Section III, 1922 [157] Trans. R.S.C. 



Cavitation in the Propagation of Sound 



By R. W. Boyle 



(Read May Meeting, 1922) 



In the theory of the radiation of sound waves from a diaphragm 

 or plate into a fîuid medium, and also of the propagation of the waves 

 through the medium, there enters a question as to whether the 

 maximum amount of energy which can be transmitted is, or is not, 

 limited by the phenomenon of cavitation. 



For example, if a diaphragm in the medium is executing simple 

 harmonic vibrations it pushes out into the medium and creates a com- 

 pressional wave before it. When the diaphragm springs back on account 

 of its elastic forces, if the static pressure of the medium at the surface of 

 contact with the diaphragm is insufficient, the medium cannot im- 

 mediately follow back with the diaphragm, which therefore caves in, 

 away from the medium, producing thereby a vacuum or partial 

 vacuum, and interrupting the rhythmic motion of the wave. The 

 greater the static pressure of the medium, therefore, the greater 

 will be the maximum possible amplitude of vibration, for any given 

 pitch of vibration, to preserve the rhythmic character of the wave. 



Again, in considering the travel of the wave through the medium, 

 if at any point the amplitude of alternating pressure in the waves is p, 

 and the static pressure there existing is po, the resultant pressure at 

 the instants of maximum displacement is po±p- It might appear 

 that p cannot exceed Po, for if it did the resultant pressure at the instant 

 of greatest rarefaction would be negative, i.e., the medium would be 

 under tension. In this condition a discontinuity at that point in the 

 medium might be produced, a vacuum or a bubble be formed, and the 

 rhythmic character of the wave interrupted. In the case of solids 

 cavitation of the kind above described could not occur. 



At very low frequencies it is doubtless true that cavitation may 

 be produced in the manner suggested, and therefore there will be a 

 maximum amplitude of displacement corresponding to a given fre- 

 quency at which energy can be transmitted in a regular wave motion. 

 But there is a difference of opinion about such laws of cavitation 

 remaining valid at higher ranpes of frequency. 



It is possible to demonstrate in very careful static experiments, 

 that a liquid can be placed under a tension, but it is contended by 

 some that in the dynamic case of wave motion it will not be possible 



