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Sound radiated in the transverse direction due to flexural vibration of the duct walls excited by the internal sound field is called the breakout noise. Breakout noise is predominant in the flexible rectangular ducts compared to other shapes due to lower stiffness. The prediction of sound radiated from the duct walls helps to understand the breakout noise phenomena. In literature, analytical models are available to predict sound radiation from the rectangular duct such as a finite-length line source, equivalent cylindrical and an equivalent plate model (EPM). In EPM, modal radiation efficiencies and total radiation efficiency of the duct is calculated by assuming it as an equivalent rectangular plate. The length and breadth of the plate are represented by the length of the duct and perimeter of the cross-section of the duct. In the present research work, Transverse transmission loss (TTL) of the rectangular duct is predicted by using total radiation efficiency calculated by equivalent plate model. TTL of the rectangular duct is also calculated with modal radiation efficiency of the breathing mode (Dominant Mode). The values obtained from both the results are compared with the TTL predicted from the finite-length source model, and all the three models are in good agreement. However, the prediction of total radiation efficiency and modal radiation efficiency of the duct in equivalent plate model (EPM) involves solving complex mathematical equations. Hence, a simplified model is proposed to predict the total radiation efficiency of the duct by using the modal radiation efficiency of simple plate mode. The major contributing mode for the sound radiation in the duct is the breathing mode, i.e., dominant symmetric mode on the four walls. This mode is equivalent to the (4,1) mode of the simple plate. Literature is available to predict the modal radiation efficiency of the simple plate. The modal radiation efficiency of simple plate mode (4,1) is used for the TTL calculations. From the comparison study, it is observed that within the cut-on frequency, the predicted results from the proposed model are in reasonable agreement with the literature data.
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