|Yim, Jack Tsz Hong
|Acoustical insertion loss of a modified plenum window in different configurations and settings
Windows -- Acoustic properties.
Hong Kong Polytechnic University -- Dissertations
|Department of Building Services Engineering
|xi, 66 pages : color illustrations ; 30 cm
|In a highly urbanized city like Hong Kong where noise from busy traffic is of particular concern, it is sometimes unavoidable to develop a land for residential purpose despite its proximity to major road or train line. Inhabitants have to close the windows of their flats to prevent the intrusion of noise from outside. However when windows are closed, no wind or natural ventilation will be available in the dwelling. It is difficult for a window to accomplish these two conflicting roles at the same time satisfactorily. There have been many studies on the sound insulation of different kinds of windows with some targeted on partially opened windows which can provide natural ventilation to a certain extent. Tong and Tang  have designed a plenum window with staggered window panes which can provide sound insulation while the gap between the window panes allows air to pass through. The present study, which is basically a continuation of the study by Tong and Tang, is to modify the design of the plenum window by tilting the angle of the inner window pane and keeping the separation of window panes unchanged to see how this modification can affect the acoustic property of a plenum window. Computer simulation was first conducted to illustrate how the dynamics of sound waves would be affected by tilting the angle of window pane. Actual noise measurement was conducted using a 1:4 scaled down physical model to find out the acoustic performance of the modified plenum window with tilted window pane at 0°, +5° and -5°. Continuous broad band white noise normalised to traffic noise spectrum in the later calculation of insertion loss was input to an array of twenty 2-inch speakers in a line to resemble a line source and 13 microphones were placed inside the model to record the SPL. Noise measurements were conducted inside a semi-anechoic chamber resembling an outdoor sound field condition. The noise measurement results show that highest IL achieved by a modified plenum is 13.1 dBA at +5° tilting angle, which is 1.6 dBA better than that of a normal plenum window (0° tilting). However positive tilting do not always improve the IL but can sometimes on the contrary cause a deterioration in IL by almost 1 dBA as compared to that of 0° tilting in large window opening scenario. Therefore whether there is improvement or deterioration in the IL depends not only on the tilting angle but also on a combination of other influencing factors: size of window opening (or % of staggering) and distance of noise source. It is understood that reducing the noise source distance from the receiver will change the angle of incident of sound wave on the window pane and this causes the IL to be reduced in general. The effect of orientation of noise source relative to the receiver is also tested and it is demonstrated that a change of 30° in orientation can increase the IL by almost 5 dBA as compared to that of parallel noise source to the receiver.
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