Noise transmission through duct divisions in air circuits, considered as three-port acoustic systems

An experimental study is presented on the characterization of the noise transmission properties of the duct junctions used in ventilation and air-conditioning air circuits, assumed as three-port acoustic systems with plane-wave sound propagation. The procedure requires the measurement and post-processing of the transfer functions between the control signals of three loudspeakers, one at each branch, and the sound pressure signal recorded at six positions (two-microphone technique). The result is the scattering matrix, which contains three reflection coefficients and six transmission coefficients. Experiments were conducted on junctions of ducts with the same diameter, with three different deviation angles for the branch take-off. Besides, several different airflow conditions were imposed for each geometrical configuration. This paper describes the methodology used and the results obtained for the cases tested, and discusses the effect of the different factors involved in the noise transmission through the duct divisions.