The effect of a subsonic mean flow on the organ-pipe type of vibration of the air in a duct of uniform area is calculated. The flow at entry to the duct is assumed to be at constant stagnation pressure, and a range of exit conditions is considered. It is found that the effect of the mean flow is to reduce the resonant frequencies by a factor of (1—M2) (where M is the Mach number in the duct) and to introduce damping of the vibration. Except when the duct is terminated by an efficient diffuser, this damping is rather large and it would require a powerful driving mechanism to produce large vibrations in the duct.