Chapter 4 — PHYSICS OF SOUND 



\ 



"\X 



REVERBERATION- 14,1 kHj <^' 

 SUB ECHO- WOkHz '^• 



DOPPLER DOWN 



/ 



REVERBERATION- I3.9KH2 



SUB ECHO-13 ekHz 



DOPPLER DOWN 



-•—UNDERWAY 

 OUTGOING PING- I^O kHz 



71.42 

 Figure 4-28.— Doppler down. 



6 KNOTS 

 NO DOPPLER 



OTS 

 MODERATE 

 .^♦^- DOWN 



NOTS 



DERATE DOWN 



•^/ Z KNOTS 

 ^1'^ SLIGHT UP 



NOTS 

 ERATE UP 

 ///// ' 

 '6 KNOTS 

 OR ABOVE 

 MARKED UP 



DOPPLER REPORTS 



To summarize what you have just learned 

 about doppler: Doppler is the difference in 

 pitch between the reverberations and the echo. 

 When the echo is higher than the reverbera- 

 tions, you will sing out "Doppler up". If it's 

 the same as the reverberations, report "No 

 doppler." If the echo is lower than the re- 

 verberations, say "Doppler down." 



When you become a really skilled operator, 

 you Avill be able to give the degree of doppler, 

 that is, how high or low it is. For instance, 

 a submarine coming directly toward your ship 

 at 6 knots returns a lilgher echo than a sub- 

 marine coming directly toward the ship at 2 

 knots. Also, a submarine coming directly toward 

 the ship at 6 Icnots returns a higher echo than a 

 6-knot submarine that is heading only slightly 

 toward the ship. 



The importance of prompt and accurate re- 

 ports cannot be overemphasized. Two details 

 count: how much speed the submarine is making, 

 plus how much it is heading toward or away 

 from the ship. The combination of these two 

 considerations is spoken of as the component 

 of the submarine's speed toward or away from 

 the ship. If the submarine's speed component 

 is more than 6 knots, report "Doppler marked, 

 up (or down)." When it is 3 to 6 ioiots, in- 

 clusive, report "Doppler moderate, up (or down)." 

 If less than 3 knots, then call it "Doppler 

 slight, up (or down)." (See fig, 4-29,) 



Figure 4-29. 



71,43 

 Doppler conditions. 



You now realize how important it is to give 

 an accurate report of doppler to the conning 

 officer. It tells whether the submarine is coming 

 toward or going away from your ship, and 

 gives tlie component of the submarine's motion. 

 Remember that any contact that does have doppler 

 must be moving in the water; if it is not a 

 surface ship, chances are that it is a sub- 

 marine or a large fish. Figure 4-30 gives an 

 idea of the value of doppler. 



HOW TO RECOGNIZE ECHOES 



Sonar Technicians learn to recognize one 

 echo from another by listening to recordings 

 of reverberations and echoes and by practicing 

 with the sonar aboard sliip. The following topics 

 contain remai'ks intended to serve as helpful 

 hints, but they cannot take the place of actual 

 practice, 



TWO TYPES OF ECHOES 



Echoes are of two types: those with doppler 

 and those without doppler. If an echo has doppler, 

 you can assume that it is a submarine or a 

 large fish or mammal — perhaps a whale. 



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