transformation projects,nuclear industrial plants,power stations,vehicles,aerospace,etc.Aiming at the low-frequency line spectrum noise,this paper designs an ultra-thin multi-order absorbing metasurface unit with a fundamental frequency below 200 Hz.The mathematical model of plane wave propagation in the low-frequency multi-stage sound-absorbing metasurface is established by the mode expansion method and the acoustic-electrical analogy method.The simulation model of the structure is established by the finite element method,and the superior sound absorption effect of the designed structure is verified by experiments,and The correctness of the two models is also verified.At the same time,in view of the current low-frequency line spectrum noise problem in air ducts,the ultra-thin design of the meta-structure surface is applied to the pipe muffler application,which can greatly reduce the thickness of the structure,realize the sound wave control with a thin layer and a small size,and expand the ultra-thin layer.The scope of application of the structure surface.Mainly involves the following work:Based on the equivalent medium theory,a low-frequency multi-stage sound-absorbing metasurface is designed.The spiral channel(Fabry-Perot cavity)is designed to reduce the thickness of the structure,and the impedance is increased by adding perforated baffles and embedded holes.The degree of freedom controls the impedance;the normal impedance of the embedded hole and the Fabry-Perot cavity is analyzed,and the impedance of each part of the meta-surface structure is intensively equivalent using the acousto-electric analogy method to realize the overall normal direction of the system,etc.Effective impedance and absorption coefficient solution.3D printing technology is used to make meta-surface experimental samples,and the sound absorption coefficient is measured experimentally.The results show that additional perfect absorption peaks can be added when the outer shape of the meta-surface remains unchanged and the original perfect absorption peak is maintained.At the same time,the peak frequency can be tuned.At the same time,the more perforated partitions with embedded holes,the higher the freedom of impedance manipulation and the stronger the tuning ability of the metastructure surface.In addition,the numerical solution of the finite element simulation and the analytical solution of the mathematical model are in good agreement with the experimental results,and both are reliable and effective.Secondly,when studying the structural parameters of the designed multi-stage sound-absorbing metasurface,it is found that the peak frequency is determined by the imaginary part of the acoustic impedance,and the absorption coefficient is determined by the real part of the acoustic impedance,and when the normalized impedance is imaginary When the part is 0 and the real part is close to 1,a quasi-perfect absorption peak will appear.At the same time,the acoustic energy dissipation of the multi-stage sound-absorbing metasurface is mainly concentrated on the embedded hole,and it works in series at each resonance frequency.Finally,the metasurface units are combined in a certain spatial arrangement,and the sound absorption characteristics of superposition coupling are studied.It is realized that the30 mm thick broadband coupled sound absorption metasurface can absorb more than 90% of the sound waves with frequency of 155-167 Hz.The effect is that the ratio of wavelength to thickness is 71.5.And the characteristics of realizing sound wave control with a thin layer and small size are applied to the pipe muffling,achieving a thickness of 30 mm at 147 Hz to isolate more than 97.9% of the sound,and the wavelength-to-thickness ratio is 77.8.The designed structure can solve the problem of noise control in the limited space in reality. |
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