Chapter 4 — PHYSICS OF SOUND 



and other fresh water sources, the salinity 

 values may fall to levels approaching zero. 

 The speed of sound increases about 4 feet per 

 second for each part per thousand increase 

 in salinity. Salinity has a lesser effect on the 

 speed of sound than does temperature, but its 

 effect is greater than that of pressure. 



Composite 



Figure 4-14 shows reasonably normal curves 

 for temperature, salinity, and pressure as a 

 function of depth in the Pacific Ocean and also 

 the resulting velocity structure. It should be 

 noted that the salinity variation plays a minor 

 part in the form of the depth-velocity curve. 

 Thus effect is almost entirely evident in the 

 first 500 feet below the surface. The tempera- 

 ture curve also shows wide valuations in the top 

 500 feet. From 2000 feet, downward, the tempera- 

 ture is nearly uniform as the water approaches 

 the maximum density point at about 40°F. The 

 pressure effect is represented by a straight line 

 as the velocity increases linearly with depth. 



On the composite curve, it easily can be 

 seen that the velocity in the top 2000 feet is a 

 somewhat skewed replica of the temperatui'e 

 curve. Below 2000 feet it follows closely the 

 straight line gradient of the pressure curve. 



DEPTH AND TEMPERATURE 



Except at the mouths of great rivers, where 

 salinity may be a determinant, the path followed 

 by sound is governed by the water temperature 

 and the pressui'e effect of depth. 



The pressure effect is always present and 

 always acts in the same manner, tending to 

 bend the sound upward. Figure 4-15 illustrates 

 the situation when the temperature does not 

 change with depth. Even though the temperature 

 does not change, the speed of sound increases 

 with depth, due entirely to the effect of pressure, 

 and the sound bends upward. 



Fig-ure 4-16 shows what happens when tem- 

 perature increases steadily with depth. When 

 the surface of the sea is cooler than layers 

 beneath it, the water has a positive thermal 

 gradient. Although this condition is unusual, 

 it does happen, and causes the sound to be 

 refracted sharply upward. In certain areas of 

 the Red Sea, between Africa and Arabia, tem- 

 peratures of well over 100°F have been re- 

 corded in depths exceeding 1 mile. Moreover, 

 the salinity of the water in those areas approaches 

 30 percent, compared to between 3 and 4 percent 

 in most ocean areas. 



When the sea grows cooler as the depth 

 increases, the water is said to have a negative 



71.29 71.30 



Figure 4-15. -Pressure tends to bend the sound Figure 4-16.- Positive thermal gradient tends 

 beam upward. to bend the sound wave upward. 



47 



