OPERATION AND APPLICATION OF SONAR DEVICES 



transducer, with a pulsating cylindrical surface of a 

 length which is large, and a diameter which is small, 

 compared to a wave length. Here the sound is emitted 

 uniformly in all planes containing a cross section ol 

 the cylinder, while in perpendicular planes the sound 

 is concentrated in a beam. 



23 APPLICATION OF UNDERWATER 

 SOUND DEVICES 



The applications of underwater sound devices in 

 naval operations may be roughly divided into two 

 categories: tactical and nontactical. 1 Tactical applica- 

 tions include the detection of surface vessels, subma- 

 rines, torpedoes, mines, and underwater phenomena 

 by surface vessels and submarines. The nontactical 

 applications include: fathometer depth determina- 

 tions by surface craft and submarines; monitoring of 

 noise output by submarines: underwater communica- 

 tion by code or voice; fundamental studies on sound 

 propagation in the ocean, on ship noises, on under- 

 water phenomena, etc., designed to aid in the tactical 

 employment of sonar devices; and finally, the calibra- 

 tion of sonar gear with standard projectors and hy- 

 drophones. 



2.3.: 



Tactical Applications 



Tactical applications of sonar gear generally in- 

 volve either echo ranging or listening. In echo rang- 

 ing, the devices emit either a pulse or a continuous- 

 wave signal, which may be of either sonic or super- 

 sonic frequency. If the signal strikes a target, part of 

 its energy is reflected back to the emitting device (the 

 projector) which receives the signal. On the basis of 

 characteristics such as frequency shift and time delay 

 between emission and reception, conclusions may be 

 drawn regarding the range, bearing, speed, and na- 

 ture of the target. In listening, any supersonic or sonic 

 signal or noise emitted by a target actuates a receiving 

 device which, from the characteristics of the signal or 

 noise (intensity, frequency, and direction), enables 

 conclusions to be drawn regarding the range, speed, 

 bearing, and nature of the target. 



Echo-Ranging Gear 



Conventional searchlight echo ranging gear as em- 



i The distinction between tactical and nontactical applica- 

 tions is, of course, rather arbitrary and by no means rigid. 



ployed in tire last war may be classified according to 

 its tactical use, as antisubmarine or prosubmarine. 

 Searchlight-type gear is in general use in both the 

 United States and British Navies. Searchlight-type 

 echo-ranging gear consists of a projector with either 

 a square or a circular diaphragm operating on the 

 piezoelectric or on the magnetostrictive principle. 

 The dimensions of the projector are, as a rule, con- 

 siderably larger than the wave length of sound 

 emitted, so that a relatively narrow beam is formed. 

 The axis of this beam may be rotated by training the 

 projector in a horizontal and sometimes in a vertical 

 plane. As a result of the application of a suitable pulse 

 voltage, the projector emits supersonic pulses of from 

 10 to 200 milliseconds duration. 



Immediately alter transmission, the projector is 

 switched from the transmitter to the receiver. Any re- 

 ceived reflected pulse of supersonic frequency is 

 either directly rectified and presented visually on the 

 screen of a cathode-ray tube or, more customarily, is 

 heterodyned to an audible frequency and presented 

 through a loud-speaker. The time delay between > 

 transmission and reception is a measure of the range 

 of the target from the projector; the orientation of 

 the projector (or beam) axis in space at the instant of 

 transmission gives the relative bearing of the target; 

 any difference between the frequency of the emitted 

 and the received pulse (the Doppler effect) is a meas- 

 ure of the speed of the target; and the quality of the 

 received (heterodyned) ptdse often throws informa- 

 tion upon the nature of the target (for example, dis- 

 tinguishes a submarine from its wake). Thus, con- 

 siderable information about the position, motion, 

 and nature of the target is obtained. 



By systematically training the projector in azimuth 

 (i.e., by following a definite search plan) it is possible 

 to sweep a sector of ocean where the presence of the 

 target is either known or suspected. In echo ranging 

 by surface craft in search of submarines, the target 

 may be found at different depths and ranges so that 

 the width of the beam of the main projector is not 

 always adequate for maintenance of contact. In this 

 case, auxiliary projectors may be employed. 



A list of representative tactical applications of vari- 

 ous searchlight echo-ranging devices follows: 



1. Detection of submarines by surface craft. 



2. Detection of submarines by harbor installations. 



3. Detection of submarines and surface craft by 

 submarines. 



4. Detection of small objects such as mines, tor- 



