Figure 7 2. Transient cavities in the vortex sheet behind a sharp-edged disk. The water flows from 



left to right. (From Ref. 31.) 



Many other kinds of surface disturbances, natural and artificial, result in under- 

 water sounds. In this section, we shall devote our attention principally to sounds made 

 by impinging solid or liquid objects. One reason for this limitation is that scientific 

 results available to us are concerned principally with such disturbances. Actually, how- 

 ever, the main features of sound production by various kinds of surface disturbances 

 are the same. For example, it will be shown that the underwater sound of sprays 

 and of rainfall is explained simply in terms of the sound produced by the impinge- 

 ment of a solitary droplet. Similarly the underwater sounds of breaking waves can 

 be at least partially understood in the same light. 



The study of splashes, cavities, and other visible aspects of water-entry 

 phenomena has occupied the attention of a great many researchers, of whom a num- 

 ber have elucidated the object of their studies by some remarkable photographs. Special 

 mention is due to the study made more than fifty years ago by Worthington [32]. He 

 produced extensive series of photographs showing consecutive stages in the motion of 

 the surface of the liquid during splashes produced by the impingement of liquid 

 droplets and spherical pellets. In 1918, Mallock [33] studied similar splashes and 

 adverted to the existence of cavities and to the relation of the size and shape of the 

 latter to the characteristic pitch of the sounds heard in air. In recent years, a number 

 of researchers have studied various aspects of water-entry phenomena in connection 

 with the behavior of missiles, usually without reference to the sound. The photographs, 

 Figure 13, were obtained by May [34] and show the cavity and the splash produced 

 by the vertical entry of a steel sphere. Recently, Richardson [35] has called attention 



258 



