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The mixing region of a jet is observed to be a complex noise generator. The noise produced is highly directional and is affected by various geometric and flow parameters as well as by conditions in the settling chamber upstream of the nozzle. Noise measurements for a family of circular model air jets ranging in diameter from 34 to 12 inches are consistent with available data for a turbojet engine. The intensity of the fluctuating pressure field near the jet is greatest at an axial distance of approximately two diameters downstream from the nozzle exit and decrease generally with increasing distance. The frequency spectrums recorded near the jet boundary are usually peaked, the peak frequencies being higher near the jet exit than at points farther downstream. These noise frequencies generally increase with increasing jet fluid velocity and decrease with increasing jet size. Hot wire surveys of turbulence (axial velocity fluctuation) in the jet stream indicated spectrums which were very similar in quality to the noise spectrums recorded just outside the jet boundary and at the same axial stations.

Some sample experimental results relating to the nature and sources of noise in jets are presented. Previous work is extended to include more detailed information about the jet structure. In this regard noise data are compared for jets having somewhat different turbulence profiles. High temperature jets are noted to have different radiation patterns and to be more efficient noise radiators than low temperature jets. Noise data for the high temperature supersonic jet of a rocket engine are included for comparison with those of subsonic jets. Some experiments have shown the noise levels at the low frequencies can be markedly reduced by the use of grids in the jet stream.

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The mixing region of a jet is observed to be a complex noise generator. The noise produced is highly directional and is a function of the jet size, velocity, density, and turbulence level. Experimental studies of the noise from a series of model subsonic jets ranging in diameter from 0.75 to 12.0 inches in diameter are compared with data for a turbojet engine. The noise generated by this engine was found to be closely related to that generated by the simple model jets, and an empirical relation is given to allow the extrapolation of available jet noise data to other operating conditions. In addition, an attempt is made to correlate the noise data with some aerodynamic measurements inside the jet, and a brief discussion is given of recent experiments with devices designed to alter the jet noise spectrum. Data are presented in a form convenient for comparison with Lighthill's theory.