The Influence of Atmospheric Turbulence on Barrier Sound Reduction

Doctoral thesis, 2001

Numerical modelling and measurements are used for studying the reduced performance of a noise barrier, caused by a turbulent atmosphere. The turbulence scatters the sound, which leads to an increased noise level in the acoustic shadow region behind the barrier. For the numerical studies, a parabolic equation method (PE) is extended to model situations with a thin screen in a turbulent atmosphere. A scattering cross-section method is also extended to take into account source and receiver positions above a ground surface, and scaling properties are used to generate an efficient prediction scheme. A substitute-sources method (SSM) is developed, which is numerically faster than the PE. Both two and three-dimensional implementations are studied, from which it is concluded that the sound level can be well predicted for a large variety of situations using a two-dimensional model. The effect of turbulence increases with frequency and with enlarged geometry. The study concentrates on road traffic noise situations, where screens or buildings are used as noise barriers. The results show that the increase in sound pressure level caused by turbulence can be large at higher frequencies and significant as an A-weighted level.