INTRODUCTION TO SONAR 



2, Your ear is sensitive to the 800- Hz note 

 for wiiich sonax equipment is designed. Your 

 ear temporarily is paralyzed by very loud signals 

 so that you may not hear the weaker signals that 

 follow. 



3. Your ear has the property of pitch dis- 

 crimination. This characteristic will aid you in 

 selecting an echo from the background of noise 

 and reverberation. 



Heavy gunfire may cause permanent deafness 

 or other injury to your ears. Use ear protectors 

 or cotton when near gunfire, and take every 

 possible precaution to protect your hearing. As 

 a Sonar Technician, you are essential to the 

 successful fighting of your ship. Your ability to 

 operate the sonar depends on the effectiveness 

 of your hearing. 



WAVES IN GENERAL 



Waves may be classified by types as trans- 

 verse and longitudinal. A transverse wave is one 

 wherein the particles of the medium through 

 which the wave is passing move at right angles 

 vertically to the wave's direction. In a longi- 

 tudinal wave the particles move back and forth 

 along the wave's direction of travel, resulting in 

 compression and rarefaction of the wave. An 

 example of a transverse wave is a water wave. 



Throw a stone into a pool. A series of circular 

 waves travels away from the disturbance. In 

 figure 4-3 such waves are diagramed as though 

 seen in cross section. Observe that the waves 

 are a succession of crests and troughs. The 

 wavelength (1 cycle) is the distance from the 

 crest of one wave to the crest of the next wave. 



The amplitude of a transverse wave is half 

 the distance, measured vertically from crest to 

 trough, and serves to indicate the intensity of 

 the wave motion. 



-WAVELENGTH 



RAREFACTION 



COMPRESSION 



TRANSDUCER 



WAVE LENGTH 



Figure 4-4. 



SOUND WAVES 



4.221 

 Longitudinal waves. 



Figure 4-3. 



71.19 

 Elements of a wave. 



Sound waves are longitudinal or compression 

 waves, set up by some vibrating object such as 

 a sonar transducer. In its forward movement, 

 the vibrating transducer pushes the water par- 

 ticles lying against it, producing an area of high 

 pressure, or compression. 



On the backward movement of the trans- 

 ducer, the water particles return to the area 

 from which they were displaced during com- 

 pression and travel beyond, producing an area 

 of low pressure, or a rarefaction. The com- 

 pression moves outward by pushing the water 

 particles immediately in front of the compressed 

 particles. The rarefaction follows the compres- 

 sion, transferring the pull produced by the back- 

 ward movement to the particles immediately 

 ahead. The next forward movement of the trans- 

 ducer produces another compression and so on. 

 In figure 4-4 the compressions are represented 



36 



