Sound absorption coefficient measurement: Re-examining the relationship between impedance tube and reverberant room methods

Abstract

Absorption is one of the most commonly used parameters in linear acoustics. It is well known that the absorption for any material will differ when the properties of the material change. These properties include: thickness, density, flow resistivity, method of mounting, etc. Previous studies have shown that the results for an absorption coefficient test are dependent on the testing method, that is, the absorption coefficients of the same material with the same properties will vary depending on the testing method. Two techniques commonly used to perform such measurements are: 1) Reverberant room method and 2) Impedance tube transfer function method. Intuitively a relationship between the results of the two measurement methods for the same material should exist. This paper aims to develop a methodology to establish and define a clear relationship between the two resulting absorption coefficients measured from samples of the same type of material. To do this, 28 polyester samples have been tested using the two aforementioned methods. A set of variables has been considered for each sample such as thickness, density, and flow resistivity. This paper presents the results of the multivariate linear regression study of the absorption coefficients and provides a new model to convert the normal incidence sound absorption coefficient measured in an impedance tube into a random incidence sound absorption coefficient.