ASA 125th Meeting Ottawa 1993 May

2aNS4. Improvement of the spatial Hankel transform method to determine the impedance of outdoor ground surfaces.

C. Verhaegen

W. Lauriks

A. Cops

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.