A  similar  case  often  arises  in  connection  with sonar. Sound waves often strike small objects in the sea, such as fish or air bubbles. These small objects cause  the  waves  to  scatter.  Each  object  produces  a small echo, which may return to the transducer. The reflections of sound waves from the sea surface and the  sea  bottom  also  create  echoes.  The  combined echoes   from   all   these   disturbances   are   called “reverberations.” Since   they   are   reflected   from various ranges, the y seem to be a continuous sound. Reverberations  from  nearby  points  may  be  so  loud that  they  interfere  with  the  returning  echo  from  a target. There are three main types of reverberation, or backward scattering of the sound wave. They are as follows: 1. There is reverberation from the mass of water. Causes  of  this  type  of  reverberation  are  not completely  known,  although  fish  and  other  objects contribute to it. 2. There is reverberation from the surface. This is   most   intense   immediately   after   the   sonar transmission;  it  then  decreases  rapidly.  The  intensity of  the  reverberation  increases  markedly  with increased  roughness  of  the  sea  surface. 3.  There  is  reverberation  from  the  bottom.  In shallow water, this type of reverberation is the most intense of the three, especially over rocky and rough bottoms. Divergence Just as the beam from a searchlight spreads out and  becomes  weaker  with  distance,  so  does  sound. The farther the target is from the sonar transducer, the weaker the sound waves will be when they reach it. This  is  known  as  spreading  or  divergence. Refraction If there were no temperature differences in the water, the sound beam would travel in a straight line. This happens because the speed of sound would be roughly  the  same  at  all  depths.  The  sound  beam would  spread  and  become  weaker  at  a  relatively constant  rate. Unfortunately,  the  speed  of  sound  is  not  constant at  all  depths.  The  speed  of  sound  in  seawater increases  from  4,700  feet  per  second  to  5,300  feet  per second  as  the  temperature  increases  from  30°F  to 85°F.  Salinity  and  pressure  effects  on  sound  speed are not as extreme as the large effects produced by temperature  changes  in  the  sea.  Because  of  the varying  temperature  differences  in  the  sea,  the  sound beam does not travel in a straight line, but follows curved paths. This results in bending, splitting, and distorting of the sound beam. When  the  sound  beam  is  bent,  it  is  said  to  be refracted. A sound beam is refracted when it passes from a medium of a given temperature into a medium with a different temperature. An example of this is a sound beam traveling from an area of warm water into an layer of cold water.    The  sound  beam  will  bend away  from  the  area  of  higher  temperature  (higher sound velocity) toward the lower temperature (lower sound velocity). As  a  result  of  refraction,  the  range  at  which  a submarine can be detected by sound may be reduced to less than 1,000 yards, and this range may change sharply with changing submarine depth. Speed of the Sound Beam As  mentioned  previously,  sound  travels  much faster in seawater than in the atmosphere. Near sea level,   sound   travels   through   the   atmosphere   at approximately  1,080  feet  per  second.  In  seawater, that same sound beam will travel at approximately 4,700 to 5,300 feet per second. There are three main characteristics of seawater that  affect  the  speed  of  the  sound  wave  traveling through  it.  These  characteristics  are  as  follows: 1. Salinity (the amount of salt in the water) 2. Pressure (caused by increased depth) 3. Temperature (the effect of which is calculated in  terms  of  slopes,  or  gradients) There is a high mineral content in seawater. The density of seawater is approximately 64 pounds per cubic foot, while fresh water has a density of about 62.4 pounds per cubic foot. This difference is caused by the salt in the seawater. Salt content in seawater is called the salinity of water. The overall effect of increasing the salinity is an increase in the speed of the sound beam in the water. This means that as the sound travels through water of varying salinity, it travels faster through the water with more salt content. Such a change in salinity is considerable at the mouth of a river emptying into the sea. Elsewhere, the difference in salinity is too small 4-3