The effect of duch Mach number upon the acoustic reflection coefficient at the inlet of a duct with mean flow is investiagted. An analysis, which models the duct inlet as a very short, one‐dimensional nozzle over which the mean flow is accelerated from rest, gives good agreement with some recent experimental results. Discrepancies between the analysis and he experimental results are discussed in terms of radiation losses at the inlet and real fluid‐flow effects within the duct.Subject Classification: [43]28.60; [43]20.30.