Figure 18. Oscillogram of sound pressure and photographs of splash produced by impinging water 

 droplet. The interval between frames is 1 3 millisec. The impact sound occurs between Frames 1 

 and 2; bubble sound begins between Frames 13 and 14. The bubble can be seen in Frames 14 

 to 18. The mass of the droplet is 56 mg; its velocity at impact, 350 cm/sec. 



entation will indicate something of the complexity of the subject and the vast oppor- 

 tunity for further research. 



V. Unsteady Flow 



The generation of sound by an unsteady flow differs in an essential way from 

 the mechanisms discussed in the previous sections. Whereas bubbles and splashes 

 inherently require the presence of both liquid and gaseous states of matter, an unsteady 

 flow can generate sound in a homogenous fluid, either liquid or gas alone. 



The recognition of the contribution to the noise from high-speed aircraft 

 made by turbulence and other forms of unsteadiness has led to many investigations 

 of this form of noise. Comprehensive reviews of these investigations have been pub- 

 lished recently by Fowell and Korbacher [37] and by Mawardi [38]. The terms "aero- 

 dynamic noise" and "flow noise" have been applied to the phenomenon; but we 

 prefer the term "unsteady-flow noise" in order to point up the essential feature 

 causing the noise and distinguishing it from other kinds of fluid-dynamic noise. 



Results obtained in the study of noise from air flows are applicable to the 



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