Numerical modelling of sound propagation to closed urban courtyards

Doctoral thesis, 2009

Because modern urban environments suffer from excessive levels of road traffic noise, access to closed courtyards is essential in order to offer urban sound environments of high quality with regard to health and perceived sound. To reach this quality, which is described by the quiet side definition, the 24-hour equivalent noise level should be below 45 dB(A). Since many courtyards do not necessarily fulfil this requirement, noise abatement schemes are of interest. Current engineering prediction methods, however, fail to accurately predict the noise level from road traffic in closed courtyards. The 2.5-dimensional equivalent source method (ESM), an accurate frequency-domain method, has therefore been developed to predict sound propagation to a closed courtyard and to evaluate noise abatement schemes in the yard. This method, an extension of the two-dimensional (2-D) ESM, allows for a point source or incoherent line source in a 3-D environment where the geometry is invariant in one direction. For two real-life courtyard geometries, the averaged effects of various noise abatement schemes are presented, mainly based on in- and near-courtyard absorption and screen treatments. Since it is also of interest to study both 3-D and time-domain effects, the 3-D extended Fourier pseudospectral time-domain (PSTD) method has been developed. This method accurately predicts sound propagation through an inhomogeneous moving atmosphere to a closed courtyard. It extends the Fourier PSTD method by allowing to model propagation media with discontinuous properties. By only requiring two spatial points per wavelength, the method is also computationally more efficient than other state-of-the-art, similarly-accurate methods in urban acoustics. It has been validated for typical outdoor sound propagation cases as well as for the closed courtyard geometry. A scale model study has been executed to validate the two developed prediction methods, and to explain the characteristics of the acoustic soundscape in courtyards. Measurements and calculations show that higher-order façade reflections greatly influence the equivalent sound level and sound decay properties in closed courtyards. It explains the relatively constant sound levels in time and space in closed urban courtyards, as well as the significance of noise from distant traffic.