Acoustic Low / Broadband Frequency Sound Absorption Performance Of Foam Metal Acoustic Metamaterials

Foam metal as porous material has good performance in controlling noise pollution,but there is a problem that the sound absorption effect is seriously affected by frequency:the absorption of low-frequency noise is insufficient,and it is difficult to achieve broadband sound absorption.While the resonant acoustic structure can achieve high absorption peaks at low frequencies.By combining the sound absorption characteristics of foam metal and resonant acoustic structure through structural design,the broadband sound absorption and low frequency sound absorption of foam metal-based acoustic materials can be realized.Two types of acoustic structures based on foam metal are proposed for broadband acoustic absorption and low-frequency acoustic absorption respectively,which are simulated by combining the theoretical model of porous materials and finite element simulation model to investigate the main factors affecting the acoustic structure with limited materials and less experimental times;then the dimensions of the acoustic structure or the acoustic performance in a specific acoustic band are optimized to obtain a broadband acoustic structure with lower thickness and the acoustic structure design dimensions that can enhance the acoustic effect at low frequencies.Vacuum-inflation percolation casting method was used to prepare aluminum foam with uniform pores,and the particle swarm algorithm was applied to inverse the five nonacoustic parameters associated with the pore structure of the foam metal for establishing theoretical and simulation models.The acoustic structure of the posterior cavity of the double-layer foam metal was established to investigate its broadband sound absorption performance.The acoustic model established by the transfer matrix method and theoretical model of porous materials(JCA model,Johnson-Champoux-Allard model)was used to calculate the absorption coefficient of the structure,and the error of the model could be controlled within 8.4%.The pore structure,metal layer thickness and cavity depth of the foam metal are important factors affecting the broadband sound absorption.The foam metal with smaller flow resistance placed in the first layer can improve the broadband sound absorption effect,increase the thickness of the first metal layer and decrease the thickness of the second metal layer;reducing the cavity depth of the first layer and increasing the cavity depth of the second layer are effective methods to improve the broadband sound absorption performance.The average absorption coefficient is 0.82 and the total thickness is 29.54 mm,which is 57% less than the thickness before the optimization,and the error between the test value of absorption coefficient and the model calculation is 1.3% and 5.9%.The theoretical absorption model was established by using the double porosity theory and the JCA model of porous materials,and the finite element simulation model established by COMSOL was used to compare with the existing literature results.Aluminum foam with high flow resistance combined with folded slit structure can form a sub-wavelength acoustic super-material with low-frequency absorption characteristics;the higher the number of cells,the more the absorption peak of the material will move to lower frequencies,and the most important structural dimensions affecting low-frequency absorption are slit width and folded layer height.The Nelder-Mead algorithm was applied to optimize the low-frequency sound absorption structures of two aluminum foams,and the experimental measured absorption coefficient curves of the structures were closer to the calculated values,which led to the improvement of their low-frequency sound absorption performance by 13.4% and 8.9%,respectively.