542 



SUMMARY 



the wake, as bubbles with radii between i2r and Rr + 

 d/J, the attenuation coefficient Ke in decibels per yard 

 may be written [see equation (7) of Chapter 32] 



if. = 1.4 X 10Sm(/?,) 



(4) 



VELOCITY IN KNOTS 



In the wake less than 500 yd behind a destroyer or 

 destroyer escort, the attenuation coefficient in the 

 horizontal direction Ke = Hw/w is about 1 db per yd 

 at 20 kc (see Section 32.3). If attenuation at other 

 frequencies by the same wake is taken into account, 

 the total amount of air is about 0.7 X 10"" cu cm per 

 cu cm of water in the wake of a destroyer at 15 knots, 

 one minute after the passage of the vessel (see Tables 

 1 and 2 of Chapter 28). 



For sound transmitted vertically upward and 

 reflected back by the surface, thus traveling twice 

 through the center of a destroyer wake about 20 ft 

 thick, the attenuation coefficient is found from the 

 equation 



ft- -^"' 



Figure 

 wakes. 



1. Initial transmission loss across destroyer where ffu, now denotes the two-way attenuation. The 



IN MINUTES 

 Figure 2. Decay of transmission loss across destroyer wakes, a = initial transmission loss in db. 



The attenuation coefficient K^ is determined by the 

 density of air in resonant bubbles of radius Rt- If 

 u(Rr)dR is the fraction of air present, in 1 cu cm of 



value of Ke observed in this case is about 3 db per yd 

 at 20 kc [see equation (13) of Chapter 32]. 

 For sound transmitted along a horizontal path per- 



