United States Department of Transportation
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Abstract:Noise measurements were taken at six barrier sites: two wooden, two metal, and one concrete barrier were studied; the sixth site had no barrier and was studied to determine the ground effect. The approach was to determine insertion losses by taking simultaneous measurements behind the barrier at different elevations over the same point. In this procedure, when the uppermost-microphone is clearly in the line of sight of the traffic, the difference between the level for the uppermost microphone and the level for one of the lower microphones is the insertion loss at the height of the lower microphone. For the measurements, three microphones were positioned at different heights on a 9.1-m (30-ft) pole and a fourth microphone (with its own support) was placed 1.5 m (5 ft) above ground level. Unfortunately, with the microphones so arranged, the uppermost microphone was in line of sight of only the measurement sites close to the barriers. Thus many of the values derived from the analysis of the data must be viewed as differential insertion losses. Both predicted and measured noise levels behind the barriers were compared, and the results led to the conclusion that the barriers were performing as they should be expected to. The principal recommendation that could be made, considering the rather limited scope of this study, was that no policy decisions should be made that would eliminate the use of any material or construction technique on the basis of its performance.
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