Page 565 kadio acoustic ranging 6224 



from image sources as explained in 6221. These reflected paths, which are reversed 

 in either phase of pressure or phase of particle velocity according to the character and 

 number of reflecting surfaces they encounter, cross the direct path and each other in 

 the medium, and at their points of intersection the pressure or particle velocity is 

 either reinforced or neutralized by an amount depending on the phase relationship of 

 the waves. This has already been mentioned briefly in 6221, but because of its impor- 

 tance in subaqueous sound ranging a fuller discussion of the effect is desirable. 



If two waves of the same amplitude and wave length arrive from different directions 

 and meet in condensation, the amplitude at this point will be increased, but if they 

 intersect when one is in condensation and the other in rarefaction they will tend to 

 neutralize at this point. In neither case will there be energy transfer or loss, however, 

 and each wave will emerge in its original direction of propagation and with its original 

 amplitude. In a water medium with a nondirective source of sound, there will be a 

 number of zones of reinforcement and neutralization of both pressure and particle 

 velocity, which are complicated by refraction. 



The most important zone of interference occurs between the sound that travels 

 directly from the source to the point of reception, and the sound that is reflected once 

 from the surface of the water. This surface-reflected sound is reversed in phase and 

 thus, at a distance from the source where the lengths of the direct and reflected paths 

 are nearl}^ equal, the pressure and particle velocity are nearly canceled. This effect 

 limits the range of both the unreflected and the surface-reflected sound, and is the 

 reason why, in actual practice in subaqueous sound ranging, the maximum distance 

 at which the unreflected sound is effective does not exceed 7 to 10 miles. 



6224. Reduction of Sound Energy 



As sound passes through a medium, its intensity decreases as the distance from the 

 source increases. This decrease in energy in a homogeneous medium, as discussed in 

 622, may be attributed to absorption, reflection, refraction, diffraction, interference, 

 and spreading. 



Absorption is the conversion of sound energy to other forms of energy because of 

 the presence of foreign matter in suspension in the medium, turbulence and viscosity 

 of the medium, and thermal conductivity of the medium. Attenuation caused by 

 viscosity may be considered as a loss of energy due to the resistance offered by the 

 medium to the passage of sound. In a water medium and at the frequencies effective 

 in subaqueous sound ranging, viscosity losses are very small in comparison with other 

 losses. Thermal conductivity of the medium allows the conduction of heat from the 

 condensed points of the wave, which are at a higher temperature than the surrounding 

 medium. These losses are very small in water, being even less than those caused by 

 viscosity. 



When the sound wave encounters the reflecting boundaries of a body of water, 

 some of the sound energy will penetrate into the media above and below and thus be 

 lost from the water medium. The transmission of energy through the surface bound- 

 ary into the air above is very slight, as stated in 6222. And transmission through the 

 bottom boundary takes place only when the angle of incidence is less than the critical 

 angle. The amount of energy penetrating the lower medium is a function of the 

 angle of incidence and the acoustic resistances of the water and bottom material. 



In a homogeneous medium, ignoring absorption, the intensity of sound from a point 

 source varies inversely with the square of the distance from the source. Sound emitted 



