A revised standard method for ground impedance measurement.

Abstract

ANSI S1.18 (1998) for ground impedance measurement has been revised to combine the previously existing template method with direct deduction of impedance from data for the complex level difference between vertically separated microphones. It has been found that, despite the improved sensitivity at low frequencies that should be achievable through use of a longer range, the combined approach ceases to give reliable information for horizontal source‐receiver separations of more than 1 m. The revised standard contains four worked examples: three grass‐covered ground surfaces and a gravel road. The first example has been used to show that although use of consecutive measurements with a single microphone produces data that is usable with the template method, the phase information is not sufficiently robust to allow direct impedance deduction. In the three grassland examples, the widely used single parameter model is found to overpredict the real and imaginary parts of ground impedance at lower frequencies. The fourth example shows that the geometries prescribed in the standard are not appropriate for determining impedance spectra of relatively hard surfaces but are sufficient to establish that the surface is acoustically hard.