Study of the Performance of Acoustic Barriers for Indiana Toll Roads

The contents of this report reflect the views of the authors who are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Federal Highway Administration and the Indiana Department of Transportation. The report does not constitute a standard, specification, or regulation. The purpose of this study was to optimize the geometry and the acoustic properties of sound barriers for traffic noise applications. The approach consisted of developing and validating boundary element predictive models, which were subsequently exercised in order to refine the barrier characteristic and determine optimized configurations. The simple geometry of a circular disk was chosen to validate the boundary element model in the first part of this work. Experiments were performed in an anechoic chamber to validate the numerical model. Complex barrier geometries were then investigated to study the effects of geometry on sound barrier performance. Boundary element models were used to quantify the accuracy of existing, approximate diffraction-based models. Diffraction-based models have been widely applied in noise control engineering applications owing to their relative ease of use. Recent research suggests that multi-path diffraction components should be summed on a phase-coherent basis instead of on an energy basis. The accuracy of a phase-coherent diffraction model has been verified against the boundary element solution and the limitations of the diffraction model are discussed for both the case of infinite length barriers and barriers of finite length. A new barrier performance metric, based on the sound power propagating within the shadow zone was also investigated. It was found that variation of barrier geometry while maintaining the surface area constant did not yield any meaningful difference in the sound power propagating within the shadow zone. The performance of straight-edge barriers with various top geometries and sound absorptive treatments was then investigated. Experiments were performed using a finite size barrier in an anechoic chamber to verify the boundary element model. Good agreement was obtained between the results from the numerical model and the experimental data. The most important finding was that absorptive treatment applied to the top of a barrier was more effective at reducing sound levels in the shadow zone than a simple increase of barrier height. The use of the boundary element method to calculate the new barrier sound power performance metric is also discussed in connection with the complex geometry. It is shown …