Parallel barrier effectiveness - Dulles noise 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 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 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 dB to 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; (3) additional verification needs to be performed with an artificial fixed-point source before it can be recommended as a viable alternative to actual highway traffic in measuring barrier effectiveness; and (4) although the 'BARRIER 2.1' computer program cannot model the Dulles test situation exactly, and actual ground impedance data were not available, the trends in the predicted insertion loss data were in good agreement with the predicted results although lower in absolute level.