Determination of the characteristics of a linear ducted sound source
  Ducted flow devices for a range of purposes, such as air-moving fans, are routinely characterised experimentally to understand their acoustic performance as part of the continuing trend for quiet, high efficiency design. The International Organization for Standardization (ISO) method 5136 is widely used in industry and academia to determine the sound radiated into a duct. This involves placing the device at the centre of a long cylindrical duct with anechoic terminations at each end to eliminate reflections. A single off-axis microphone is used on the inlet and outlet sides that can theoretically capture the plane-wave mode amplitudes but this does not provide enough information to fully account for higher-order modes. In this study, the 'two-port' source model is formulated to include higher-order modes and applied for the first three modes as a proof of concept. This requires six independent surface pressure measurements on each side or 'port'. The resulting experimental set-up is much shorter than the ISO rig and does not require anechoic terminations. The relative importance of the higher-order modes has been considered and the inaccuracies when using the ISO method to find source sound power has been analysed. 
Timothy J. Newman, Anurag Agarwal, Ann P. Dowling and Ludovic Desvard
physics.flu-dyn