Free field measurements of the absorption coefficient for nonlocally reacting sound absorbing porous layers.

A simple asymptotic approximation with two parameters (the plane wave reflection coefficient and a correction factor) has been derived by Brekhovskikh and Godin [Acoustics of Layered Media II. Point Sources and Bounded Beams (Springer, New York, 1992)], for the spherical field reflected by nonlocally reacting surfaces. In the present work, an expression for the correction factor is obtained for the case of a homogeneous and isotropic porous layer. A free field method for evaluating reflection and absorption coefficients with this approximation is presented. The evaluation of the absorption coefficient at normal incidence is performed on a porous layer on a rigid backing, and compared to measurements performed using a two microphone technique and an impedance tube.