Indirect Measurement of the Bulk Properties of Acoustic Absorbing Samples

In this paper, a new procedure is investigated to measure bulk properties of acoustic absorbing samples that are not clamped on the sides. Tests have been performed with a PU probe and with a sound source that is emitting spherical sound waves above a large slab of a homogeneous material. Multiple tests have been performed for different configurations, i.e. with and without a backplate as well as with two sample thicknesses, for several sound source heights. A method is presented that uses a combination of tests performed under different conditions in order to obtain the complex characteristic impedance and complex wavenumber of the sample. The method utilizes a plane wave model to calculate the bulk properties for each sound source height. Erroneous estimates are found since this model does not correct for spherical waves. However, depending on the height of the sound source the results are affected by near field effects differently. Tests at different heights are combined, and the plane wave values are obtained using an extrapolation technique. The results of the method are investigated through tests and simulations.