Abstract: Equations of acoustic energy flux and transmission loss in a duct with a by-pass are derived from the continuities of sound pressure and volume velocity of particles at the joints of the duct and the by-pass,assuming all the sound waves considered here are plane ones. Computation results on a ndcrocomputor show that acoustic energy circulation exists in certain frequencies for a given duct. Transmission losses of 4 duct configuratfons are also presented and explained from the vieWPoint of active noise control.