Characterization of fluid machines as acoustic multi-port sources

The aim of the work reported in this paper was to study the characterization of fluid machines (e.g., fans, pumps) as sources of fluid-borne sound in connected pipe or duct systems. Of special interest is the possibility of extending earlier published methods beyond the plane wave region: i.e., to include higher order modes effects. For machines which can be modelled as an acoustic multi-port source, a method which makes it possible to measure the source data is suggested together with a modal decomposition technique. In order to verify and test the measurement method and modal decomposition technique, measurements were first carried out on a small test-rig without flow, with an ordinary loudspeaker as a source. Then the method was tested on an axial fan mounted to the end of a duct. In both cases ten microphone positions, divided equally between two duct cross-sections, were used to separate the waves (five modes) into left- and right-going components. It was verified that the measured source data can be used to predict the sound field created by the fan at a given acoustic load. (C) 1996 Academic Press Limited