Measurements of acoustic properties of synthetic 3-D printed grass blades to predict sound attenuation of tall grass-covered ground

Tall grass covering the ground can greatly affect acoustic transmission loss over long propagation ranges. Understanding and predicting the sound attenuation introduced by this type of vegetation is of both naval and aerospace relevance. Tall grass fields are common near both shorelines and air- or spaceports. In this study, the effect of the grass blades is assumed to be decoupled from the absorption of the ground. A numerical simulation replaces the space occupied by the blades with an equivalent fluid volume that has an attenuation coefficient equal to that of synthetic grass blades. Grass blades are modelled with rectangular parallel stripes. A 5 mm thick disc frames the blades to form a sample that fits in a cylindrical impedance tube. Several samples are designed, and 3-D printed to have blades oriented at 0 or 45 degrees with respect to the propagation of the acoustic wave. The samples are stackable modular components that can be combined to create multilayered samples. Complex characteristic impedances and wavenumbers of different multilayered samples are measured through a series of impedance tube experiments. The measured wavenumber is then used as input in a parabolic equation solver to quantify the effect of a tall grassy field on acoustic transmission loss over a propagation range of 300 m.