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The extensive research on flow noise carried out in various laboratories during the past ten years has been concerned mainly with jet noise, with particular emphasis on jet engine noise. The flow velocities involved in these research programs generally has been close to sonic. Despite this considerable effort in the jet noise field, comparatively little is known about the noise produced by turbulent flow in duct systems. This problem is considerably more complex than the jet noise problem since many more parameters affect the flow and the noise. Not only will the duct geometry and wall conditions influence the flow structure, but the presence of (transverse) duct resonances can markedly affect the noise field.

In view of this complexity, research on flow noise in duct systems for the purpose of obtaining practical engineering data, is apt to be limited merely to empirical work which simply involves the measurement of the noise spectra produced by various typical duct elements. The present work is of this type. However even with this limited objective, the problem still involves experimental difficulties concerning the measurement of acoustic power and the identification of the relevant noise sources as well as the suppression of the irrelevent ones.