A signal transducer mounts to a straight tube to measure the flow velocity of fluid flowing therein. The transducer operates at a frequency above cut-off allowing multiple modes to exist, and is mounted with a wedge of material having a sound speed less than the sound speed of the fluid, coupled to the conduit, and angled to launch a signal into the fluid at an acceptance angle that sends the lowest order mode acoustic signal axially through the fluid. Extraneous modes present at generally lesser magnitude convert to the desired mode, resulting in propagation of a coherent burst that is readily detected by a receiving transducer located further along the conduit. The wedge may be formed of hard plastic such as an acrylic, PVC or PEI having a shear wave velocity less than 1400 meters per second, and the launch geometry results in the attenuation, mode conversion or cancellation of higher mode signal energy components. The principal signal is strong and well defined, arriving in a window before any interfering signals. Upstream and downstream signals are thresholded, and processed to provide a flow measurement of high accuracy.