39 



1. INThODUCTI ON 



During the last generation or two it has been noticed 

 from time to time by different observers that when a charge of 

 explosive is set off under water, the sound received by a hydro- 

 phone some distance away consists of a number of sharp "pokes", 

 which decrease gradually in amplitude and become more and more 

 closely spaced in time. The intervals between the pokes are 

 greater the greater the charge of explosive and the smaller the 

 depth of the charge below the surface; at shallow to moderate 

 depths the intervals which have been observed range from one or 

 a few hundredths of a second for detonating caps to around one 

 second for charges of several hundred pounds of explosive. It 

 'has been convincingly shown oy Willis that the periodic 

 phenomenon involved is one of radial pulsations of the gas bubble 

 produced by the explosion. i^'urther investigations of this pulsa- 

 tion phenomenon have shown that its interest is not limited to 

 the field of the acoustics of explosive sound: it appears es- 

 sential to a detailed understanding of the "plumes" of water 

 thrown up over an underwater explosion; in some cases it may have 

 an i.nporcant effect on the damage inflicted by an underwater 

 explosion on neighboring structures; it may provide a useful re- 

 search tool in the experimental study of explosions and other 

 problems in hydrodynamics. Moreover, bubbles formed by explosions 

 are not the only ones which may be made to undergo radial pulsa- 

 tions of large amplitude: an example is the collapse of cavitation 



Sefi tVift article by Willis 5 n this Volume, 



