I. THE UNDERWATER SPARK SOUND SOURCE 



The theory of the underwater spark as a sound source 

 has been considered for some time without conclusive 

 findings as to its value and limitations, or the optimum 

 design of the equipments involved. The principle of this 

 sound source may be stated briefly as follows: an underwater 

 electrical discharge causes a sharp increase in temperature 

 of the water between the electrodes. The water vaporizes 

 and forms a gas bubble which expands and collapses, and 

 radiates acoustic waves. The nature of the radiation is 

 dependent upon a number of factors. The following sections 

 discuss some of the major theoretical and mechanical 

 considerations involved, and the development and testing 

 of a workable model of the underwater spark sound source. 



DESIGN CONSIDERATIONS 



Several years of experiment made it apparent that much 

 had to be learned about the development and construction of 

 suitable electrodes and gaps before a satisfactory underwater 

 spark sound source, producing high acoustic levels in the 

 lower frequency region, could be achieved. Experience at 

 NEL and other laboratories showed that there was consider- 

 able variability from one spark discharge to another. 



Construction of the Gap 



A basic fact about spark gaps, operated underwater, is 

 that the power into the load is equal to the current squared 

 times the resistance, or 



P = I^ R 



Therefore, it is necessary to make the resistance of the 

 gap as large as possible. When two electrodes are short- 

 circuited by water the resistance is low rather than high 

 as is desired. It is apparent that the gap should be as wide 

 as possible to raise this value. A m.ethod for getting high 

 power that at first seems effective is to use a wide gap and 

 very high voltages to cause the gap to break down. Another 

 way is to raise the gap resistance by using the best dielectric 



