INTRODUCTION TO SONAR 



CONTROLLING FACTORS 



The speed of sound wave travel through the 

 water is controlled by three conditions of the 

 sea. They are: temperature, which takes the 

 form of slopes and gradients; pressure, caused 

 by increased depth; and salinity, or the salt 

 content of the water. 



Temperature 



With presently operational sonars, tempera- 

 ture is by far the most important of the factors 

 affecting the speed of sound in water. Depending 

 on the temperature, the speed of sound increases 

 with increasing temperature at the rate of 4 to 8 

 feet per second per degree of change. Inasmuch 

 as the temperature of the sea varies from 

 freezing in the polar seas to more than 85°F in 

 the tropics, and may decrease by more than 

 30°F from the surface to a depth of 450 feet, 

 it Is clear that temperature has a great effect 

 on the speed of sound. Remember: The speed 

 of sound increases when the temperature of 

 water increases. 



Pressure 



Sound travels faster in water under pres- 

 sure. Pressure increases as depth increases, 

 so the deeper a sound wave travels, the faster 

 it travels. Pressure effect on transmitted sound, 

 although rather small in comparison to tempera- 

 ture effects, cannot be neglected. The speed of 

 sound increases about 2 feet per second each 

 100 feet of depth. 



Salinity 



Sea water has a high nalneral content. Salt 

 content is spoken of as the salinity of the water. 

 The weiglit of the higher density sea water is 

 about 64 pounds per cubic foot; that of fresh 

 water is about 62.4 pounds per cubic foot. 

 This variation is the result of the salt content 

 in the sea water. 



The overall effect of increasing the salinity 

 of water is to increase the speed of the sound, 

 which means that when sound passes through 

 water that varies in salinity, it travels faster 

 in the saltier water. In the open ocean, the 

 values of salinity normally lie between 30 and 

 35 parts per thousand. In the region of rivers 



PRESSURE 



(PSD 

 2500 5000 



SALINITY 



(0/00) 

 33 34 35 



TEMPERATURE 



( = F) 



10,000 



142 146 150 297 404.5 512 4800 4900 5000 

 VELOCITY CHANGE VELOCITY 



(FT/SEC) (FT/SEC) 



35.5(71) 

 Figure 4-14. — Normal curves for pressure, salinity, and temperature. 



46 



