Exhaust Noise Field Generated in the JT3D Core Engine—Noise Floor Presented by the Internal Noise Sources

Abstract

An experimental study has been conducted to determine the strength of the acoustic radiation generated in the combustion and turbine stages of a JT8D core engine and transmitted out through the primary jet exhaust duct. The acoustic field within the tall pipe duct extension was measured by an array of microphones flush-mounted on the extension wall. The interpretation of the cross-power spectral densities of the microphone signals by means of a theoretical model of the sound propagation within the duct permitted the acoustic energy transmission out of the nozzle exit to be determined. The results show that the internally generated noise is a dominant noise component from the core engine at larger angles from the jet axis for the case of low engine power settings. In general, although the internally generated core engine noise component lies well below the JT8D engine jet-noise and fan-noise components at the higher engine power settings, it represents an additional noise parameter that must be considered in the application of fan-noise and jet-noise suppression techniques. [Work accomplised when the author was a member of the Aircraft Noise Staff of the Boeing Company, Seattle, Washington.]