Abstract
Two ground impedance models have been used to compare calculated and measured excess attenuation during outdoor sound propagation over various ground surfaces. A single parameter model uses available semi‐empirical relationships between the specific flow resistance and the characteristic impedance of fibrous materials to estimate the normal impedance of ground surfaces assuming the medium to be of semi‐infinite extent. A three‐parameter model is based on the specific flow resistance and thickness of a layer of porous material situated over a semi‐infinite finite impedance backing also characterized by its specific flow resistance. While the single parameter model was found adequate for modelling lawn grass and asphalt surfaces, the layered model provided significant improvement for exposed earth and thin snow layers over ground. For snow layers, low flow resistance layers over higher a flow resistance backing gave good agreement. For the exposed ground, a thin, high flow resistance layer over a more permeable backing proved to be most suitable. [Work supported by NSF.]
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