APPLICATION OF UNDERWATER SOUND DEVICES 



pedoes, landing obstacles, and shoals by surface 

 craft, submarines, and swimmers. 



Listening Gear 



The second tactical application of sonar gear in- 

 volves listening, in which any supersonic or sonic 

 signal or noise emitted by the target and reaching the 

 hydrophone 3 is picked up and presented to the opera- 

 tor. If the incident signal is nol ol an audible fre- 

 quency to begin with, the resultant hydrophone out- 

 put may be heterodyned to an audible frequency. 

 Intensity, frequency, and direction of incidence of 

 the signal enable conclusions to be drawn regarding 

 the nature, speed, range, and bearing ol the target. In 

 particular, if a directional hydrophone 11 is used, the 

 bearing of the target may be determined from the 

 direction of orientation of the hydrophone axis at the 

 position of maximum response. 



The following specific tactical applications of lis- 

 tening may be listed: 



1. Sonic and supersonic listening for submarines 

 from antisubmarine craft, harbor installations, other 

 submarines, and expendable devices, sono buoys, etc. 1 

 Supersonic listening is perhaps preferable to sonic 

 listening in these cases because of the high directivity 

 generally obtainable with supersonic gear. 111 Super- 

 sonic hydrophones on antisubmarine craft, harbor 

 installations, and submarines can also pick up the 

 emitted echo-ranging pings of enemy submarines. 



2. Sonic and supersonic listening for surface craft 

 from submarines. Sonic and supersonic listening, 

 usually with more or less directional hydrophones, is 

 widely used by submarines to detect the noise output 

 of merchant craft in convoys, of antisubmarine craft, 

 and of other enemy warships. Submarines also often 

 overhear on supersonic listening gear the echo-rang- 

 ing pings of antisubmarine craft. 



3. Supersonic listening for torpedoes from surface 



craft and submarines. Cavitation noise from a moving 

 torpedo may be detected by directional supersonic 

 listening gear, and appropriate evasive action may be 

 taken. 



J Hydrophones used in practice are of a variety of construc- 

 tions, sizes, and shapes, but usually operate on the piezoelectric 

 or magnetostrictive principle. 



k The directional receiving hydrophone may be the projector 

 of the supersonic echo-ranging gear, hooked up electrically for 

 signal reception. 



1 A sono buoy is a device containing a hydrophone and a 

 radio transmitter. When the hydrophone receives a signal from 

 a target (the submarine), it is transmitted by means of a radio 

 link to patrolling antisubmarine air or surface craft. 



"' The high directivity is desirable for two reasons: it leads to 

 a greater bearing accuracy and minimizes the sell and ambient 

 noise pickup of the gear. 



2.3.: 



Nontactical Applications 



Nontactical applications of sonar gear include the 

 use of standard projectors and hydrophones for the 

 calibration of other kinds of sonic and supersonic de- 

 vices," the determination of physical parameters ol 

 sonar devices, and analysis of the physical parameters 

 ol a particular type of gear to determine its suitability 

 for the tactical or other purpose at hand. A somewhat 

 related nontactical application is the use of standard 

 projectors and hydrophones for testing echo-ranging 

 and listening gear installed on antisubmarine craft 

 and on submarines, and for permitting a submarine 

 to monitor its own sonic and supersonic output. 



Other nontactical applications of sonar gear in- 

 clude fathometer depth determination by surface 

 craft and by submarines. Fathometer gear, similar in 

 construction and operation to supersonic searchlight 

 echo-ranging gear, emits short supersonic pulses, and 

 then receives and mechanically records reflections 

 from the ocean bottom. Sonar gear (standard hydro- 

 phones and projectors) may also be used to stuch the 

 sound output ol disturbances as well as to determine 

 the sound-absorbing and reflecting properties of vari- 

 ous materials. A further nontactical application of 

 transmitting projectors and receiving hydrophones 

 involves the use of code or speech-modulated super- 

 sonic signals for underwater communication between 

 surface and subsurface craft. 



Finally, an important nontactical application of 

 sonar gear involves its use in fundamental studies 

 of sound propagation in the sea under various oceano- 

 graphic, surface, and bottom conditions, with the at- 

 tendant study of surface and bottom reflection, refrac- 

 tion, attenuation, scattering, and reverberation. The 

 study of various types of noise background (ship's 

 noise, ambient noise, and target noise) and of the 

 reflecting power of various targets should be men- 

 tioned in this connection. Such fundamental studies 

 are useful in determining the relation of gear opera- 

 tional efficacy to gear parameters with a view toward 

 optimum sonar design. 



» Calibration usually involves the determination of the axis 

 response, directivity, efficiency, and power output of the device. 

 (See Chapter 4.) 



