The present invention relates to a noise barrier upper diffraction sound reduction device using a Helmholtz sound absorber having a variable function of resonance frequency; It is installed horizontally on the upper end of the soundproof wall is installed on one side of the track that the train moves, characterized in that the through-hole is formed in communication with the space portion in the upper portion of the chamber in which a plurality of space portion is partitioned. According to the present invention, the diffraction sound reduction device using the Helmholtz resonance principle at the top of the soundproof wall is provided to reduce the diffraction sound at the top of the soundproof wall, thereby compensating the limitation of the performance of the conventional upright soundproof wall. In particular, the present invention has the advantage of overcoming the technical limitations of the existing railway soundproof wall performance, and to expand the soundproofing wall effect and sound absorbing position according to the noise source location and characteristics according to the speed of the train operation.
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