A Scale Model Study of Parallel Urban Canyons

Abstract

Shielded urban areas are of importance regarding urban citizens' annoyance and adverse health effects related to road traffic noise. This work extends the existing knowledge of sound propagation to such areas by a scale model study, rather than by model calculations. The scale model study was executed for two parallel urban canyons at a 1 to 40 scale, with a point source located in one canyon. Cases with acoustically hard façades and absorption and diffusion façade treatments were investigated. To correct for excess air attenuation of the measurements, a waveletbased method has been applied. The measurement results in the shielded canyon show that, in contrast to the directly exposed street canyon, the levels and the decay times are quite constant over the length of the canyon. The energy-time curve in the shielded canyon is characterized by a rise time, which can be related to the sound pressure level. The rise times and decays can be explained by separate reflection, diffraction and diffusion processes. A closed courtyard situation enlarges the level difference between acoustically hard façades and applied façade absorption or diffusion treatments at both the directly exposed and shielded side. A comparison between measurements with two different diffusion mechanisms, horizontal and vertical diffusion, reveals that vertical diffusion yields lower levels at the shielded side compared to horizontal diffusion for the investigated situations.

Using Acoustic Diffusors to Reduce Noise in Urban Areas

“…Fort he sound decay,t he vertical arrangement givesalittle bit longer RT30 (0.63 s) than for the horizontal and regular alignment (0.57 sa nd 0.55 srespectively). This result is coherent with those obtained by Hornikx and Forssén [3], showing that vertical diffusion givesal onger sound decay than horizontal diffusion, since more energy is scattered out of the street in the case of diffusion elements oriented horizontally.More accurately,t he sound decay is also al ittle bit more linear and shorter for the RE configuration, than for the HO and the VE configurations. It suggests that it is more efficient to use a2 D-regular diffusors arrangements likeR E, instead a1D-regular arrangement.…”

“…This decrease of the sound decay for street with diffuse reflection is in agreement with the experimental observation done by Hornikx and Forssén [3], about the effect of diffusion on the reverberation time RT10 in as treet. Indeed, it wass hown that the RT10 is longer for specular façades than for façades with diffusion patches.…”

“…When the distance from the sound source increases, this behavior is reversed: specular reflections are still energetic, while, for other reflection laws, due to the multiple diffuse reflections through the openings, the sound field decreases. It can be noted that these results about the far field effect are consistent with other studies on the effect of diffusion, as well as the effect of the scattering coefficient, on the sound propagation in street [2,8,3]. Indeed, it was also shown that the effect of diffusion on the sound pressure levela ppears as an increase at short distances from sources, and as adecrease at larger distances.…”

“…This results is also confirmed by the RA configuration (2Dr andom)t hat gives similar results to the RE configuration. Fort he sound attenuation, the difference is also very low, as already shown by Hornikx and Forssén [3], for the vertical and the horizontal diffusion. However, it slightly seems more efficient to use av ertical arrangement of diffusors to reduce the noise leveli nt he street (ΔL = 0.92 dB), instead ah orizontal one (ΔL = 0.71 dB), butalso instead aregular one, although this last configuration produces the best sound decay.I tc onfirms that the best sound attenuation is not correlated with the best sound decay systematically.…”

“…In ar ecent paper,H ornikx and Forssén have investigated the use of oriented patches (absorption and diffusion)asafaçade treatment to reduce noise in shielded canyons. As an interesting result, they have shown that vertical diffusion yields lower levels at the shielded side compared to horizontal diffusion [3].…”