Noise barriers with reactive surfaces

The boundary element numerical modelling method is used to study the insertion loss of rectangular, T-shaped and cylindrical edged noise barriers with rigid, absorbing and soft surfaces. The most efficient design is a T-shape with a soft upper surface, it is found that a uniform series of wells in the upper surface of a T-shaped barrier produce insertion loss values equal to those of a soft surface over a significant range of frequencies. At low frequencies the insertion loss is less than that expected for a rigid upper surface. The use of wells tuned to two different frequencies widens the spectral range of the soft surface effect. It is found that the performance of the wells can be described accurately in the numerical model by a simple analytical expression for the specific impedance of a plane surface. Experimental measurements on scale models are compared with the results of numerical modelling. (C) 1997 Elsevier Science Ltd.