  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.
