In a uniform rigid-walled duct, plane waves propagate with axial phase speed (1±M) times the sound speed, according to the inviscid plug-flow model. The difference between the downstream and upstream phase speeds provides a measure of the flow rate; the same concept can be extended to realistic profiles of M (Mach number) across the duct section, although the lowest-order mode is not a plane wave any more. Calculations were carried out to provide a means of converting phase speed differences to flow rates, assuming that propagation is confined to a single low-order mode (i.e., near-axial propagation). The results were then compared with phase speed measurements made in a cylindrical steel tube, through which air was pumped at a controlled steady flow rate. Close but not perfect agreement was found, which raises the question of whether our neglect of thermoviscous phenomena is justified, particularly near the duct walls. [Work supported by British Gas.]