Estimation of the accuracy of measured acoustical parameters of porous materials with an acoustical method

Abstract

Acoustical parameters such as tortuosity, viscous and thermal characteristic lengths, or even static thermal permeability are often required to describe the dynamic behavior of porous materials. Using a middle frequency method, based on the use of a standing waves tube, the direct measurements of the resistivity and porosity, and the accurate measurement of the dynamic density and bulk modulus, these intrinsic parameters can be obtained from analytical inversion of Johnsons and Lafarges models. The interests of the method lie in the simplicity of the apparatus and on the clear separation of viscous and thermal dissipative contributions. This last point specially helps to understand the physics of the medium and check the validity of the assumptions made: the material should not have a strong elastic behavior, and it must fit the models used in the inversion process. In this presentation, the robustness of the method is discussed and estimations of the measurement uncertainties are given. The occurrence of systematic errors coming from the use of semi-phenomenological models for determining intrinsic parameters is tackled. The measurement procedure, and the experimental set-up are detailed, and results obtained on materials with very different properties are presented.