Final Report - Technical assessment of the effectiveness of noise walls

This initiative of International INCE deals with noise walls--the outdoor barriers erected in parallel with highway and rail lines, and in other areas (such as airport runways), where there is a demand to reduce the noise levels of surface transportation sources. There is worldwide interest in the control of noise by the erection of such barriers. Walls are composed of wood, metal, masonry, earth, and other materials, both opaque and transparent. Most of the walls that have been erected to date completely block the sight lines between vehicles and roadside housing. The cost of installation usually exceeds USD one million per kilometer. In some countries, governmental authorities have authorized the use of highway construction funds for the erection of noise walls. When building a new highway or widening an existing highway, the construction of noise walls is required in some jurisdictions when the predicted noise levels of the road traffic exceed defined governmental guidelines. The key questions are: how valid are the traffic noise predictions, and how effective are the noise walls acoustically after they have been erected? Over the years, a number of analytical studies have facilitated the prediction of the noise reduction afforded by such barriers. It is reported, however, that barriers may not always perform acoustically as well as intended. The principal objective of this study is to obtain a global view of the effectiveness of noise walls-the outdoor barriers erected in parallel with highway and rail lines, and in other areas (such as airport runways). The report summarizes the scientific basis of noise barriers, including measures of barrier efficiency, the physical phenomena involved (including effects associated with the propagation and effects associated with the noise wall as well as different barrier shapes), and the various models used to predict barrier performance. Different barrier materials are briefly described. The measurement of barrier effectiveness is also discussed. A section discusses the three main application areas where barriers are used: road traffic noise, railroad noise, and ground-based aircraft operations.