21st International Congress on Sound and Vibration 2014, ICSV 2014, Beijing, 13-17 July 2014 ; The duct section containing side-branches mounted flush with the duct wall can produce strong and broadband sound transmission loss provided that the lengths of these side- branches are appropriately arranged and when there is no air flow along the duct. However, in the presence of a turbulent air flow, the flow separation at the mouths of these side- branches and the shear layers so generated will excite the side-branches. Such excitation has been shown to have downgraded the performance of Helmholtz resonators and thus a downgrade of the side-branch array in the presence of a flow is expected. In this study, an experiment is setup to investigate how the air flow speed will affect the frequency spectrum of the sound transmission loss across a side-branch array with branch lengths so arranged that they decrease linearly from one to half the duct width. Eleven side-branches, each with a width of 0.1 the duct width, are used. The sound transmission losses are measured using the 4- microphone method. The results show that the degradation of acoustical performance of the side-branch array (strength and bandwidth) is in-significant until a threshold is reached. This threshold is around 8 mIs under the current experimental setup. The sound transmission losses then drop quickly with increasing air flow speed, but these fast drops occur within narrow bandwidths. The rate of sound transmission loss reduction increases with air flow speed. Narrowband sound amplifications can be observed when the flow speed reaches 20 mIs, which is around the flow speed limit of a practical flow duct. ; Department of Building Services Engineering