| Sound is pervasive in all aspects of life.Noise,that is the unwanted sound is not only an environmental pollution,but also affects the health of human body,such as hearing disorder,rest disorder,sleep disorder,etc.In particular,the medium and low frequency sound wave with long wavelength is difficult to be controlled and the attenuation process is slow,so it has become one of the important hidden dangers affecting the health of urban residents.It can be seen that the problem about noise has become more and more complex and serious,at the same time,people have been pursuing a better living environment and diversified lifestyles.Therefore,it is necessary to deal with the noise problem.At present,this issue was mainly dealt with from two aspects,one is to control the noise sources,and the other is to make interference in the noise transmission routes.The control of noise sources,such as the no honking signs on some roads,construction time restrictions and so on.However,the source of noise is wide,such as traffic noise,industrial noise,construction noise,social noise,etc.,which are unavoidable and difficult to be controlled.Therefore,how to effectively interfere with and block the transmission path of noise has become the main research content in the noise control filed.Generally,we use the sound insulation materials or sound absorbing materials to interfere with the transmission path of sound waves.Sound insulation materials mainly isolate the noise from the receiving objects,which still exist in essence.Sound absorbing materials could convert acoustic energy into other energy such as heat energy,mechanical energy and so on.Therefore,the noise can be fundamentally reduced and eliminated.Fiber sound absorption materials have been widely used in the field of sound absorption,especially nonwoven materials,due to their advantages such as extensive sources and low cost.However,the absorption efficiency of nonwoven alone is relatively unsatisfactory at the middle and low frequency part（100-2500 Hz）,and that is exactly what’s happened with highly sensitive for auditory sense.Therefore,in order to improve the living environment,there is an urgent need to develop a kind of thin,light and low-cost noise reduction material that can absorb sound waves in a wider range.Considering that the fiber aggregate material made of coarse fiber is not ideal for low-frequency sound absorption,we chose fine fibers made by electrostatic spinning to prepare sound absorption materials.Electrospinning process can continuously produce fine and randomly oriented nano fibers,with a specific surface area of 10-1000 times that of traditional acoustic fiber materials,which is conducive to the improvement of sound absorption performance to a large extent.The research object of this work is nanofiber net,microperforated fiber net and the composite sound absorption structure with nonwoven materials.From the perspective of acoustic materials,it is mainly composed of microperforated materials and fiber porous materials.Therefore,the sound absorption ability is at least the result of the synergistic effect of two sound absorption mechanisms.This topic mainly discussed the sound absorption performance and mechanism of the nanofiber net,micro perforated fiber net and its composite sound absorption structure with nonwoven materials through the combination of experimental testing and theoretical analysis.In addition,the linear relationship between the sound performance of micro perforated fiber net and its structural parameters was revealed through linear analysis.The gradient structure multilayer sound absorbing material was prepared on the basis of nonwoven materials,and its impedance characteristic was analyzed to establish and improve the corresponding sound absorption model,which provided a new idea for the design of composite sound absorbing materials to broaden its application in the field of sound absorption and noise reduction.The main research contents of this topic include the following four parts:（1）This work selected the common polyvinyl alcohol（PVA）spinning solution to prepare nanofiber net,which is universally applicable,and then compounded with melt blown nonwovens to prepare a light layered sound absorbing material.The influence of additives（PEO,Ti O2）on the sound absorption performance were discussed.The results showed that the composite nanofiber net made of the mixed spinning solution containing additives can improve the sound absorption performance to a certain extent,but there was an optimal amount for the added content,which had little effect on the sound absorption effect after exceeding the critical value.At the same time,the bending and tensile properties of the composite nanofiber net were also studied to ensure that its mechanical properties could meet the requirements of daily application.The results showed that the composite nanofiber net still represented satisfied sound absorption capacity after 1000 times of bending,which also proved the effectiveness and practicability of the composite nanofiber net as a sound absorption material,laying a foundation for the subsequent research.