sound  wave.  The  frequency  (in  hertz)  of  the  sound wave is the number of wavelengths that occur every second. FACTORS  AFFECTING  THE  SOUND BEAM The  particular  sound  waves  of  interest  to  the sonar  operator  are  the  waves  that  leave  the  sonar transducer in the form of a beam and go out into the water in search of a submarine. If the sound beam finds a target, it will return in the form of an echo. The  use  of  sonar  equipment  depends  on  the presence and the recognition of an echo from a target. Detection  of  the  echo  depends  on  the  quality  and relative strength (loudness) of the echo compared to the strength and character of other sounds, since they tend to mask or cover it. The  sonar  operator  should  know  what  factors  can weaken the sound beam as it travels through water, what factors in the seawater determine the path and speed of the sound beam, and what factors affect the strength   and   character   of   the   echo.   Any   signal strength  lost  during  the  beam’s  travel  through  the water is known as “transmission loss.” Some of the factors determining transmission loss are discussed in the  following  paragraphs. Absorption and Scattering Some of the sound energy emitted by the source will  be  absorbed  while  passing  through  the  water. The amount absorbed this way depends on the sea state. Absorption  is  high  when  winds  are  great enough   to   produce   whitecaps   and   cause   a concentration of bubbles in the surface layer of the water. In areas of wakes and strong currents, such as riptides,   the   loss   of   sound   energy   is   greater. Therefore, echo ranging through wakes and riptides is difficult   because   of   the   combined   effect   of   false echoes,  high  reverberations,  and  increased  absorp- tion.  Absorption  is  greater  at  higher  frequencies  than at  lower  frequencies. Sound waves are weakened when they reach a region  of  seawater  that  contains  foreign  matter,  such as  seaweed,  silt,  animal  life,  or  air  bubbles.  This foreign matter scatters the sound beam and causes loss   of   sound   energy. The   practical   result   of scattering  is  to  reduce  echo  strength,  especially  at long range. Reflection Echoes occur when the sound beam hits an object or  a  boundary  region  between  transmission  mediums in such a manner as to reflect the sound or to throw it back  to  its  origin. Reflection   of   sound   waves sometimes happens when a wave strikes a medium of different  density  from  that  through  which  it  has  been traveling. This  will  occur  in  cases  where  the  two mediums are of sufficiently different densities, and the  wave  strikes  at  a  large  angle.  This  happens because the sound wave travels at different speeds through the two different densities. For example, a sound  wave  traveling  through  seawater  is  almost entirely reflected at the boundary of the water and air. The speed of sound in seawater is about four times greater than the speed of sound in air, and the density of water is more than 800 times greater than that of air. Therefore, practically all of the sound beam will be  reflected  downward  from  the  sea  surface. Similarly, when a sound wave traveling through the seawater strikes a solid object like a submarine, the difference in the density and the sound velocity in the two mediums is such that all but a small amount of the sound beam will be reflected. That portion of the beam  that  strikes  surfaces  of  the  submarine perpendicular to the beam will be reflected directly back to the origin as an echo. In calm seas, most of the sound energy that strikes the water surface from below will be reflected back down into the sea. A scattering effect occurs as the sea  gets  progressively  rougher. In   these circumstances,  part  of  any  sound  striking  the  surface is lost in the air, and part is reflected in scattering directions in the sea. In water less than 600 feet deep, the  sound  may  also  be  reflected  off  the  bottom.  Other factors being equal, the transmission loss will be least over a smooth, sandy bottom and greatest over soft mud.  Over  rough  and  rocky  bottoms,  the  sound  is scattered, resulting in strong bottom reverberations. Reverberation When sound waves echo and re-echo in a large hall,  the  sound  reverberates.  Reverberations  are multiple reflections. Lightning is an example of this from nature. When lightning discharges, it causes a quick, sharp sound; but by the time the sound of the thunder  is  heard,  it  is  usually  drawn  out  into  a prolonged  roar  by  reverberations. 4-2