546 



SUMMARY 



35.3.3 Angular Variation of the 

 Echo Level 



When a wake is insonified by a stationary trans- 

 ducer and the echo is recorded by a different hydro- 

 phone at several positions, the average echo level thus 

 determined may show moderate variations with posi- 

 tion of the hydrophone even after corrections for 

 range to the wake, measured along the hydrophone 

 axis, have been applied. This angular variation of the 

 echo level has not been investigated experimentally; 

 however, a simple theoretical estimate of the order of 

 magnitude of this effect can be made for long pulses 

 and may be useful [see equations (72), (73), and (76) 

 of Chapter 28]. 



For pulses longer than the width of the wake meas- 



ured along the sound beam, the echo level should be 

 proportional to 



cos/3 



cos a. -\- cos /3 



(16) 



if the wake is highly opaque (total attenuation across 

 the wake more than a few decibels) ; and proportional 

 to 



sec a (17) 



if the wake is acoustically transparent (total attenua- 

 tion less than 1 db). In equations (16) and (17), 8 

 denotes the angle between the transducer axis and a 

 line perpendicular to the wake, as illustrated in 

 Figure 4; a is the corresponding angle between the 

 axis of the hydrophone and a line perpendicular to 

 the wake. 



