| With the rapid development of social economy,the problem of noise control in civil engineering,industrial engineering and military environment is becoming more and more difficult.Most of the noise is in the middle and low frequency range,and the low frequency noise will not only affect the use of equipment,but also affect the normal physiological function of the human body.Therefore,the realization of noise absorption and control in the low frequency range is an important issue to be solved urgently.The traditional sound absorption materials such as fibrous and foam porous materials have the disadvantages of poor sound absorption performance,large volume and poor durability in the low frequency range.Traditional resonant sound absorbing materials have disadvantages such as large back cavity depth,low frequency acoustic absorption amplitude and narrow sound absorption bandwidth,which become the shackles of its development in the field of low frequency noise controlIn order to break the shackles of poor low frequency absorption performance of traditional acoustic materials and fill in the blank of low frequency noise control,acoustic metamaterial came into being,because of its flexible designability,rich tuning degree of freedom,small size control advantages of large wave length,it provides a new technical solution for the field of structural noise reduction,and has a good application prospect.However,it is still a very challenging problem to realize low frequency acoustic wave absorption control on the premise of ensuring the overall thickness of the sound absorption structure is thin.Based on the sound absorption principle of coiling-up channel structure and Helmholtz resonator,three acoustic metamaterial thin laminates with nearly perfect sound absorption characteristics in the low frequency range are designed and studied in this paper.The main work completed in this paper and the research results obtained are as follows:1、By introducing the coiled-up space into the design of Helmholtz resonator,a metamaterials composed of finger coiled-up cavity is designed.Firstly,through the combination analysis of particle velocity field and sound pressure level field nephogram,as well as the analysis of surface acoustic impedance,it is revealed that the sound absorption mechanism of the metamaterial laminate is caused by the perfect match between the acoustic impedance of the structure surface and the characteristic impedance of the air.The sound energy is dissipated not only because of the resonance caused by the mass entry of sound waves into the inner folded cavity,but also because of the heat-viscosity effect caused by the friction between sound waves and the inner wall of the narrow cavity.Secondly,the influence rule of different parameters on sound absorption performance was explored through geometric parametric scanning.Finally,the low frequency sound absorption performance of the design structure was verified by sound absorption experiment.The results show that the minimum sound absorption frequency of the designed structure can reach nearly perfect sound absorption(0.994)at 348 Hz,and the controlled wavelength is more than 41 times of the metamaterial thickness(24mm).The design method of broadening the bandwidth is discussed: By combining two half-cell units with different folding numbers in parallel to form a complete unit,two acoustic peaks are generated by coupling the original acoustic single peak,and the normalized bandwidth can exceed 14.5%.2、By introducing the coiled-up space into the design of Helmholtz resonator,a metamaterials composed of coiled-up cavity is designed.Also,through the combination analysis of particle velocity field diagram and sound pressure level field nephogram,as well as the analysis of surface acoustic impedance,it is revealed that the sound absorption mechanism of metamaterial is due to the fact that the structure perfectly matches the characteristic impedance of air.Therefore,the combined action of spiral cavity resonance and heat-viscosity effect makes the structure achieve nearly perfect sound absorption performance.After geometric parametric scanning,the influence rule of different parameters on the sound absorption performance was obtained,and finally the low frequency sound absorption performance of the structure was verified by experimental tests.The results show that the minimum sound absorption frequency of the designed structure can reach nearly perfect sound absorption(0.997)at 218 Hz,and the controlled wavelength is more than 66 times of the metamaterial thickness(24mm).The design method of broadening the bandwidth is discussed: By combining two half-cell units with different folding numbers in series to form a complete unit,on the basis of the upper low frequency sound absorbing peak being basically unchanged,a lower frequency sound absorbing peak is generated by coupling,and the normalized bandwidth can exceed 25.5%.3 、 Based on the Helmholtz resonance principle,a Helmholtz acoustic metamaterial is designed.The maximum sound velocity and maximum sound pressure level inside the structure were analyzed by numerical simulation calculation,and the former was verified by surface acoustic impedance analysis.It was revealed that the sound absorption mechanism of metamaterial is that the structure achieves perfect impedance matching with air,and the Helmholtz resonance condition is achieved.A large number of sound waves enter the cavity and compressed the air,forcing the air column of the neck tube to rub against the inner wall and dissipate sound energy.Based on sound absorption principle,important geometric parameters were extracted and their influence on sound absorption performance was analyzed.Finally,the correctness of numerical simulation was verified by experiments.The results show that the minimum sound absorption frequency of the designed structure can reach nearly perfect sound absorption(0.999)at 328 Hz,and the controlled wavelength is more than 44 times of the metamaterial thickness(24mm). |
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