Parallel barrier effectiveness, Dulles noise barrier project.

In an effort to minimize the cost and maximize the effectiveness of highway noise barriers, the Federal Highway Administration (FHWA) and a National Pooled Fund Panel (made up of 14 states) funded a field study program on an experimental highway noise barrier. A test barrier was constructed in 1984 at a site at Dulles International Airport in Chantilly, Virginia. The study, conducted from May 1989 to August 1989 by the U.S. Department of Transportation, Research and Special Programs Administration, Transportation System Center (U.S. DOT/RSPA/TSC), focused on the use of absorptive treatment and tilting as a means of improving the insertion loss of two parallel highway noise barriers. Measurements were conducted with both controlled moving point sources (trucks) and an artificial fixed-point source (speaker system). Results show (1) the addition of absorptive treatment to the roadside face of two vertical, parallel, highway noise barriers eliminated multiple reflections and was found to improve the insertion loss (2–6 dB); (2) tilting proved to be an effective alternative to absorptive treatment in eliminating the multiple reflections and subsequent degradation in performance of two vertical reflective barriers; and (3) use of an artificial fixed-point source is not a viable test of barrier effectiveness.