INTRODUCTION TO SONAR 



Direct Path 



Theoretically, if the sound waves were not 

 affected by velocity gradients, all the sound 

 waves would be straight lines, and would travel 

 in a direct path at whatever angle they left the 

 source. In actual practice, however, the sound 

 beam follows a path, or paths, as determined 

 by the sea condition at the time. 



VELOCITY- 



RANGE- 



30 MILES 



Sound Channel 



Figure 4-21 illustrates a combination of 

 two gradients of equal slope, one negative and 

 one positive. Their junction is a point of mini- 

 mum velocity. If a sound source transmits at 

 this depth of minimum velocity, all of the sound 

 beams that start in an upward direction will be 

 bent back down, and those that start downward 

 will be bent back up. When such a condition 

 occurs, we have what is called a sound channel. 

 The depth of minimum velocity is called the 

 axis of the channel. In this symmetrical situa- 

 tion, a beam that starts out downward will rise 

 as high above the channel axis as it went below 

 it, and then will be bent downward again. Sound 

 will remain in the channel as far as the channel 

 exists, and will suffer very little loss as it 

 progresses through the channel. 



Sound channels are a rarity in shallow water 

 (under 100 fathoms), but are always present 

 in the deep water areas of the world. The depth 

 of the axis of the channel is about 350 fathoms 

 in the central Pacific and somewhat over 500 

 fathoms in the Atlantic. In the polar regions, 

 where the surface water is materially colder, 

 the axis of the channel lies nearer the surface. 



71.34 

 Figure 4-21. — Strong negative and strong positive 

 gradients forming a sound channel. 



51.8 

 Figure 4-22. — Convergence zone. 



Convergence Zone 



Another effect that is closely related to the 

 deep sound channel is called the convergence 

 zone effect. The convergence zone effect is 

 now applied to long-range active sonar, but this 

 effect long has been used by submarine sonar 

 operators. If the sound source is placed near 

 the surface instead of near the axis of the 

 sound channel, the path followed by the sound 

 beam looks somewhat like that in figure 4-22. 



Sound energy from a shallow source travels 

 downward in deep water. At a depth of several 

 thousand feet, the signal is refracted due to 

 pressure, and returns to the surface at a range 

 of about 30 miles. The surface zone is from 

 3 to 5 miles wide. 



The sound that reaches the surface in the 

 first convergence zone is reflected or refracted 

 at the sui'face, and goes through the same 

 pattern again. It produces a zone approximately 

 6 miles wide at 60 miles, and another 9 miles 

 wide at about 90 miles. Experienced Sonar 

 Technicians are familiar with this convergence 

 zone effect. Often they have picked up strong 

 noise signals from targets that appear suddenly, 

 show up strongly for a few minutes, and then 

 disappear. 



A surface or near- surface contact detected 

 in the first convergence zone will have about 

 the same signal strength as a target detected 

 at 3 miles when no zone is present. Minimum 

 depth required along the path of the sound 

 beam is about 1000 fathoms. The usual re- 

 quirement is 2000 fathoms for conducting con- 

 vergence zone searches. The minimum depth 

 required is related to the surface velocity of 

 the sound, and increases as the velocity in- 

 creases. 



50 



