INTRODUCTION TO SONAR 



FREQUENCY (HERTZ) 

 500 1000 5000 10,000 



55 FT 

 100 FT 

 200 FT 



35.51 

 Figure 4-11. — Depth effect on cavitation noise. 



generators, gasoline pumps, blower motors, and 

 portable power tools. Any (or all) of the afore- 

 mentioned equipment may be operating at any 

 one time, adding greatly to the noise radiated 

 by the ship, and therefore entering the sonar 

 receiver. If the combination of these noises 

 becomes too great for effective sonar search, 

 ask your watch supervisor to request the OOD 

 to secure all nonessential equipment. Another 

 noise source is circuit noise, generated within 

 the sonar equipment itself. This noise may be 

 in the form of a 60-hertz hum, staticlike noises 

 from electric cables due to improper cable 

 shielding, or leakage currents from other sources 

 entering the receiver stages. Unlike ambient 

 noise, however, this type of generated noise may 

 be controlled. 



Marine Life 



Cavitation noise versus depth at a constant 

 speed (170 rpm) is shown in figure 4-11. The 

 upper curve is the same as the upper curve 

 in figure 4-10. As the submarine goes deeper, 

 the cavitation noise decreases and moves to 

 the higher frequency end of the spectrum in 

 much the same manner as though speed had 

 been decreased. This decrease in noise Is caused 

 by the Increased pressure with depth, which 

 tends to compress cavitation noise. The curves 

 are not exactly the same as those shown in 

 figure 4-10, but the similarity Is readily apparent. 



Machinery Noise 



Machinery noise is produced aboard ship by 

 the main propulsion machinery and by any or 

 all of a large number of auxiliary machines 

 that may or may not be connected with the 

 main propulsion system. Machinery noise usually 

 1 s produced by rotating or reciprocating machines . 

 Most noise of this type Is generated by dynamic 

 imbalance of the rotating portion of the machinery 

 that causes a vibration within the machine. Such 

 a vibration may then be transmitted through the 

 machine mounts to the hull, from where it is 

 radiated Into the water as acoustic energy. 



Shipboard Noise 



In addition to the machinery discussed so 

 far, there are many other shipboard noise sources 

 over which you probably will not have any 

 control. Some of the noise sources aire fire 

 and flushing pumps, air compressors, refrigera- 

 tion machinery, air conditioning systems, diesel 



A sonar operator may hear many strange 

 sounds during his time on watch. He may have 

 to listen to (and try to identify) the source of 

 whistles, shrieks, buzzes, pings, Icnocks, crack- 

 lings, and other weird noises not ordinarily 

 associated with the sea. Most of these noises 

 probably come from different species of fish, 

 but only a few have been identified positively. 

 Following are descriptions of a few marine 

 life noises. 



Porpoises give out a whistling sound and 

 sometimes a sound like a chuckle. (Submarine 

 hydroplanes and rudders sometimes also give 

 off a whistling sound.) These mammals are 

 found In all ocean areas of the world. 



Snapping shrimp are common around the 

 world between latitudes 45°N and 45°S, usually 

 in waters less than 30 fathoms deep. As you 

 approach a bed of snapping shrimp, you hear 

 a buzzing sound. As you go closer, the sound 

 resembles fat sizzling on a fire, then becomes 

 similar to that given off by burning brush. 



Whales, which are found In all oceans, give 

 off a variety of sounds. Including knocks, groans, 

 pings, and one resembling a swinging rusty gate. 

 The knocking sound of the sperm whale resembles 

 the noise of hammering. Sperm whales and other 

 large species seldom are heard in waters 

 shallower than 100 fathoms. Blackflsh, which 

 are similar to whales, emit a whistling sound 

 like a porpoise, but clearer in tone. 



The preceding examples of marine life noises 

 are just a few that you will encounter. Do not 

 let the strange sounds mislead or distract you. 

 A valid target may be just beyond the whale you 

 are listening to. Available tapes and films 



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