This paper reports the results of a set of experiments involving the passage of flow through a constriction - a single orifice - located in the middle of a duct. Geometric variations during testing include the length of the duct, orifice size, and orifice thickness. Acoustic oscillations in the duct were measured by systematically varying the flow rate for each combination of geometric variables. Hot wire measurements were performed, in the region downstream of the orifice, under select conditions. The results indicate the excitation of discrete sharp tones of high amplitude that abruptly shift frequencies as the flow rate is increased within a certain range. The excited frequencies are related to fairly high harmonics of the natural acoustic modes of the full duct with both ends open and the half duct with one end closed and the other end open. Simultaneous measurements from the hot wire and the microphone show the same frequency. This suggests the prevalence of a "lock-on" type of phenomena generally witnessed in flow-structure oscillatory interactions. The effect of increase in the orifice diameter is to decrease the range of frequencies when the sharp tones are heard, as also to decrease their amplitudes. Some evidence is presented that suggests the critical role played by the thickness of the orifice in exciting these sharp tones. © 2000 by the authors.