A Study Of Sound Absorption And Insulation Performance Of Multi-layer Kapok Felt Composite Material

The vehicle noise may seem to be more prominent in EV（electrical vehicle）due to the absence of masking by the engine noise,and EV has higher medium and high frequency noise level than ICE（internal combustion engine）vehicle,so passengers are more sensitive to the noise of EV.The sound package could be used for noise control of EV.Most traditional materials used in sound package are heavy and non-renewable.The heavy solutions of the sound package will reduce the range and increase cost directly,so excellent acoustic performance and lightweight solutions of sound package will be required by OEM.It is important to develop a new lightweight and renewable nature fiber acoustic material for EV sound package.this is of great help to reduce the weight,replace the use of petrochemical materials,and reduce CO₂emissions.Kapok fiber is a natural,eco-friendly and renewable plant fiber,which has a large hollow structure.Kapok felt made by kapok fiber is used as the research object in this dissertation.A study of sound absorption and insulation performance of single-layer and multi-layer kapok felt composite material are carried out considering the microscopic pore morphology of kapok felt.The results of this dissertation can be applied to the development of kapok fiber acoustic materials for sound package of EV.The main contents of this dissertation are summarized as follows:（1）The microscopic pore characteristics of kapok felt and its influence on acoustic performance were analyzed.Kapok felts with different bulk densities are made by airlaid process.The pore morphology of kapok felt with different bulk densities is observed by using SEM（scanning electron microscopy）.A classification method for the pores of kapok felt is proposed.The flattening characteristics of kapok fiber in the felt is quantitatively analyzed by a compression test.The sound absorption coefficient of kapok felt before and after compression is tested by impedance tube.The test results show that the flattened kapok fiber has a great influence on the sound absorption performance.The sound absorption coefficient of the compressed material below 4000Hz is lower than the uncompressed material.The causes of the attenuation are analyzed.The diameter ratio of the large pores and the middle pores in the kapok felt becomes larger after compression test,which weakens the acoustic coupling between large pores and middle pores.According to the acoustic coupling characteristics of the double-porosity material,the middle pore contributes less to the sound absorption of kapok felt.（2）Test and calculation methods of non-acoustical parameters of kapok felt are studied.First,five non-acoustical parameters of kapok felt with different bulk densities are tested.A simple formula for calculating porosity is given and the cause of calculation errors is analyzed.The airflow resistivity of kapok felt is calculated using empirical and theoretical models commonly used in fibrous materials,and compared with measurements,the results shows that the prediction accuracy of all existing models is low.These models cannot be used to predict airflow resistivity of kapok felt.New empirical model of airflow resistivity for kapok felt is given by fitting the test values.New theoretical models of the airflow resistivity prediction of kapok felt with circular and flat fiber cross-sectional shape are proposed.The models are optimized by the flattening ratio,which is introduced to describe the state between the two fiber cross-sectional shapes.The results shows that the optimized model has high prediction accuracy in the wide range of bulk density.The empirical formulas for predicting the non-acoustical parameters by the compression ratio of kapok felt are given.（3）An Optimized model called Kapok-JCA（Johnson-Champoux-Allard）model is built.The sound absorption coefficient of kapok felt is calculated using the existing empirical and theoretical acoustic models,and compared with measurements,it seems that existing models have low prediction accuracy.It’s found that the characteristic length needed to be corrected by analyzing the error of the calculation of sound velocity in the audible frequency range.The kapok felt is simplified to a double-porosity equivalent model based the multi-level pore size characteristics of kapok felt,and the effective diameter of pores in kapok felt is calculated and the relationship between the equivalent Nussen number and bulk density is obtained.A method to optimize the characteristic length of the equivalent model is proposed considering the acoustic coupling in different pores and the slippage effect in small pores,then the Kapok-JCA model is obtained,the comparison results show that the calculation accuracy of the Kapok-JCA model is higher than JCA model in the frequency range above1000Hz.（4）The sound absorption coefficient and insertion loss of multi-layer kapok felt composite material are tested,and the test data analysis is carried out.The comparison result shows that the insertion loss of double-layer kapok felt composite material is significantly higher than that of cotton felt composite material.The design principles of sound absorption and sound insulation performance of multi-layer kapok felt composite material are proposed.The sound absorption coefficient and insertion loss of multi-layer kapok felt composite material are calculated by using Kapok-JCA model and Kapok-JCA limp model based on transfer matrix method.The calculation error is analyzed.Finally,the applicable scenarios of Kapok-JCA model and Kapok-JCA limp model are given for multi-layer kapok felt composite material with different acoustic reactance of surface layer and different acoustic reactance ratios of two adjacent layers of kapok felt.