Chapter 4 — PHYSICS OF SOUND 



REFRACTION 



If there were no temperature differences 

 in the sea, the sound wave would travel approxi- 

 mately in a straight line, because the speed of 

 sound would be roughly the same at all depths. 

 As indicated in figure 4-12, the sound would 

 spread and become weakened by attenuation at 

 a relatively constant rate. 



Unfortunately, however, the speed of sound 

 is not the same at all depths. The velocity of 

 sound in sea water increases from 4700 feet 

 per second to 5300 feet per second as the 

 temperature increases from 30° to 85°F. As 

 will be seen later in the chapter, salinity and 

 pressure also affect sound speed, but their 

 effects usually are small in relation to the 

 large effects commonly produced by tempera- 

 ture changes. Because of the varying tempera- 

 ture differences in the sea, the sound does not 

 travel in a straight line. Instead, it follows 

 curved paths, resulting in bending, splitting, 

 and distortion of the sound beam. 



When a beam of sound passes from one 

 medium in which its speed Is high (such as 

 warm water) into one in which its speed is 

 low (such as cool water), the beam is refracted 

 (bent), A sound beam bends away from levels 

 of lilgh temperature and high sound velocity, 

 and bends toward levels of low temperature 

 and low sound velocity. Figure 4-13 illustrates 

 the refraction of a sound beam. As a result of 

 refraction, the range at which a submarine can 



be detected by sound may be reduced to less 

 than 1000 yards. Tliis range may change sharply 

 with changing submarine depth. 



QUENCHING 



In strong winds and heavy seas, the roll 

 Emd pitch of the echo ranging sMp make it 

 difficult to keep the sound directed on the target. 

 Additionally, the turbulence produces air bubbles 

 in the water, weakening the sound waves. Occa- 

 sionally this envelope of air bubbles blankets 

 the sound emitted by the transducer. Sonar 

 operators can tell, by a dull thudding sound, 

 when the sound beam is being sent out into air. 

 This action is known as quenching. 



PROPAGATION OF SOUND 

 IN THE SEA 



We have discussed the various basic phe- 

 nomena that cause power loss in transmitting 

 sound: divergence, attenuation (absorption, and 

 scattering), reflections, reverberations, refrac- 

 tion (bending), and quenching. Now, we must 

 consider the structure of the sea as an acoustic 

 medium, and learn the effects of this structure 

 on the transmission of sound. 



Of the many conditions affecting sound wave 

 travel through the water, the following factors 

 influence its speed. 



MAXIMUM ECHO RANGE 



71.26 

 Figure 4-12. — Sound travel in water of constant 

 temperature. 



71.27 

 Figure 4-13. — A refracted sound beam. 



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