A study of the influence of fluid motion on the emission of higher‐order acoustic modes from a stationary source in a duct is presented. In particular, the ratio between the upstream and downstream higher‐order mode amplitudes is calculated. This ratio has been measured for the (1,0) mode in a rectangular duct at flow velocities from zero to 0.25 times the sound speed. The frequency dependence has also been measured at a fixed flow Mach number of 0.1.

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