Abstract
Noise reduction ΔL by an acoustic barrier for straight line source is treated by reduced scale model experiment. The line source used in the experiment has 3. 8m in length and radiates a test noise mechanically. The line source is an incoherent line source. Sound attenuation by a semi-infinite acoustic barrier in a free field was obtained experimentally about this line source. The results were plotted about the relation between ΔL for line source and the Fresnel number N, where N is given by N=2δ/λ and δ is the maximum path-length difference in this case. All experimental values spread in a relatively narrow range, and we can obtain a curve on the average(Fig. 12). This curve shows values lower than those of Maekawa's data, by 3〜5dB which were presented for a point source, and is similar to Rathe's experimental values, which were measured near the tracks of a railway line. In addition, our curve shows a similar tendency to the curve calculated by Kurze and Anderson about an incoherent line source. It was proved by this study that the noise reduction ΔL for a line source can be obtained approximately as the sum of individual ΔL's for sound emanating from a few points on the line source. It may be quite well to consider that the figure providing a relation ΔL vs. Fresnel number N is applicable for design of a barrier for traffic-noise(e. g. coming from cars or trains).
