Acoustical protection of a plenum window installed with sound absorptions

Traffic noise has long been an important issue in environmental protection in Hong Kong. Urban re-development to make room for more residential units has made the issue more challenging as the areas to be re-developed were usually used for industrial or commercial purposes in the past where noise control has not been as stringent as that for residential areas. Owing to this problem, a new PolyU hostel has installed with plenum windows which can provide an additional 8 dBA acoustical protection when compared to the conventional side-hung window design. In this study, a site measurement is carried out to explore how the acoustical protection of such plenum window design can be improved. Strong sound absorption materials are used to cover the ceiling, floor and the mullions of the plenum, while micro-perforated sheets are placed in front of the inner window glass pane with a 50 mm air gap in between. The outer window glass pane remains untreated. The noise reduction under various combinations of these sound absorption have been tested. The maximum noise level difference between the indoor and the outdoor is 24 dBA.