CHOICE OF TESTING GEOMETRY 



51 



170' 



180 



260 



270 



280 



--0.5 



- -0.4 



-0.3 



-O.J 



ISO 



— 0° 

 180°-- 



■0° 



160° 



4; 90° 



+ 6 0° 



- 40° 



-20° 



- -10° 



--5° 



--0.05 

 --0.04 



- -0.03 

 ■0.02 



-0,5 

 --0.4 



-7 0.3 

 --0.25 



h/r 



Figure 1 1. Collection chart lor surface reflections at low frequencies for nondirectional instruments. 



Instructions: 



Let li = depth of hydrophone and projector. 



i = testing distance. 



\ = wave length. 



p = RMS pressure at hydrophone with surface present. 



p = RMS pressure at hydrophone if surface were not present. 



Calculate h/r and r/\. Proceed to the alignment chart on right and place a straight edge so that it crosses the scale 

 marked h/r at the calculated value and the scale marked r/\ also at the calculated value. Read the angle " at the 

 intersection of the straight edge and the scale marked a. Proceed to the diagram on left. Find the circle with center 

 at A whose indicated value corresponds to the calculated value of h/r. Find the point of intersection of this circle 

 with the radial line corresponding to the value of « previously ohlained. Then the value indicated on the circle, with 

 center at B, on which this point lies will he the value of 



20 log t. 

 Interpolate if necessary. 



Example. Let /; = 5 ft, r = 10 ft, frequency equals 150 c. 



h 



Then 



= 0.5, 



X = 



4800 

 150 



32 ft in water, 



From the alignment chart: a = 47°. 



From the diagram: 20 log 



: 2.6 dl). 



- = 0.313. 

 X 



