Chapter 8 



EXPLOSIONS AS SOURCES OF SOUND 



8.1 



INTRODUCTION 



EXPLOSIVE SOUND differs from sinusoidal sound 

 both in the intensity which can be achieved with 

 it and in the fact that it consists of one or more pulses 

 of extremely short duration. These characteristics 

 have prompted many suggestions for the employ- 

 ment of explosive sound in communication and echo 

 ranging, few of which, however, have so far been 

 utilized in practice. The survey given in this chapter 

 and the next of what is known about explosive sound 

 is partly designed to facilitate an understanding of 

 the possibilities and limitations of explosive sound 

 in such applications. The study of explosive sound 

 can be useful in another way, however, in that it can 

 supply valuable additions to our information about 

 the nature of the sea and its bottom, and about the 

 causes of many of the phenomena observed in sound 

 transmission. The possibilities of explosive sound as 

 a research tool have accordingly been kept in mind in 

 the selection of material for these chapters. 



To understand the complex phenomena which ac- 

 company the propagation of explosive sound in the 

 sea one ought to begin by finding out as precisely as 

 possible just what the explosive disturbance is like, 

 originally, before it has been propagated to any ap- 

 preciable distance. Fortunately, much has been 

 learned about explosions and the pressure disturb- 

 ances which they create in the water near them. A 

 detailed survey of what is known about underwater 

 explosions would require a volume in itself ; however, 

 an effort will be made in this chapter to summarize 

 briefly those parts of our knowledge of underwater 

 explosions which have a bearing on the use of ex- 

 plosions as sources of sound. In this chapter, there- 

 fore, we shall be concerned with the disturbance at 

 comparatively short ranges from the explosion, where 

 its characteristics are presumably little affected by 

 the departures of sea water from the concept of a 

 pure homogeneous fluid. Most of the information in 

 this field has been obtained in the course of experi- 

 ments directed toward the elucidation of the damag- 



ing effects due to explosions. With this information 

 as background, we shall be able, in Chapter 9, to dis- 

 cuss the propagation of explosive sound through siz- 

 able distances in the sea where departure of the 

 medium from homogeneity, effects of the bottom, 

 and other factors are important. 



8.2 SEQUENCE OF EVENTS IN 



UNDERWATER EXPLOSIONS 



An explosion is a process by which, in an extremely 

 short space of time, a quantity of "explosive" ma- 

 terial is converted into gas at very high temperature 

 and pressure. This conversion is due to a chemical 

 reaction which converts the explosive material from 

 a thermod3mamically unstable state to a more stable 

 one with the evolution of a great amount of heat. 

 This reaction, when initiated at one point of a mass 

 of explosive, propagates itself rapidly until all the 

 mass is involved. The propagation may take place 

 in either of two ways, called respectively burning and 

 detonation. In burning, the contact of the hot gaseous 

 products of the reaction with the untransformed por- 

 tion causes a reaction to take place at the surface of 

 the latter, the rate of transformation being slow- 

 enough so that the boundary between transformed 

 and untransformed material advances with a speed 

 slower than the speed with which the pressure gener- 

 ated by the reaction is propagated through the mass. 

 In detonation, on the other hand, the reaction takes 

 place so rapidly that it can keep up with the pressure 

 wave, which in this case is known as a detonation 

 wave. These two processes, which will be discussed 

 more fully later, permit explosive materials to be 

 divided into two rather well-defined classes: ex- 

 plosives which detonate, commonly called high ex- 

 plosives, and explosives which merely burn, for which 

 we shall use the term propellants since the most im- 

 portant explosives of this type are used to propel 

 projectiles from guns, or as rocket fuels. A given 

 quantity of high explosive will radiate considerably 

 more acoustic energy when it is set off than will a like 



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