Research On Sound Absorption Properties And Mechanisms Of Multi-level Bionic Coupling Materials

With the development of industrial manufacture, transportation and urban architecture,noise pollution has become a big public hazard of polluting the human society environment,the development of new sound absorption materials is an important way to reduce noisepollution. At present, the mainstream development trendency of sound-absorbing material isthe combinative application of material improvement and new acoustic structure to improvethe absorption property of sound-absorbing material.In this manuscript, rice hull-polyurethane composite porous materials were prepared bytaking polyurethane as basic material and crop byproducts-rice hull as filler, and analyze theinfluence of rice hull for the acoustic performance of polyurethane. A rapid method wasdeveloped for determining all the characteristic parameters of porous materials by means ofacoustic measurements. Tested the characteristic parameters of polyurethane and ricehull-polyurethane, and analyzed the influence of various parameters for the acousticperformance, and also conducted a preliminary exploration of the sound absorptionmechanism of rice hull-polyurethane composite materials. According to the part of previousachievement of owl bionic noise reduction that our team did, a bionic coupling multi-layerabsorbing structure was established based on the coupling sound absorption characteristicsof owl skin and coverts, MATLAB and acoustic software ACTRAN were applied tocalculate its acoustic performance and analyze the influence of different parameters of eachlayer on the absorption coefficients of this model, and finite element method was used toanalyze the influence of the bionic model for the sound filed in the cableway cabin andexplored the noise reduction effect of multi-layered bionic structure in transportation.In the respect of material, a testing program was developed by taking the temperature,total mass and the ratio of the polyol and MDI as factors, analyzed the influence of variousfactors for the acoustic performance of polyurethane foam and identified the optimumformula of the polyurethane preparation. Rice hull-polyurethane compound were preparedby taking the optimized formula as base material and adding different amount of rice hull.Attenuated Total Reflection-Fourier Transform Infrared spectroscopy method and scanningelectron microscopy were applied to analyze the influence of rice hull for the chemical andphysical structure of the compound. The results indicated that rice hull did not participate inthe chemical reactions of polyurethane and only affected its physical structure. Rice hullsignificantly affected the pore formation and pore size distribution. The testing of theacoustic performance of materials showed that rice hull improve the sound absorption performance at low frequency without increasing the thickness of the material, and theinsulate performance showed opposite trend compared with the sound absorption coefficientof materials.In order to further study the acoustic performance of polyurethane foam and rice hull–polyurethane compound, a unique test bench was established based on Johnson-Allardmodel and Lafarge-Allard model. The bench was able to simultaneous determine all thecharacteristic parameters of porous material rapidly with only one sample, solving thecomplex and time-consuming drawbacks of conventional test methods. The basic principleof this method was as following: firstly, changed the different optional boundary conditionsbehind the materials, then tested the correspondent surface impedance and sound absorptioncoefficient of the material, then used MATLAB and least square method to solve all thecharacteristic parameters. Flow resistance and porosity of the material were determined bymeasuring the surface impedance at low frequencies, while tortuosity, characteristic lengthand thermal permeability were determined by the adjustment of Johnson-Allard model andtest values of absorption coefficients or the adjustment of Lafarge-Allard model and testvalues of effective density and bulk modulus. The reliability of the method was verified bythe comparisons with the test results of other laboratories or the experimental and predictedvalues. Finally, the test bench was used to test the characteristic parameters of preparedpolyurethane foam and rice hull-polyuretane composites, further analyzed the soundabsorption performance and acoustic mechanism. The biggest advantage of this method wasrapid and with no need for other specialized test equipment, which provided a new idea forthe test of characteristic parameters of porous materials.In case of the material structure, three bionic models were designed based on themulti-level organization structure and morphological characteristics of skin and coverts ofospreys, including trapezoidal ribbed surface, backing with a cavity and multi-layer couplingbionic models. MATLAB and acoustic analysis software ACTRAN were applied to analyzethe acoustic performances and influence of different parameters of each layer on absorptioncoefficients of this model. The acoustic performance analysis of different surfaces showedthat: the porous material backed with a cavity improved the absorption performance at lowfrequency and the absorption coefficients curve moved to low frequency. The trapezoidalribbed surface effectively improved the sound-absorption performance of materials in theintermediate and high frequency. The porous materials with trapezoidal ribbed surface andbacked with a cavity showed the best acoustic performance, the absorption coefficients curvemoved to the low frequency and significantly increased the absorption coefficients in theentire frequency band. In case of the multi-layer coupling structures, the bionic modelshowed the best sound absorption cability. The average normal incidence absorptioncoefficient reached0.778with frequency range from0to2000Hz while0.85from200to2000Hz. The significant improvement of absorption coefficients can be mainly due to the Helmholtz effects of micro-silt plate and flexible micro-perforated membrane, and thecombination with porous materials lead to even better absorption performance in broadband.Cableway cabin was taking as research object, the multi-bionic coupling model andcontrast models were pasted on the seats and carpet of the cabin, ACTRAN was applied tocalculate the acoustic modals of the cabin cavity, to analyze the changes of the sound field ofthe cabin with or without sound-absorbing material and the SPL near the people’s ears, andthe contribution of each segment was also calculated. The results revealed that: the vibrationmode of each modal of the cabin cavity showed symmetric distribution in the two sides ofhorizontal and vertical due to the horizontal and vertical symmetry of the acbin structure,and the higher the frequency, the more complex the deformation. The SPL near the earssignificantly reduced after the five models were added, of which, model2and5showed thebest sound absorption efficiency. As expected, absorption material was added at all parts wasmost effective to reduce the SPL in the cabin, in which, the carpet contribution seems to bepredominant. The SPL curve moved to low frequency, indicating the absorption materialincreased the mass of the system. The sound absorption provided by the sound absorbingstructure at low frequency is very low, but improved as the frequency increases. There wasalmost no energy scattering for multi-layer structure below50Hz.