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a b s t r a c t This paper presents an experimental, numerical and analytical study of the open roof effect on acoustic propagation along a 3D urban canyon. The experimental study is led by means of a street scale model. The numerical results are performed with a 2D-Finite Difference in Time Domain approach adapted to take into account the acoustic radiation losses due to the street open roof. An analytical model, based on the modal decomposition of the pressure field in the street width mixed with a 2D image sources model including the reflection by the open roof, is also presented. Results are given for several frequencies in the low frequency domain. The comparison of these approaches shows a quite good agreement until f = 100 Hz at full scale. For higher frequency, experimental results show that the leakage, due to the street open roof, is not anymore uniformly distributed on all modes of the street. The notion of leaky modes must be introduced to model the acoustic propagation in a street canyon.

This work deals with numerical modeling of sound propagation in street canyons with at building facades. The street is seen as an open waveguide and two 3D wave models are used : a parabolic equation model and a modal expansion model. The comparison between the models shows a very good agreement. Then, the study focuses on the radiation condition at the opening of the street. In usual energetic approaches as ray tracing, the opening is assumed to be perfectly absorbing. This assumption is realistic at high frequencies, however the reection phenomenon caused by the geometric discontinuity at the opening is still an open question at low frequencies. This possible reection is investigated through three parametric studies of the acoustic longitudinal energy ux decay along the street. The rst study shows that the approximation of total absorption at the street open ceiling is no longer valid when the ratio η between the street width and the wavelength is small. The second study shows that the eect of source height is weak except under restrictive conditions: when η is small and when the source height is not small compared to the wavelength, the approximation of total absorption is acceptable for an elevated source. At least, the results of the last study show that the error made by assuming a perfectly absorbing ceiling is not negligible compared to the error made by considering perfectly reecting walls.