Sound attenuation in a circular duct is investigated by measurement and numerical calculation for using a slit-like short circular expansion chamber, termed 'slit' here, that performs as a resonator muffler in the duct. Since its resonant frequency depends on its depth, some depth-varying configurations of it are introduced in order to widen its effective frequency range. First, the performance of a set of serialized slits is examined. This configuration is composed by the slits located in series in the duct, having different diameters from each other and concentric to the duct. While this type of slit basically shows the summative performance of each composing slit, a certain interaction among slits occurs if the resonant frequency of each slit comes close to each other. Second, a single slit eccentric to the duct is examined. In every case of it treated here, two resonant frequencies are obtained. While the lower resonance seems to relate to the (1,0) mode in the slit, the higher one seems to relate to the corresponding concentric slit. Finally, a set of serialized slits eccentric to the duct is considered. Its property becomes considerably complicated against the two cases mentioned above. In certain configurations of it, however, a good performance for sound reduction is obtained. (C) 2003 Elsevier Ltd. All rights reserved.