63 HYDROGRAPHIC MANUAL PaGE 574 



temperature and salinity of the water at any depth are known, the velocity at that 

 depth may be found from tables. Velocities so determined are extensively used in 

 correcting echo soundings and are occasionally used in R.A.R. 



The velocity of sound in any medium may be found from Newton's fundamental 

 equation which is: 



_, , . _ /Elasticity of the medium 

 ^~\ Density of the medium 



In the case of an extended medium, the bulk modulus of elasticity is used. This modu- 

 lus can be determined in a physical laboratory by measuring the change in volume pro- 

 duced by known forces. The condensations and rarefactions in a sound wave take 

 place so rapidly that there is little time for the heat developed in condensation, and 

 the cooling in rarefaction, to be transferred to the surrounding medium. The change is 

 not isothermal — it is adiabatic and involves the ratio of specific heat at constant pressure 

 to the specific heat at constant volume. Hence the velocity equation must be modified 

 to allow for this fact. 



The velocity of sound in sea water may be calculated from the temperature and 

 salinity of the water and the hydrostatic pressure. An increase in temperature increases 

 the elasticity but reduces the density; both of these changes increase the velocity, but 

 the adiabatic correction is reduced. Salinity also affects both terms of the equation, 

 an increase in salinity causing an increase in both the density and the elasticity, the 

 net effect being an increase in the velocity. Hydrostatic pressure also increases both 

 the elasticity and the density, and results in an increase in velocity. In addition to 

 increasing with depth, hydrostatic pressiu"e varies slightly with latitude because of 

 the change in gravity. However, corrections for latitude may be safely ignored in 

 R.A.R., since the maximum variation from the corrections given in table 34 is but 0.5 

 meter per second for a depth of 5,000 fathoms. 



The velocity of sound increases with changes in the physical characteristics of the 

 water by the following approximate percentages: 



(a) Each 1°C. increase in temperature causes an average increase in velocity of 0.2 percent. 



(b) Each 1.0 °/oo increase in salinity causes an average increase in velocity of 0.1 percent. 



(c) The increase in pressure for each 100 fathoms of depth increases the velocity 0.22 percent. 



Remembering that the velocity of sound in water is approximately 1,500 meters per 

 second at temperature 14° C, salinity 35.0 °/oo, and surface atmospheric pressure, the 

 above percentages may be used to compute approximate velocities for other values of 

 temperature, salinity, and depth. 



Temperature is the most important physical characteristic of sea water affecting 

 the velocity of sound (see 632). To compute the velocity accurately the vertical and 

 horizontal distribution of temperature in the water medium must be known, so the 

 temperatures must be measured in situ. The density must also be measured to find 

 the salinity, although it has relatively less effect on the velocity. If the depth of water 

 is known, the change in velocity due to pressure can be readily computed. 



An important fact to bear in mind is that the velocity of sound is greatly affected 

 by the presence of suspended particles, not in solution. Sea water always contains 

 such suspended matter, although in the open ocean it is encountered in such minute 

 quantities it may be ignored. At some places along the coast, particularly near the 

 mouths of large rivers, it is present in appreciable amounts, but its effect on the velocity 

 of sound is practically indeterminable, except by experimental tests. 



