1000 



Figure 11. Spectrum of sound radiated by a growing and collapsing vapor cavity in a compressible 

 liquid. The dash lines show the asymptotes corresponding to the shocks indicated in Fig. 10. 



gas in the cavity affect the amplitude of the shock. A calculation would be necessary 

 to ascertain the effect in a specific case.* A more important reason for the discrepancy, 

 however, is the fact that real cavities may not collapse as spheres. Ample evidence, 

 both theoretical and experimental, indicates that collapsing cavities undergo radical 

 distortions in shape and may even break up into clouds of smaller bubbles. The 

 cavities which produce the noise represented in Figure 8 resemble those shown in 

 Figure 12 which is taken from Ref. [31]. A quantitative description of the motion and, 

 especially, of the development of the shock for any case other than that of spherical 

 symmetry would be very difficult, however. All that can be said is that, despite the 

 uncertainties concerning details, the main features of the sound generated by cavita- 

 tion seem to be accounted for. 



IV. Surface Disturbances 



The airborne sound which accompanies the splash made by a droplet or a 

 solid object falling onto the surface of water is well known. It is rather to be expected 

 that concomitant sounds exist in the water below the surface and, indeed, such is 

 the case. This is not to say that there is any resemblance or necessary relation between 

 the two sound fields — only that disturbances of the surface which make sounds in air 

 generally produce other sounds in the water. 



* Dr. T. Brooke Benjamin has indicated that theoretical work on a related question, 

 the development of the pressures about a collapsing cavity containing a significant amount 

 of gas, is underway at King's College, Cambridge. Results indicate that a shock will develop 

 in the vicinity of a gas-filled collapsing cavity if the pressure at maximum compression exceeds 

 a value of about 2000 atmospheres. This pressure is slightly greater than that reached at the 

 first collapse of the gas bubble formed by an underwater explosion. 



257 



