INTRODUCTION TO SONAR 



Reflection takes place whenever the sound 

 hits the boundary between sea and air (sea 

 surface) and between sea and bottom, and when 

 it hits a solid object, such as a submarine. 

 The amount of energy reflected depends on the 

 object's density, size, shape, and aspect. More 

 energy is reflected by a submarine broadside 

 to the sound beam than by one that is bow on. 



Surface Effect 



Because the density of water is several 

 hundred times that of air, practically all of a 

 sound wave is reflected downward when it strikes 

 the surface boundary. This effect is true only 

 when the surface is quite smooth, however. 

 When the surface is rough, scattering takes 

 place. 



Bottom Effect 



The bottom of the sea reflects sound waves, 

 too. In deep water, this aspect need not be 

 considered, but in waters of less than 100 

 fathoms, the sound may be unwantedly reflected 

 from the bottom. Other considerations being 

 equal, transmission loss is least over soft 

 mud. Over rough and rocky bottoms, the sound 

 is scattered, resulting in strong bottom re- 

 verberations. 



REVERBERATIONS 



You probably are acquainted with the effect 

 in an empty room where your voice seems to 

 echo all around you as you talk. After you 

 stop talldng, the sound continues to bounce 

 around the room from wall to wall until it 

 finally is absorbed by the walls and the air. 

 The sound level in the room while you are 

 talking is higher than normal because of the 

 reverberation effects. The same effects can 

 also be observed in the ocean. Reverberation 

 in the ocean usually is divided into three cate- 

 gories. They are reverberations from the mass 

 of water, from the surface, and from the bottom. 



The following discussion on reverberations 

 may seem like a repetition of the topic on re- 

 flections, but the two phenomena are not the 

 same. Although reverberations are reflections, 

 all reflections do not become reverberations. 

 All the processes contributing to reverberation 

 are rai* .in 'iire, with the result that 

 reverberation amplitudes vary over wide limits. 



Moreover, reverberation level is proportional 

 to source level and to pulse length. Another 

 point to remember is that when you are in 

 company with another ship, you may hear re- 

 verberation effects from her sonar in addition 

 to your own. 



From Mass of Water 



Reverberation from the mass of water is 

 called volvune reverberation, which was men- 

 tioned in the discussion on scattering. Suppose 

 a short pulse of sound is sent out from a station- 

 ary underwater source, which is immediately 

 replaced by a listening hydrophone. As the 

 pulse of sound travels through the water, it 

 encounters various particles that reflect and 

 scatter the sound. Because almost all of these 

 particles are much smaller than a wavelength 

 of the sound, they do not reflect the sound as 

 a flat mirror reflects light. Instead, they absorb 

 energy from the sound wave and reradiate this 

 energy in all directions. Some reradiated energy 

 from each particle returns to the hydrophone 

 at the source location and is heard as a gradually 

 fading tone at the same frequency as the source. 



From the Surface 



Some of the sound energy from the source 

 strikes the surface, the point of impingement 

 moving farther and farther from the source as 

 the sound travels. If the surface were perfectly 

 flat, this sound energy would be reflected as 

 though from a mirror, and would bounce away 

 from the source in accordance with the laws 

 of reflection. But the surface is not perfectly 

 smooth, and each wavelet tends to reflect the 

 sound in all directions. Some reflected sound 

 returns to the hydrophone, adding to the re- 

 verberation. 



From the Bottom 



In general, reverberation effects from both 

 the mass of water and the surface are small 

 compared to bottom reverberation. The bottom 

 is usually much rougher than the surface. Thus, 

 more of the sound is reflected in other directions 

 than those expected of a reflecting mirror. If 

 the water is fairly deep, no bottom reverberation 

 occurs for quite some time after the pulse, 

 because the sound must be given time to reach 

 the bottom and be reflected. Normally, a sharp 

 rise eventually occurs in the reverberation level 

 after the source is cut off. 



44 



