CHAPTER 6 



PRINCIPLES OF SONAR 



The earliest sonar equipment was a passive 

 device, a simple hydrophone lowered into the 

 water and used to listen for noise created by a 

 submarine. The only indication of a target was 

 an audio tone. Bearing accuracy was doubtful, 

 and ranges were strictly guessworlc. 



Today's sonar equipments provide highly 

 accurate ranges and bearings. They present 

 information both visually and aurally. Both active 

 and passive types of equipment are used. 



Although specific sonars and related equip- 

 ments (except a fathometer and a tape recorder) 

 are not covered in this chapter, basic principles 

 of operation given herein are applicable to 

 all sonar equipments. 



SONAR SYSTEMS 



Two general types of sonar systems are 

 employed for the detection of targets. They are 

 referred to as active and passive sonars. 



The active type of sonar is capable of trans- 

 mitting underwater sound pulses that strike 

 targets and are returned in the form of echoes. 

 Echoes returned indicate the range and bearing 

 of the target. 



Passive sonars do not transmit sound. They 

 merely listen for sounds produced by the target 

 in order to obtain accurate bearing and estimated 

 range information. 



Active sonar systems normally are associated 

 with surface ships, whereas passive systems 

 usually are associated with submarines. Newer 

 surface ships, however, employ separate passive 

 systems in addition to their active sonars. Sub- 

 marines, although still relying primarily on 

 passive systems, also employ active sonars. 



ACTIVE SONAR 



A simplified active sonar system is shown 

 in figure 6-1. In this set the sonar transmitter 

 consists of a high-frequency audio oscillator and 



SONAR 



CONTROL 



INDICATOR 



RECEIVER 



TRANSMITTER 



TRANSDUCER 



71.47 



Figure 6-1. — Simplified active sonar system. 



an amplifier. The transmitter feeds a short, 

 powerful pulse to the transducer for transmission 

 into the water. The signal is transmitted in 

 360° of azimuth, 



A transducer is a device used to convert one 

 form of energy into a different form. In a sonar 

 set, the transducer converts electrical energy 

 into acoustical energy and reconverts the sound 

 echoes to an electrical signal, thus acting as 

 both a loudspeaker and a microphone. 



Just as important to the sonar system as the 

 transmitter and transducer is the receiver. It 

 functions on the superheterodyne principle. In 

 this unit the small, high-frequency electrical 



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