This paper describes the noise reduction effects of the shape and the thickness of a device on the top of barriers. The analysis was made by Boundary Element Method (BEM). The results show as follows.
(1) The effect of the barrier with a reflective cylinder is similar to that of the same width simple barrier. While, in the case of the barrier with an absorptive cylinder, the effect is greater than that of the same width simple barrier.
(2) The most effective barrier is double barrier, and the next one is the barrier with Y-shaped device.
(3) At the near site of the highway bridge, the barrier with an absorptive mushroom device is more effective than that with Y-shaped device.

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