（2）In order to further improve the low-frequency sound absorption performance,we combined with the characteristics of the microperforated plate,and a series of micro perforation fiber net with different micro perforation diameters and perforation rates were prepared by micro perforating process.This is a kind of new sound absorption structure,in order to study the main factors that influence the sound absorption performance,the impedance tube method was used to measure the sound absorption coefficient of the micro perforated fiber net at different cavity distances.The thickness,weight per unit area,perforation rate,micro perforated diameter and the air permeability of the micro perforated fiber net were also tested.The single factor analysis method was used to study the linear correlation between each factor and the average sound absorption coefficient of the micro perforated fiber net.When the specimens were close to the rigid wall,the average sound absorption coefficient of the micro perforated fiber net was relatively low,and the value were all below 0.2,which did not have engineering practical value.When a certain cavity depth was set between the sample and the rigid wall,the average sound absorption coefficient had been significantly improved.When the thickness of the air layer was 0,1 cm,2 cm and 3 cm,the correlation coefficients R between the average sound absorption coefficient and the perforation rate were 0.654,0.561,0.464 and 0.661 respectively,and the correlation coefficients R with the perforation diameter were 0.265,0.540,0.523 and 0.467 respectively.The linear correlation coefficient between the average sound absorption coefficient of the micro perforated fiber net and its thickness,gram weight per unit area and air permeability were relatively low,and the correlation coefficient R were less than 0.2.Through multiple regression analysis of the experimental data,the multiple linear regression equation between the average sound absorption coefficient of micro perforation fiber net and its structural parameters was obtained,and the calculated prediction value was compared with the experimental value.The results showed that the multiple linear regression equation could also predict the average sound absorption coefficient of the micro perforated fiber net well even when here was no cavity,whose average sound absorption coefficient was unsatisfactory at this point.After setting a certain cavity depth,its linear correlation was better and the correlation coefficient R were above 0.9 at different cavity depths.Besides,basically all data points felled within 95%of the prediction interval.In addition,three samples with the same structure were used to verify the multiple linear regression equation,and the results showed that it could predict the average sound absorption coefficient of the microporous fiber net well.The average sound absorption coefficient can be used to characterize whether the acoustic material has sound absorption performance.The establishment of multiple linear regression equation in this part could quickly and simply judge whether the micro perforated fiber net possess sound absorption capacity according to its structural parameters.（3）The acoustic electrical analogy model and transfer matrix model were commonly used to analyze and predict the sound absorption performance of micro perforated structures.It can be known from the research and discussion in the previous part that the sound absorption coefficient of the micro perforation fiber membrane prepared in this project was closely related to the perforation diameter and perforation rate,and was little related to the thickness,weight and air permeability.In light of this,we can say that the micro perforation fiber net prepared in this work was mainly depend on the micro perforation sound absorption mechanism.The sound absorption performance of the micro perforated fiber net was tested by impedance tube method,and the micro perforated sound absorption model was used to analyze the sound absorption performance.The results showed that the acoustoelectric analogy model can better predict the sound absorption performance before the peak value of the microporous fiber net compared with the transfer matrix model,while the transfer matrix model can better predict the peak value.In addition,it can be found that the sound absorption band of the micro perforated fiber net prepared in this study was wider than the predicted value,and the sound absorption performance on the right side of the peak value was also better than the predicted value of the micro perforated model.It should be pointed out that although the macro structure of the micro perforated fiber membrane prepared in this work is similar to that of the micro perforated plate,it was composed of fibers in essence.Therefore,on the basis of the micro perforated sound absorption model,a new model for predicting the sound absorption performance of the micro perforated fiber net was established by considering the vibration effect of the membrane panel,the heat conduction effect and the viscous effect of the flexible motion of the fiber frame.It was found that the new model can effectively improve the prediction accuracy and had a better coincidence with the test values compared with the existing equivalent circuit model and transfer matrix model.At the same time,the work of this part also showed that the membrane panel vibration effect of the fiber net and the flexible vibration of fiber material were also important indicators that could affect the acoustic performance of micro perforated fiber net,as are the micro perforated parameters.（4）In order to keep a pleased sound absorption performance of the material in a wide frequency range,the micro perforated fiber net was composited with the needle punched nonwoven material to make a kind of composite sound absorber structure.The effects of the perforation parameters,the placement position of the fiber net and cavity depth on the sound absorption performance of the micro perforated fiber net composite nonwoven material were discussed.The sound absorption coefficient of micro-perforated fiber net composite nonwovens was measured by the impedance tube method,and the composite sound absorber model was established by combining the Johnson-Allard porous sound absorption model and the improved micro-perforated fiber net sound absorption model.The results show that the composite sound absorber model can better predict the sound absorption performance of micro-perforated fiber mesh composite nonwovens. |
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