This paper describes an experimental study of noise in ventilating ducts and the quieting effect of a sound absorbing duct lining. The attenuation in decibels per foot length of duct was measured directly, using a loudspeaker as a sound source, for both lined and unlined rectangular ducts of various shapes and sizes, and for various frequencies. The relation of the attenuation to the cross-sectional dimensions of the duct and to the absorption coefficient of the lining was found to agree more closely with the formula derived theoretically by Sivian than with any of the other formulae which have been proposed. Measurements were also made of the noise of a fan of 5000 c.f.m. capacity connected to the system. It was found that the noise generated by the friction of the air against the inside walls of the duct, either lined or unlined, for velocities as high as 1500 f.p.m., was negligible in comparison with the noise created by the fan blades. Tests were made on the quieting effect and the resistance to air flow of a special baffle structure introduced in the system. The frictional resistance of the absorbent lined duct, as compared to that of the unlined duct, was also measured.