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An experimental investigation has been conducted to determine the source and nature of disturbances in the settling chamber and test section of a Mach 5 wind tunnel. Various changes in the air supply piping to the wind tunnel are shown to influence the disturbance levels in the settling chamber. These levels were reduced by the use of an acoustic muffler section in the settling chamber. Three nozzles were tested with the same settling chamber, and hot-wire measurements indicated that the test section disturbances were entirely acoustic. Significant reductions in the test section noise levels were obtained with an electroplated nozzle, utilizing boundary-layer removal upstream of the throat. The source of test section noise is shown to be different for laminar and turbulent nozzle-wall boundary layers. = acoustic origin = stagnation condition downstream of normal shock = value at wall = freestream in nozzle or settling chamber = stagnation value

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