The dependence of the noise output of the jet engine upon the eighth power of the exhaust velocity has been firmly established both theoretically and experimentally—at least for the higher subsonic speeds. The advent of the bypass engine and the continued increase of the bypass ratio of these engines have produced a very considerable reduction in the radiated noise. In some instances, however, the reduction has fallen short of that expected, on the basis of an eighth‐power law. It seems probable that other noise sources having a lower velocity exponent now dominate the radiated power. In order to investigate the transition and the nature of these sources, an experimental study using model air jets has been initiated. The effects of both fixed and rotating flow spoilers upon the radiated power have been investigated. Some results of this study are presented in this paper and conclusions drawn regarding the application of these to the real‐life problem of predicting and controlling the noise of the modern jet engine. [Work supported by National Aeronautics and Space Administration, Langley Field, Virginia.]