Self-induced waves in a conduit with corrugated walls. II. Experiments with air in corrugated and finned tubes

Abstract

Metal bellows were found to produce a pure note when air was passed through them over a small range of velocity. An axial rod, restricting the stream to a narrow annular cross-section, was usually essential for speech, and sound was produced also by a narrow stream between the bellows and an external pipe. More powerful notes were obtained when the bellows inside the pipe were replaced by helically finned tubes. The pressure distribution in this arrangement was examined. The phase velocity of the waves inside the pipe was much less than the velocity of sound, and this effect is explained by applying the theory of wave propagation in periodic structures. The ratio of the average frequency, at which an air particle passed the protuberances, to the frequency of the sound emitted was usually about 1·6.