Prediction of outdoor ground effect

• The proposed method of predicting outdoor ground effect should improve on existing schemes in respect of characterising ground impedance. • It allows for the effects of roughness on land and water. • It proposes characterising turbulence using heat flux and friction velocity. • It proposes a Fresnel zone method based on pressure squared to allow for changes in ground impedance with range. A classification of outdoor ground surfaces for calculating the attenuation due to destructive interference between direct and ground-reflected sound is proposed which uses a physically admissible ground impedance model with parameters of porosity, flow resistivity, and, for some ground surfaces, layer depth. This impedance model is shown to enable good fits to short range level difference measurements over a wide range of ground surfaces. Ranges of parameter values are suggested in each class to allow for variations in soil depth profiles, moisture content and surface roughness. Also, methods are proposed to account for mean ground roughness heights of less than 0.1 m and for waves on a water surface. The influence of atmospheric turbulence on ground effect is included through a coherence factor which assumes a Kolmogorov turbulence spectrum with parameters for which values can be calculated if heat flux and friction velocity are known. To account for changes in impedance along the propagation path, a Fresnel zone method is proposed which weights pressures squared since this method has been shown to compare better with 2D Boundary Element Method predictions than a method that weights excess attenuation.