  This paper presents an experimental, numerical and analytical study of the
effect of open roof 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 a horizontal plane mixed with a 2D image sources model to
describe the attenuation along the street, is also proposed. Results are given
for several frequencies in the low frequency domain (1000-2500 Hz). The
comparison of the three approaches shows a good agreement until f=100 Hz at
full scale, the analytical model and the 2D numerical simulation adapted to 3D
permit to modelize the acoustic propagation along a street. For higher
frequency, experimental results show that the leakeage, 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 modelize the acoustic propagation
in a street canyon.
