Chapter 9 



TRANSMISSION OF EXPLOSIVE SOUND IN THE SEA 



9.1 



INTRODUCTION 



HAVING ESTABLISHED in Chapter 8 the nature of 

 explosions as sources of sound, we are ready to 

 consider the results of experiments showing how 

 pulses of explosive sound are affected by transmission 

 through moderate and long distances in the ocean. 

 The amount of experimental data available in this 

 field is scanty by comparison with that which has 

 been accumulated on sinusoidal soimd, and since ac- 

 curate recording of explosive pulses is possible only 

 if very careful precautions are taken, one must be 

 cautious in drawing conclusions from this work. 

 Nevertheless, these experiments demonstrate strik- 

 ingly the utility of explosive sound as an aid to funda- 

 mental research on the nature of the ocean as an 

 acoustic medium. Whereas in experiments using long 

 pulses of sinusoidal sound the signal received at the 

 hydrophone is usually inextricably compounded out 

 of directly transmitted sound, scattered sound, and 

 sovmd reflected from the surface or bottom, the ex- 

 tremely short duration of explosive pulses makes it 

 possible, in many cases, to distinguish between the 

 contributions of these different mechanisms by virtue 

 of the differences in time of arrival. Another charac- 

 teristic difference between explosive and sinusoidal 

 sound is that dispersion effects, which depend upon 

 the phases of the various component frequencies in 

 the arriving sound, can easily be studied with a 

 transient disturbance, but are practically impossible 

 to measure with single-frequency somid. The disper- 

 sion accompanying the transmission of sound through 

 sea water alone, although doubtless present, is very 

 minute, and has never been detected; in shallow- 

 water transmission, on the other hand, dispersion 

 phenomena are important and can be made to give 

 useful information about the bottom. Other ad- 

 vantages of explosive sound which are significant in 

 certain types of experiments include the high in- 

 tensity attainable and the fact that explosive sources 

 are relatively easy to manipulate and can be fired 

 at great depths. 



The experiments to be discussed in this chapter 

 shed light on a variety of problems of sound propa- 

 gation in the ocean. For example, the variation from 

 shot to shot in the sound intensity received at a dis- 

 tance is found in Section 9.2.5 to be much smaller 

 than that which is observed for sinusoidal sound, 

 especially when the path of the sound lies entirely in 

 isothermal water. This suggests that most of the 

 variation observed with sinusoidal sound is due to 

 some sort of interference phenomenon. Another ex- 

 ample is provided by the estimates of attenuation of 

 low-frequency sound, or at least of an upper bound 

 to it, given in Section 9.3.2; these estimates are made 

 on the basis of experiments which include ranges up 

 to hundreds of miles. Other interesting results of ex- 

 periments with explosive sound up to the present in- 

 clude the occurrence of a reflection coefficient near 

 unity for the free surface of the ocean at directions 

 of incidence surprisingly close to the horizontal (Sec- 

 tion 9.2.1), the comparison of observed intensities 

 with the predictions of ray theory (Section 9.2.2), the 

 occurrence of diffraction (Section 9.2.3), the deduc- 

 tion of details of the bottom strata from shallow- 

 water experiments (Section 9.4), etc. 



Before discussing the experimental material in de- 

 tail it will be worth while to say a few words regard- 

 ing the technique of measuring and recording ex- 

 plosive sounds. A systematic discussion of experi- 

 mental techniques would be beyond the scope of this 

 volume; but to enable the reader to form a balanced 

 opinion on past and future experiments with ex- 

 plosive sound, some of the pitfalls and stumbling 

 blocks in this field should be pointed out. In the first 

 place, if actual pressure-time curves are to be re- 

 corded, special attention has to be given to uni- 

 formity of the response of the hydrophone and re- 

 cording circuit, both in amplitude and in phase, over 

 a wide range of frequencies. Because of the very 

 short time scale when small charges are used, trouble 

 may be caused by the finite time of transit of the 

 sound wave across the hydrophone, and by diffrac- 

 tion around the hydrophone and its supports. Changes 



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