INTRODUCTION TO SONAR 



type, affording improved sensitivity and detection 

 range. 



At any instant of RDT, the signal is the 

 resultant of phased excitation of several con- 

 tiguous transducer staves, resulting in a source 

 level improvement that is characteristic of direc- 

 tional transmission. Excitation of the transducer 

 staves is caused by the output of a transmit 

 scanner, which operates much like the video 

 scanning switch of a conventional scanning sonar. 

 Transmission is accomplished in a sector (up 

 to 300° wide), which is centered on the bow or 

 on a selected bearing, depending on the type of 

 operation chosen by the operator at the control 

 unit. 



PASSIVE SONAR 



Passive sonar, as its name implies, depends 

 entirely on the target's noise as the sound 

 source instead of the returned echoes of a trans- 

 mitted signal. Target detection is achieved at 

 great ranges through the use of highly sensitive 

 hydrophones. 



Although passive sonar is usually associated 

 with submarines, many surface ships now have 

 a passive system that utilizes their active sonar 

 transducers. During the interval between sound 

 pulse transmissions, the transducer acts as a 

 hydrophone, allowing the sonar operator to moni- 

 tor a broader frequency spectrum than normally 

 is possible. Passive reception does not interfere 

 with the reception of pulse echoes. During trans- 

 mission, however, the passive feature is inter- 

 rupted. 



Hydrophones 



A hydrophone is a device used to listen for 

 underwater sounds. In operation it is similar 

 to the transducer of active sonar equipment 

 when converting sound energy to electrical energy. 

 Two general types of hydrophones may be em- 

 ployed— electrostrictive and magnetostrictive. 

 Modern hydrophones are of the electrostrictive 

 type, consisting of ceramic elemsnts that operate 

 on the piezoelectric principle. When the elements 

 are struck by a sound wave, the vibrations set 

 up are converted to an electrical signal, ampli- 

 fied, and displayed at the operating console. 



Single Line Hydrophone 



Knowledge of the single line type of hydro- 

 phone, although it is not in general use today, 

 will aid you in understanding how modern hydro- 

 phones operate. 



SOURCE 



\ = \ = 



SOUND SOURCE 



SOUND SOURCE 



71.54 

 Figure 6-8. — Operation of the RLI. 



A typical early type uses the magnetostrictive 

 effect for sound detection. The hydrophone, which 

 is trainable through 360°, is a line type whose 

 nickel tubes are arranged horizontally in a line. 

 It is divided electrically into right and left halves. 



Weak signals from the hydrophone are fed 

 to an audio amplifier, then to bandpass filters 

 that remove undesired frequencies from the 



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