AEROGRAPHER'S MATE 3 & 2 



change in frequency of a signal resulting from 

 relative motion between the source and the 

 receiver is known as doppler effect. Figure 

 12-25 illustrates doppler effect. 



Each sound wave produced by the whistle 

 is given an extra "push" by the motion of the 

 train. As the train comes toward you, the 

 resultant effect is an increase in pitch, caused 

 by compression of the waves. As the train 

 moves away from you, the sound waves are 

 spread further apart, resulting in a lower pitch. 



Because doppler effect varies inversely with 

 the velocity of sound, the effect is much less 

 marked in the sea than it is in the air. 



LOW 



HIGH 



Figure 12-25. — Doppler effect. 



209.26 



APPLICATION OF SOUND 

 IN SONAR OPERATION 



One of the primary applications of knowl- 

 edge of sound characteristics is in SONAR 

 operations. SONAR is an acronym for Sound 

 Navigation and Ranging, Sound is used in SONAR 

 operations in one of two basic ways: active 

 ranging and passive ranging. 



In passive sonar operations, sound generated 

 by a submarine target is detected and evaluated 

 by an antisubmarine unit, such as a destroyer. 

 By applying the doppler principle, a sonar 



operator can determine if the submarine is 

 closing or moving away from the destroyer. 

 In active operation the destroyer sends out 

 a package of sound waves (pulses), which will 

 reflect or bounce from the submarine. By 

 knowing the direction the sound travels, the 

 sonarman can determine the target bearing. 

 A measure of the time required for the sound 

 pulse to reach the target and return gives an 

 indication of the range to the submarine. 



To effectively use SONAR for submarine 

 detection, the sonar operator must make maxi- 

 mum use of the various properties of sound 

 discussed previously. Any sound the sonar 

 detector hears that is not part of the sound 

 package from the target is interference. The 

 major source of interference in sonar 

 operations is background noise. 



Background noise is generated by various 

 sources and is classified by the general type 

 of source. Ambient noise is the noise that would 

 be present in the sea if neither the submarine 

 nor the destroyer were present. Ambient noise 

 includes sea surface noise caused by agitation 

 of th3 sea surface. Noise is created as water 

 falls from the crest of a wave back to the sea 

 surface. The intensity of sea surface noise 

 increases as wave height increases. Another 

 type of ambient noise is biological noise created 

 by marine life. Biological noise varies in 

 frequency from the high pitched sounds of the 

 porpoise to the low-frequency noise caused by 

 repeated clicking of claws in a shrimp bed. 



In addition to ambient noise, background 

 noise includes self noise. Self noise is noise 

 created by the ship on which the detector is 

 mounted. The flow of water along the hull of a 

 moving destroyer creates noise which inter- 

 feres with evaluation of the target signal. A 

 new ship with solid seams and tight bolts and 

 rivets would generate less self noise than an 

 older ship. 



288 



