6232 HYDROGRAPHIC MANUAL PaGE 568 



bottom of uniform slope, a greater number of reflections will be required than in the 

 case of a uniform depth equal to the average depth at these two points ; if the source 

 and point of reception were interchanged, fewer reflections would occur. Where the 

 bottom slope is comparatively steep and the source of sound is where the water is 

 deep, the sound may never reach a distant point of reception up the slope. For each 

 successive bottom reflection the angle of incidence is decreased by an amount equal 

 to the angle the bottom slope makes with the horizontal. After a number of surface 

 and bottom reflections have taken place the path of sound may reach normal incidence, 

 and on succeeding reflections may actually reverse its horizontal direction of propagation 

 and return toward the source. Furthermore, before normal incidence is reached by 

 successive reflections, the angle of incidence becomes less than the critical value, and 

 appreciable transmission of energy into the bottom may take place in successively 

 greater amounts, resulting in great attenuation of sound energy, particularly in the 

 case of an extended gradual slope. This condition is even more aggravated in shoal 

 water where more reflections will take place in a given distance than will in deep water. 

 This partly explains the difficulty in sound transmission over shoals, and from deep 

 water on the Continental Slope to shoal water on the Shelf. 



Little of a practical quantitative nature is known about the influence of the bottom 

 configuration and material on the reflection of sound. What is known has been de- 

 duced principally from observations made while surveying by R.A.R. Even these 

 results are obscured by other influences on sound transmission to an extent which makes 

 it difficult to interpret the degree of influence which the bottom configuration and 

 material have. However, certain facts have been fairly well established from years of 

 observation. Where the bottom is smooth the material has little influence on sound 

 transmission, as used for purposes of distance measurement. This implies that the 

 angle of incidence, in practice, is nearly always equal to, or greater than, the critical 

 angle (see 6221 and 6222). Where the bottom is irregular the distance at which sub- 

 aqueous sound ranging is effective is very much reduced from what could normally be 

 expected with smooth regular bottom. Sound reflected from such a bottom is scat- 

 tered in many directions and the amount of sound energy which travels in a general 

 radial direction from the source is very much reduced with each reflection. Such 

 irregular bottom surfaces are common ofl' the Pacific Coast, in the waters surrounding 

 Alaska, and along the edges of the Atlantic Continental Shelf. 



In general, reflections from the water surface are more definite than bottom reflec- 

 tions. The reflection coefficient between water and air is constant, the only variable 

 being the degree of roughness of the water surface. Except when the water surface is 

 extremely disturbed, there is little effect on the reflection of sounds produced by sub- 

 aqueous explosions. Tliis is because of the predominant low-frequency fundamental 

 component of the subaqueous explosion. The wave length of this low-frequency 

 fundamental component is long as compared to the size of the irregularities of the 

 water surface (see 621). 



6232. Refraction of Sound 



In a heterogeneous aqueous medium the propagation of a sound wave is influenced 

 principally by the temperature distribution within the medium and, to a lesser extent, 

 by changes in salinity and other physical characteristics which cause a change in ve- 

 locity. Where velocity varies with depth in the medium, refraction takes place and the 

 path of the sound wave is altered from normal linear propagation. Due to this fact, 

 the maximum effective distance in subaqueous sound ranging is reduced, because of 



