Prediction of Traffic Noise Shielding by City Street Canyons

Doctoral thesis, 2004

Reducing the sound level on the exposed building facades due to traffic noise in cities is difficult and expensive. Creating access for the inhabitants to a quiet side can be an alternative method for reducing the annoyance. Therefore it is of interest to predict the level on shielded positions such as courtyards. This is however difficult using traditional methods. Distant sources contribute to the level, and multiple reflections can be very important. The equivalent sources method is used here to make predictions for canyon-like geometries. This method is extended to include effects of diffusion, absorption and atmospheric turbulence in order to improve the predictions. Substantial decreases on quiet side sound levels have been shown when introducing absorption and diffusion, and small increases have been shown due to turbulence. Measurements indicate that the level is relatively constant for courtyards throughout a city area, and a very simple model called the flat city model is proposed to explain this effect. This model assumes that all sources and receivers are located on a flat rigid plane. The effect of shielding by buildings is introduced as a correction term determined from measurements, and this term is within a relatively small range (6-10 dB) for all the areas studied.