Experiments were conducted in a Rijke tube by varying the surface roughness of the walls, the heat transfer to the air in the tube and by placing a conical horn at the open end of the tube. A sub-harmonic frequency, corresponding to the open end acting partially as a closed end, was shown to be present. Placement of a horn at the open end prevented the tube from sounding. The influence of the roughness of the walls on the pressure oscillations is discussed. ©Freund Publishing House Ltd.