623 HYDROGRAPHIC MANUAL PaGE 566 



from a point source will therefore travel with equal intensity in all directions in a homo- 

 geneous medium. The wave front is a spherical surface of continuously increasing 

 area, in which the intensity of the sound varies inversely with the increase in surface 

 area of the sphere. This diminution of energy per unit of area of wave front is said to 

 be due to spreading. In a bounded medium a sound wave will not travel far before its 

 spherical wave front encounters the boundaries of the medium and is reflected, or be- 

 fore the direction of propagation is changed by refraction. However, even after reflec- 

 tion and refraction the intensity still varies inversely with the square of the distance 

 of travel from the assumed point source. 



623. Fropagation in a Heterogeneous Water Medium 



The propagation of sound in an ideal medium with uniform physical characteristics 

 as discussed in 622, is sometimes encountered in subaqueous sound ranging but, un- 

 fortunately, the medium is usually heterogeneous in most ocean areas, with some of the 

 physical characteristics of the water medium varying in the distances through which 

 sound must be transmitted. In such a water medium with constantly changing 

 physical characteristics, the propagation of sound is indeed complicated. 



In a water medium with a heterogeneous temperature condition, such that the 

 temperature varies from place to place and from surface to bottom, the velocity of 

 sound will be similarly nonuniform (see 632 and table 32 in 9611). This variation of 

 velocity with temperature causes the path of the refracted sound wave to be different 

 from the path in a homogeneous medium, adding greater complexity to the nature of 

 propagation. The attenuation of sound due to viscosity differs only slightly, however, 

 at different temperatures— it is slightly less at a high temperature than at a low tem- 

 perature because warm water is slightly less viscous than colder water. 



Moreover, a variation of salinity in the water medium, either from place to place 

 or at different depths, causes a slight change in the velocity. With an increase in sa- 

 linity of 1 part in 1,000 parts of water, by weight, the velocity increases only 1.3 meters 

 per second on the average, the increase varying slightly with the average temperature 

 of the medium. Normally this change in velocity is so small that it causes only a slight 

 refraction of sound as compared to the refraction caused by the change in velocity due 

 to temperature. (See 63.) An increase in salinity increases the density of a water 

 medium and because of the increased density and the consequent increase in velocity 

 due to density, there is a very slight decrease in attenuation. 



All sea water contains some suspended materials, but only where large rivers dis- 

 charge into the sea is the amount of suspended material enough to affect the velocity 

 of sound, and then only slightly. But the attenuation factor is increased in propor- 

 tion to the amount of foreign matter in suspension. 



Most sea water contains various proportions of dissolved gases caused by aeration 

 and the photosynthesis of marine organisms. Gas, if present in suspension in sufficient 

 quantity, reduces the normal velocity of sound by a large amount. It has been stated 

 that if the proportion is only 1 part in 10,000 parts of water by volume the velocity will 

 be reduced 40 percent. This reduction of velocity will cause greatly increased refrac- 

 tion of sound and will also cause reflections within the medium. In addition, aeration 

 causes increased attenuation because of conversion of acoustic energy into heat and 

 because of diffuse reflection and refraction. 



Small variations in the average depth of water apparently have very little effect 

 on the propagation of sound, but if the source of sound is located in deep water and the 



