HIGH-FREQUENCY ACOUSTIC RADIATION FROM A CURVED DUCT OF CIRCULAR CROSS-SECTION

Sound propagation through bends in ducts has been studied extensively in the case of ducts of rectangular cross section without the presence of airflow, and for plane-waves or low frequencies. However, high-frequency tonal sounds may radiate from helicopter turboshaft engines or aircraft auxiliary power units equipped with a curved inlet or exhaust duct of near-circular cross section. Tests were conducted in an anechoic chamber to investigate sound propagation through such curved ducts. Pure-tone sounds were generated at one end of a curved duct by a spinning mode synthesizer; there was no airflow through the duct. In-duct analysis of propagating acoustical modes near the entrance and exit of the duct showed that a single mode at the entrance was split of the bend into all the possible propagating modes. Directivity patterns of farfield sound were not axisymmetric around the duct exit centerline. Prediction of asymmetric directivity patterns was accomplished by assuming that the modes were time-coherent instead of incoherent as implied by the classical analysis of Tyler and Sofrin. Asymmetric farfield directivity patterns of the discrete-frequency sound radiated from a curved duct must be measured in several planes. Asymmetric directivity could be utilized to control the discrete-frequency fan or compressor noise radiated in certain directions from a curved inlet or exhaust duct. (C) 1994 Institute of Noise Control Engineering.