Sound propagation in woodland

Abstract

A review of past research into sound propagation in woodland is presented. The attenuation of sound in woodland is small between about 800 and 2000Hz and greater at low and high frequencies. Attenuation measurements made in three contrasting woodlands are presented and compared with theoretical models. Propagation models using simple one- and two-parameter impedance models are used to calculate appropriate ground parameters for the prediction of impedance of the woodland soils. The ground parameters varied on different days in a single stand due to differences in moisture content and compaction. The overall differences between the stands are not significant. The woodland soil has a considerably lower impedance than other outdoor ground surfaces such as grassland or sand. A theoretical model for the attenuation of sound by thermoviscous absorption and scattering within an array of cylinders is assessed by means of a model experiment with wooden rods in an anechoic chamber. An input density 60% lower than the actual density gives a good agreement with measured attenuation. This modified model also predicts the attenuation by the cylinders in the presence of a ground surface. The scattering model is compared with the high frequency attenuation measured in the , woodland, using sampled trunk densities and radius, this underpredicts the observed attenuation, particularly in the stands with a dense branch and foliage structure. Addition of a second. dense, array of non rigid scatterers gives a good agreement with the measured data, thus modelling the scattering and absorbing effects of trunks, branches and leaves, in the high frequencies. Finally, a combined model is presented in which the attenuation caused by ground interference effects. at low frequencies. is added to a prediction of attenuation by the scattering model. across the whole frequency range. This model reproduces the frequency dependence of the attenuation of sound in woodland