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RADIO ACOUSTIC RANGING 



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the water and bottom, where composed of certain materials, offers a surface from which 

 most of the energy of the somid wave will be reflected. 



The laws governing the reflection of a sound wave in water are the same as the laws 

 that govern reflections in any other medium. When a sound wave meets a reflecting 

 surface its direction of propagation is changed. For example in A, figure 122, the 

 incident ray of the sound wave meets the reflecting surface at a and is reflected in the 

 direction of the dash line. The direction after reflection depends on the angle of inci- 

 dence i between the incident path and the normal an to the reflecting surface. The 

 path of the reflected wave will always be inclined to this normal at an angle of reflection 

 r, which is always equal to the angle of incidence. 



Each point in the sound wave W from a source S (see B in fig. 122) will be re- 

 flected from the reflecting surface as the wave advances. As successive points between 

 fli and 6i in the incident wave W reach the reflecting surface between points a and 6, 

 the direction of propagation of the wave will be changed, and the reflected wave WR 

 may be considered to be the resultant of a number of hemispheric wavelets between 



.^<^^ 



\ 



S' 

 Figure 122.— Sound reflected from a rigid surface— 4. Change of direction of path. B. Wave front retained after reflection. 



a-i and 62, whose sources lie on the reflecting surface between a and h. This is known as 

 Huygens' construction for reflected waves. 



The source of the reflected wave may be considered to be at point /S", the acoustic 

 image of the soiu-ce, symmetrically located on the opposite side of the reflecting surface 

 with reference to the source. By considering the reflected wave to be propagated 

 from the image source, the phenomenon of reflection may be more readily visualized. 

 Wliere reflection is due to a medium of greater radiation resistance the source and its 

 image are to be considered as in phase, but where reflection is due to a medium of 

 lesser radiation resistance the converse is true. Radiation resistance is the product 

 of the density of the medium and the velocity of sound in the medium. The radiation 

 resistance, or acoustic resistance, of air is less than that of water, and that of water is 

 less than that of the ocean bottom. 



Where a sound wave is reflected from a surface, a change of phase of pressure or 

 a change of phase of particle velocity occurs. Assuming a sound source in water, if 

 the reflecting surface is that of a denser medium, as at the bottom boundary, the 

 change will be in the phase of particle velocity; but if the reflecting surface is that of 

 a less dense medium, as it is between water and air, the change which occurs will be 

 in the phase of pressure. In each case there will be a phase reversal. 



The relative amount of acoustic energy that will be reflected where a sound wave 

 meets the bounding surface of a second medium, is a function of the angle of incidence 

 and the radiation resistance of the two media. This function is such that, in the case 



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