A new method of measuring the acoustic intensity of plane waves in a duct with flow is described. In this method, the acoustic transfer function between two locations in the duct is used to determine the acoustic intensity. In the case of no flow, the transfer‐function formulation reduces to a cross‐spectral relation similar to the relation used to measure acoustic intensity in three dimensions. In contrast to the relation in three dimensions, however, the cross‐spectral formulation for a duct is not limited in accuracy by the microphone spacing. The new method has been verified experimentally with a series of laboratory tests. Test results obtained by the transfer function method reveal that the net acoustic power transmitted along the pipe increases as the Mach number increases. The acoustic power radiated from the pipe opening, however, remains unchanged with increasing Mach number. This difference between the transmitted and the radiated power appears to be due to sound absorption caused by vorticity shedding at the pipe opening.