On the use of leaky modes in open waveguides for the sound propagation modeling in street canyons

Abstract : One is interested in a multimodal approach to describe the sound propagation within an urban, U-shaped, street canyon being considered as an open waveguide in which the sound may propagate. The key point in such a multimodal formalism is the choice of the basis of local transversal modes on which the acoustic field is decomposed. For a waveguide with a simple and bounded cross-section, a complete orthogonal basis can be analytically obtained. The case of an open waveguide is more difficult, since no such a basis can be exhibited. However, an open resonator, as for example the U-shaped cross-section of a street, presents resonant modes with complex eigenfrequencies, owing to radiative losses. This work first presents how to numerically obtain these modes. Results for the transverse modes are also compared with solutions obtained by the Finite Elements Method (FEM) with Perfectly Matched Layers (PML). Then, examples are treated to show how these leaky modes can be used as a basis for the modal decomposition of the sound field in a street canyon.