ECHOES FROM SURFACE VESSEL WAKES 



529 



Table 8. Dependence of wake strength on pulse length. 



quency. From Table 7, it can be seen that the average 

 difference between wake strength at 60 kc and wake 

 strength at 24 kc is 16 db. But the reality of this 

 phenomenon is doubtful, because use of this same 

 underwater sound equipment in measurements of tar- 

 get strengths of submarines has yielded results at 60 

 kc which are also 10 to 20 db above the 24 kc results, 

 thus contradicting theoretical expectations (see Sec- 

 tion 23.6.2). It is also important that measurements 

 on submarine wakes made with different equipment 

 and discussed before (see Table 4) show a decrease of 

 W with increasing frequency rather than an increase. 

 In a separate set of careful experiments on surface- 

 vessel wakes, which are summarized in Table 9, a 

 single instance was found where there was a marked 

 difference of opposite sign between wake strength at 

 60 kc and wake strength at 24 kc. The existence of an 

 isolated but well documented instance like this where 

 an apparent trend is contradicted must be given con- 

 siderable weight when conclusions are drawn about 

 the frequency dependence of the wake effect. 



The decay rate of surface wakes shows very little 

 dependence on frequency between 24 and 60 kc. The 

 average decay rates of the wakes described above are 

 1.36 db per minute at 24 kc, and 1.18 db per minute 



20 



111 

 o 30 



z 



Ui 



a. 



UJ 



u 



> 

 < 



m 



IT 



< 



ui 



> 

 o 

 m 



< 



o 

 a 



± 



> 



UJ 



;-io 



5 10 15 



AGE OF WAKE IN MINUTES 



20 



Figure 8. Echo level as function of age of wake, for 

 various ping lengths at 24 kc. Wake vessel: Small 

 carrier at about 15 knots. 



at 60 kc. The standard deviations of these measure- 

 ments are 0.59 db per minute at 24 kc and 0.69 db per 

 minute at 60 kc. This difference in averages is so much 



