### 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
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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.