12. SOUND IN THE SEA 



P. ViGOUREUx and J. B. Hersey 



1. The Nature of Sound 



Sound is produced by changes of jDressure in elastic media. If, for instance, 

 the pressure at a point in a large volume of water is momentarily increased by 

 the explosion of a small detonator, the pressure change is communicated in all 

 directions giving rise to a spherical wave travelling with a velocity which 

 depends on the elasticity and the density of the water ; an observer in the water 

 can hear the disturbance as the wave passes him. 



Strictly speaking the word "sound" is restricted to those components of a 

 disturbance which are audible. The slow pressure changes due to tides or to sea 

 waves, for example, are not referred to as sound. Also waves that have travelled 

 long distances from earthquakes or heavy explosions contain frequencies which 

 lie partly below and partly overlapping the frequency range of sound. These 

 are treated elsewhere in this text. But it will be convenient here to include 

 under "sound" all elastic disturbances of frequency higher than, say, 20 c/s. 

 True, the upper limit of audibility is some 20 kc/s and higher frequencies can 

 be "heard" only by frequency changing, but the similarity of the methods of 

 generation and of the laws of propagation makes it profitable to treat the whole 

 range together. The upper limit is in any case fixed by Nature, for the absorp- 

 tion increases so much with frequency that even if sound energy of thousands 

 of megacycles per second could be generated, it would be absorbed by the water 

 before reaching the detector. 



The change of pressure due to the wave at any point is called excess pressure 

 or acoustic pressure ; it is accompanied by movement of the particles of water 

 at the point, and the velocity of this motion is called particle velocity. 



The treatment of sound propagation is greatly simplified if it is assumed that 

 the excess pressure is small compared with the static pressure. This condition 

 is always fulfilled at great distances from even relatively powerful sources like 

 explosive chemicals. At close range, however, the assumption would be in- 

 correct and a more complicated treatment is necessary. 



2. Propagation of Sound in Water 



A. The Wave Equation 



The relation between acoustic pressure, time and position in a homogeneous 

 elastic medium is obtained from the wave equation, 



</; = C2 V2(/,, (1) 



where c is the velocity of propagation and ^ is a velocity potential from which 

 the acoustic pressure p and the particle velocity v are given by 



p = p^ (2) 



V = -V<f>, (3) 



where p is the density. 



[MS received July, 1960] 476 



