New information was published in 1983 and 1984 on an important acoustical property of duct design, the duct wall noise breakout transmission loss. Breakout transmission loss information is the starting point for predicting the acoustical performance of a duct wall and predicting the noise levels expected near the duct of an air-handler installation. The low transmission loss of a rectangular duct often allows the radiation of rumble noise, even when the duct is well-designed and acoustically treated, while the high transmission loss of a round duct wall prevents radiation of low frequency rumble even without acoustic treatment. This paper uses test data and new information on transmission loss to show (1) why some large air handler installations have rumble noise problems with rectangular ducts, (2) why treatment of rectangular ducts is not satisfactory, and (3) how to obtain noise control using a round duct.

Units: Dual