Precise model measurements versus theoretical prediction of barrier insertion loss in presence of the ground

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Abstract
Recent work on barrier performance, aimed at quantifying effects due to interference and scattering by atmospheric turbulence, has also indicated systematic deviations between measured results of diffraction and those predicted by widely used theories embodying known approximations. Modelexperiments indoors (no atmospheric turbulence and a known, controllable ground impedance) show systematic deviations, independent of frequency, up to 12 dB when compared with either first‐order approximations of Macdonald’s diffraction theory or that of Kirchhoff. More exact evaluation of the diffraction integral [e.g., Hadden and Pierce, J. Acoust. Soc. Am. 6 9, 1266–1276 (1981)] or diffraction theory based on layer potential theory [e.g., Daumas, Acustica 4 0, 212–222 (1978)] provide agreement with measured results within ±1.5 dB under all experimental configurations investigated. An empirical correction based solely on such geometrical considerations as distance of source and receiver to the barrier and height of barrier, i.e., on the value of the angle of diffraction at the edge of the barrier, provides similarly good agreement. To obtain precise theoretical predictions of barrier performance under a wide range of configurations it is suggested that one use either calculations based on the recent formulation of Hadden and Pierce or some simple approximate theory plus this empirical correction based on diffraction angle.

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