Jorge Parrondo 1 TENTH INTERNATIONAL CONGRESS ON SOUND AND VIBRATION, ICSV10 NOISE TRANSMISSION THROUGH DUCT DIVISIONS IN AIR CIRCUITS, CONSIDERED AS 3-PORT ACOUSTIC SYSTEMS Jorge Parrondo, Eduardo Ruiz, Iván García, Julián Martínez, Joaquín Fernández ( * ) Universidad de Oviedo, Departamento de Energía (EDZE), 33203-Gijón, Spain ( * ) Universidad de Extremadura, Departamento de Electrónica e Ingeniería Electromecánica, 06071-Badajoz, Spain parrondo@correo.uniovi.es Abstract An experimental study is presented on the characterization of the noise transmission properties of the duct junctions of ventilation and air-conditioning air circuits, assumed as three port acoustic systems. The procedure requires the measurement and post-processing of the transfer functions between the control signals of 3 loudspeakers, one at each branch, and the sound pressure signal recorded at 6 positions (two microphone technique). This paper describes the basis of the methodology and presents some of the results obtained for branch take-offs with three deviation angles and under different air flow conditions. INTRODUCTION Air circuits for ventilation or air-conditioning systems may conduct the noise generated at the fans or other sources towards the ventilated rooms. Usual regulations for thermal installations in buildings impose maximum background noise values for each room type, in terms of overall noise levels (dBA) or referred to RC or NC indexes. In order to meet the noise specifications, many air circuits have to include silencers, either reactive or purely dissipative. Optimal selection of the silencers at the design stage of the system requires the previous calculation of the noise levels induced in the ventilated rooms.