Page 573 radio acoustic ranging 63 



depth and may partly account for the rapid attenuation of sound in shoal water in 

 contrast with deep water. The amount of absorption of this type is far from con- 

 stant — the condition of the sea surface, the season of the year, and other factors 

 affect the amount and depth of gas penetration. 



Reflection and refraction at the boundaries of the medium account for some loss 

 of sound energy. As stated in 6222 and 6224, the sound energy leaving the water 

 medium at the boundaries is quite small. A greater impedance to sound transmission, 

 connected with reflection, is that due to the scattering of sound from the reflecting 

 boundaries. This scattering reduces the sound energy traveling in the general direc- 

 tion of the point of reception. Scattering is caused by irregularities of the bottom and 

 surface boundary. This is called diffuse reflection. 



The influence of diffraction on subaqueous sound ranging is not fully known. It 

 seems probable that some diffraction is caused by certain types of bottom and other 

 factors, but it is believed that its effect is negligible. 



Experience has disclosed that sound will be attenuated less when traveling through 

 water of a uniform temperature. Such a condition is approached in the waters off the 

 Pacific Coast where the transmission of sound is excellent in contrast with that in waters 

 off the Atlantic Coast during the summer. But during the winter the surface waters 

 off the Atlantic Coast cool sufficiently to make the temperature more nearly uniform from 

 surface to bottom and then sound is transmitted nearly as well as in the Pacific. Trans- 

 mission of sound through water of nonuniform temperature will be subject to refraction 

 of a complex character, which will lead to greater loss of effective sound energy due to 

 diffuse reflection, absorption, and the creation of complex interference patterns. 



63. VELOCITY OF SOUND 



To determine a distance by the transmission of sound in any medium it is necessary 

 not only to measure the travel time accurately but also to know the effective velocity 

 at which the sound travels. For use in echo sounding and R.A.R., electromechanical 

 instruments are available with which the travel time of sound may be measured very 

 accurately, but methods of determining the velocity are less reliable. 



In echo sounding the sound wave passes vertically tlu-ough the water from near 

 the surface to the bottom and the determination of velocity is comparatively simple, 

 for it is only necessary to calculate the mean velocity for the vertical column of water 

 (see 561). 



For R.A.R., the effective horizontal velocity between the source and the point of 

 reception is required, and to find this is more difficult because the sound wave is seldom 

 propagated horizontally in a direct path, but generally follows a refracted and reflected 

 path, as described in 6231 and 6232. For plotting purposes, the apparent horizontal 

 velocity of sound is used (see 6343), and this can be determined experimentally by 

 measuring the travel time of the sound wave between two points a known distance 

 apart. In R.A.R. surveys, frequent measurements are made over suitable known 

 distances thi'oughout the project area to determine this apparent velocity. 



The apparent velocity is seldom used for echo sounding, however, principaUy 

 because of the difficulty of measuring great depths directly with sufficient accuracy 

 to determine it. Instead, the velocity is calculated from, or taken from, tables based 

 on the physical characteristics of the water, which can be measured in situ. If the 



