INTRODUCTION TO SONAR 



NO DOPPLER 



OOPPLER DOWN 



Figure 4-30. 



71.44 

 Value of doppler. 



If the echo has no doppler, it is from the 

 beam of a submarine, from a stopped sub- 

 marine, from the submarine's wake, or it is 

 not from a submarine. If an echo without doppler 

 is from the beam of a submarine, it will be 

 sharp and clear, because the side of a sub- 

 marine presents a large reflecting surface. 



Sharp, loud echoes, without doppler, are 

 received from any large, solid surface. The 

 larger the area of the target, the greater the 

 strength of the echo. 



A vreak, mushy echo, without doppler, is 

 heard from such objects as riptides, kelp beds, 

 schools of small fish, and wakes that are breaking 

 up. The weak, mushy sound is caused by the 

 numerous small reflections that combine in 

 random fasliion to form an irregular echo wave. 



Often, you will receive the worst type of 

 echoes from the stern of a submarine. The 

 target area is small and is broken into multiple 

 surfaces by the screws, the wake, and the like, 

 so that there is a combination of reduced power 

 and interference. Don't dismiss weak, poor- 

 sounding echoes as nonsubmarine. Experienced 

 sonar operators will tell you that although an 

 echo from dead astern of the target is hardest 

 for an operator to identify, it can be done 

 with a little perseverance. 



The strength of the echo depends on the 

 power of the incoming wave. This power depends 

 on the sound output of the transducer, the size. 



position, and movement of the target, and the 

 conditions of sound transmission in the sea. 



You probably will hear a lot of different 

 sounds while echo ranging. A stationary object 

 near the surface produces good echoes, but 

 there will be no doppler. (Once in a while, a 

 strong current or tide will cause an echo to 

 have a doppler effect, but such an occurrence 

 is unusual.) A whale may have echo character- 

 istics closely resembling those of a submarine. 

 Echoes also may be heard from large fish, 

 such as blackfish. Schools of herring or other 

 small fish sometimes run large enough to re- 

 flect a strong echo. In shallow water, as along 

 the Continental Shelf on the eastern coast of 

 the United States, hulls of long- sunken ships 

 may retvirn a very convincing echo. Because 

 Nazi U-boats were so effective in this area 

 during World War II, the shelf is littered with 

 dead ships capable of hoaxing the unsuspecting 

 sonar operator into believing his contact is 

 actually a submarine. 



Often you will hear strange chuckling and 

 whistling sounds in an area abounding in por- 

 poises or dolphins. If the water is shallow and 

 the bottom is hard, rocky, or covered with 

 coral heads, sound waves are reflected strongly. 

 Often these reflections return as loud and clear 

 as echoes from a target, but they have no 

 doppler. 



From time to time, you may pick up strange 

 sounds that are difficult to identify. Submarine 

 hydi'oplanes and rudders frequently give off a 

 noticeable whistling sound. Submarine pumps 

 and other maciiinery can be heard rattling or 

 thumping if the macliines are not silenced. You 

 also can hear a torpedo. 



Torpedo noise is distinct from all others. 

 When a torpedo is fired from a submarine, the 

 first indication might be the sound of escaping 

 air, quicldy followed by a sound like a propeller 

 noise. The rhythm is too rapid for the beats 

 to be counted (as you can count those of a ship's 

 propeller), and it increases swiftly in intensity 

 as it approaches, resembling the whine of a 

 jet engine, but not as high-pitched. 



ARTIFICIAL TARGETS 



Submarines are capable of putting artificial 

 targets into the water to confuse the tracking 

 ship. They are of two general types: targets 

 that are self-propelled, wliich can return an 

 echo with doppler; and targets that are sta- 

 tionary, providing excellent sound-reflective 

 qualities, but usually without doppler. Practice 



54 



