Artificial refraction of sound propagating outdoors by a sonic crystal noise barrier with increasing cylinder diameter over height

Paper in proceeding, 2012

Upward refraction of sound propagating outdoors by a finite set of two-dimensional cylinders has been studied in the homogenization region. All cylinders in the field are located above a ground plane and have their longitudinal axes orientated parallel to the ground. By increasing the cylinder radius with height, a medium is created which effective sound speed decreases over height. With a decreasing sound speed profile a cylindrical wave propagating inside such a structure can be refracted upwards, consequently creating a shadow zone behind the object. Acoustic phenomena for continuous graded index media (GRIN CM), predicted using the Finite Element Method (FEM), are demonstrated. Using modified homogenization formulas the results are then matched with a qualitatively equivalent graded index sonic crystal (GRIN SC), predicted with Multiple Scattering Theory (MST). In order to gain a reasonable insertion loss the barrier thickness must be chosen in accordance with the wavelength of the incoming field and should be in the order of a few wavelengths. Furthermore, it has been shown that the performance of these refractive structures is determined by the shape of the effective sound speed profile in relation to the point of excitation.