City-Train noise reduction in urban area by using acoustic mini-screens made of metamaterials

Abstract

Noise and vibration from trains and tramlines pass-by's within urban context are always cause of complaints by citizens, due to the high level of annoyance especially when the vehicle activity involves night-time periods. This paper deals with the acoustic design of mini screens composed of metamaterials to be placed at the edge of railways for shielding the noise from trains crossing urban areas. Acoustic measurements have been undertaken by using the beamforming approach along with linear array microphones in order to identify whether the noise is the most originated. It has been identified that the friction between wheels and railways is the source of noise other than the weight of carriage that moderately impact the results. The measured results show that the noise source is located in the trolley area, precisely at a level higher than the rolling rails. This conclusion has been investigated by a reverse engineering method on microphones array data and based on diffraction theory. Moreover, it has been considered the variability of noise emission from convoys in transit by applying normalisation of the energy contents through time-frequency analysis (wavelet). Further studies have been focused on the location and geometry of the mini screens based on the best acoustic performance, which can achieve a reduction of 6-8 dB for distance of 10 m and 100 m across the horizontal plane, and up to 9 dB for distance of 40 m on vertical plane.