The noise reduction afforded by ducts or parallel baffles depends not only on the physical properties of the duct and its lining, but also upon the angle of incidence of the sound waves impinging on the ducts. The noise reduction per duct width or per unit length provided by a duct for sound waves impinging at several angles of incidence is found to vary with the length of the duct. The variation of noise reduction per duct width decreases with increasing duct length. For sufficiently long ducts, however, the noise reduction per duct width approaches a constant value For a fairly wide range of duct lengths, the noise reduction can be expressed asa+bl, wherebis the noise reduction per duct width,lis the length of the duct measured in duct widths, andais a source distribution correction factor. Several sets of experimental data are presented to show thatamay be comparable in size toblfor typical duct lengths. Several implications of a noise reduction of the forma+blare discussed. Procedures are given for estimating the noise reduction for ducts of finite length by use of Morse's theory and the data presented here. Some unsolved problems are discussed. [Supported in part by the U. S. Air Force under Contract Nos. AF 33(616)‐2151, AF 33(616)‐3335, and AF 33(616)‐3938.]