A Method for Identifying Tortuosity, Viscous Characteristic Length and Thermal Characteristic Length of Kapok Mixed Fiber Porous Materials 2023-01-1058

Tortuosity, viscous characteristic length and thermal characteristic length are three important parameters for estimating the acoustic performance of porous materials, and it is usually measured by ultrasonic measurement technology, which is costly. In this paper, a method for identifying the tortuosity, viscous characteristic length and thermal characteristic length for the porous fiber materials mixed with kapok fiber and two kinds of other fiber materials is proposed. The tortuosity is calculated by using the porosity and high-frequency normal sound absorption coefficient of porous materials. According to the normal sound absorption coefficient curve of porous materials under plane wave incidence, viscous characteristic length and thermal characteristic length are identified through the Johnson-Champoux-Allard-Lafarge (JCAL) model and genetic algorithm by using the measured parameters, the calculated tortuosity and static thermal permeability. The measured parameters include static airflow resistivity, porosity, Young's modulus, Poisson's ratio, structural loss factor, thickness and density. The error between the parameters identification results and the experiments is analyzed. The error between the simulations and the experiments of the normal sound absorption coefficient under plane wave incidence is very small, which proves the accuracy of this method. The relationship between sound absorption performance of kapok mixed fiber porous materials, and tortuosity, viscous characteristic length and thermal characteristic length is investigated. Assuming the frequency range is located between 1000Hz to 4000Hz under the plane wave incidence, taking the average normal sound absorption coefficient of the materials as the objective function, the JCAL model and genetic algorithm are then used to obtain the optimal parameters of kapok mixed fiber porous materials.