title: Transmission characteristics of a tee-junction in a rectangular duct at higher-order modes.
abstract: Numerical and experimental studies were undertaken to characterize the noise transmission and scattering properties in higher-order modes across the tee-junction of a rectangular duct used in ventilation and air-conditioning systems. To measure these properties, a formulation of a transmission matrix based on the transfer function and a two-microphone method was devised. The measurement of modal sound transmission and scattering coefficients is demonstrated for a duct element in a rectangular duct. The results of numerical simulations were verified by experiments. The results show that sound transmissions of fundamental mode and higher-order modes across the main duct are high at the eigen-frequencies of the main duct and sidebranch. Weak modal coupling of the branch-modes and the traveling wave in the main duct is observed at or very close to the eigen-frequencies of the sidebranch, which shifts excitation of the higher-order branch-modes at higher frequencies. A decrease in sound transmission and increase in sound scattering into higher-order modes occur with excitation of the axial branch-mode. Excitation of the longitudinal branch-mode due to branch-end reflection also results in lower sound transmission of higher-order modes across the junction along the main duct.
authors.affiliation: Architectural Engineering Program, University of Nebraska-Lincoln, 1110 South 67th Street, Omaha, Nebraska 68182-0681, USA. slau3@unl.edu
authors.forename: Siu-Kit
authors.initials: SK
authors.lastname: Lau
authors.forename: Kwan-Hao
authors.initials: KH
authors.lastname: Leung