Method for controlling boundary condition effects on the measurement of acoustic properties of small samples in tubes

Abstract

As standard ASTM E2611 reveals, the normal incidence sound transmission loss measured on a small sample in an acoustic tube is not only a property of the material, but it is also strongly dependent on boundary conditions (generally unknown) and on the way the material is mounted. This article proposes an experimental method to control the effects of the lateral boundary conditions in an acoustic tube. The main objective is to deduce the properties of a "client element" (material sample) from the measured global acoustic properties of a patchwork composed by the "client material" and a known "host support". Three patchwork configurations have to be distinguished: patchworks with and without an impervious and rigid interface between the elements, and patchworks composed by elements which cannot be identified as equivalent fluids. For each of these configurations, it is proposed to use a specific method based on the Mixing Rule Method (MRM) or on the Parallel Transfer Matrix Methods (P-TMM or dP-TMM) used in reverse way. Numerical and experimental validations are proposed in acoustic tubes on convenient configuration: a material sample surrounded by an air ring. This configuration allows reducing the material elasticframe behavior to leave a limp-frame behavior. The proposed methods allow removing the effect of the lateral air ring host surrounding the material. For homogeneous materials the two methods based on MRM and dP-TMM give similar good results. For non-homogeneous materials or for materials which cannot be modelled as equivalent fluids only the method based on dP-TMM gives good results.

Estimation of acoustical materials sound absorption coefficient under oblique incidence plane wave and diffuse field via Allard model inversion

“…In the case of porous materials with an elastic frame, the lateral boundary condition between the sample and the tube wall can have a significant impact on the measurement of the sound absorption coefficient. 53 Indeed, if the sample diameter is too small compared to the tube diameter, air leaks may occur between the sample and the tube wall. 54,55 On the other hand, if the diameter of the sample is too large, the vibration of the sample frame and the pore compression will alter the measurement of the sound absorption coefficient.…”

“…54,55 On the other hand, if the diameter of the sample is too large, the vibration of the sample frame and the pore compression will alter the measurement of the sound absorption coefficient. [56][57][58][59] Note that an alternative method 53 has been proposed to reduce the effects of the lateral boundary condition in the tube. This method allows to estimate the sound absorption of material when a sample of this material is tested in the tube with a small lateral air cavity (same as a lateral air ring).…”