Chapter 25 

 SUMMARY 



25.1 DEFINITION OF TARGET STRENGTH 25.1.3 



Transmission Loss 



FOR THE PURPOSES of discussing the reflecting 

 characteristics of different vessels, the target 

 strength T of a target is defined by 



T = E - S + 2H, (1) 



where E is the echo level, S the source level, and H 

 the one-way transmission loss from the source to the 

 target, all in decibels (see Section 19.1.3). For most 

 targets, T is independent of range at ranges much 

 greater than the dimensions of the target (see Sec- 

 tions 20.4 and 23.4), but may change with the chang- 

 ing orientation of the target relative to the sound 

 beam (see Section 23.1). 



25.1.1 Echo Level 



The echo level E is defined by 



E = 20 log pe, (2) 



where p^ is the rms pressure of the echo, in dynes per 

 square centimeter averaged over a few cycles (see Sec- 

 tion 19.1.3). If the rms pressure is not constant dur- 

 ing the echo, E is defined as the peak rms pressure. 



25.1.2 



Source Level 



For directional supersonic projectors, the rms pres- 

 sure p of the sound on the axis of a projector is in- 

 versely proportional to the square of the range r, as 

 long as the range is much greater than the dimen- 

 sions of the target and as long as the range is small 

 enough so that attenuation and surface reflection 

 may be neglected. Under these conditions, the source 

 level S is defined by 



<S = 20 log p + 20 log r, (3) 



where p is the pressure of the sound on the axis of 

 the projector, in dynes per square centimeter, at a 

 distance r, in yards, from the projector (see Section 

 19.1.3). 



The difference between the pressure level of the 

 transmitted sound at some point, and the source level 

 is called the transmission loss H from the projector 

 to that point (see Section 19.1.2). 



25.1.4 Average Values 



Since both E and H often fluctuate by as mucH as 

 10 db from pulse to pulse, it is customary to use the 

 average echo amplitude in determining E, and the 

 average pressure amplitude at the range r in deter- 

 mining H, in equation (1), where the average ampli- 

 tude is the average of a number of peak rms ampli- 

 tudes, if the rms amplitude is not constant over each 

 echo (see Section 21.6.4). 



25.1.5 Target Strength of Sphere 



A sphere reflects a plane wave equally in all direc- 

 tions (see Section 19.2.2). The target strength of a 

 sphere is 



T = 20 log - (4) 



where A is the radius of the sphere in yards (see Sec- 

 tions 19.2.1, 19.2.2 and 20.4.1). This formula is accu- 

 rate to 0.5 db if the range to the sphere is greater 

 than ten times its radius, and if the wavelength of 

 the sound is less than the radius of the sphere. 



25.1.6 Target Strength of a General 



Convex Surface 



The target strength for specular reflection from 

 any convex surface is 



r = 10 log 



<-t)(-t) 



(5) 



where Ai and A2 are the principal radii of curvature 

 of the target surface at the point where the surface is 



434 



