Akoestiek en Warmtegeleiding, K. U. Leuven Celestijnenlaan 200 D, 3001 Leuven, Belgium
The method of measuring the impedance of a ground surface based on the spatial Hankel transform, can be divided into three parts. First the pressure field is sampled on two horizontal lines near to the surface, from beneath a cylindrical symmetric sound source till infinity. Next a Hankel transform is performed on the data of each horizontal line. Finally the impedance of the surface for an incident angle ranging from 0 to 90 deg, is calculated. In practice, due to the finite maximum, measuring distance oscillations occur in the impedance curve as a function of angle of incidence. This effect can be avoided by using a spatial window before transforming the data. A Kaiser--Bessel window is selected to give the best results. A side effect of using a window is a shift from the real impedance values. Further improvement can be obtained by calculating more data points for a horizontal value larger than the maximum measuring distance. The method and its improvements will be illustrated with measurements on outdoor ground surfaces.