Estimating sound absorption coefficient under various sound pressure fields by combining an automated test bench to sound field reproduction and advanced post-processing techniques.
Abstract
The absorption coefficient of sound absorbing materials is usually obtained via impedance tube or reverberant room measurements. While impedance tube tests are restricted to normal incidence on small samples, reverberant room tests provide diffuse field absorption but require large samples and can result in overestimated absorption coefficients. This work investigates alternative methods estimating material’s absorption coefficient under oblique plane wave or diffuse field incidence.
A sound field reproduction approach was previously proposed. It allows estimating the absorption coefficient of absorbing materials under a synthetized acoustic field. It involves a virtual monopole array and two microphones above the materials. It shows good agreement with Transfer Matrix Method (TMM) simulations and needs smaller samples than the reverberant room method. It is closer to actual mounting conditions than the standard techniques and does not require a specific room. No overestimation of the absorption coefficient is observed. However, below 400 Hz, results are unreliable, attributed to measurements uncertainties and to the simplified spherical wave model above a locally reacting material used to calculate the sound reflection coefficient.
This paper aims at circumventing these limitations by using (i) an automated test bench reducing measurement uncertainties, (ii) an improved sound source and, (iii) a more general propagation model, namely Allard’s model, which is inverted to identify the materials’ complex density and wavenumber. Those properties are then used to calculate the diffuse field or oblique plane wave sound absorption coefficients based on two techniques. The first one relies on plane wave sound reflection coefficients. The second one uses a virtual sound field reproduction technique combined with a power definition for the absorption coefficient. Tests are conducted on five different materials in a semi-anechoic room and the two proposed approaches are compared with impedance tube and small cabin measurements. The results obtained are discussed along with experimental uncertainties.
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