490 



TECHNIQUE OF WAKE MEASUREMENTS 



PR WS 



qpmnwwp 



PR WS PR WS 



+— f 



>- — ^ 



>^— f 



PR WS 



pwiSnifVfvimFPP 



PR W S 



i'fc 



■+*-« — ^ 



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PR WS 



PR WS PR WS 



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P = PING 



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hM* 



-f- 



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m 



W = WAKE ECHO 



R> REVERBERATION 



S= SPHERE ECHO 



Figure 7. Oscillograms of wake echoes from E. W. Scripps. 



taken, and the averaged peak amplitude is squared to 

 obtain the echo intensity. The resulting average is 

 different both from the average peak echo intensity 

 and the average of the intensity over the entire echo. 

 Since the spread of peak amplitudes may be as much 

 as 10 db, this difference may be appreciable. The 

 difference between average peak amplitudes and 

 average intensities is discussed in Section 34.3.1. 



Finally, from the measured peak amplitudes the 

 echo-strength is computed according to the formula : 



E - S = 20 logle - 20 log k , 



where E is the echo level in decibels above 1 dyne per 

 sq cm, S the source level, defined as the sound level 

 1 yd from the projector on its axis, also in decibels 

 above 1 dyne per sq cm, and Ae is the average peak 

 amplitude of the echo as measured on the oscillo- 

 gram. The constant k on the right side of this equa- 

 tion has to be determined by calibration of the re- 

 ceiving equipment; specifically, k is the amplitude 

 measured on the oscilloscope with an incident wave 

 whose pressure is 1 dyne per sq cm and with the same 

 receiver gain at which Ae is recorded. To determine 

 S and k, an auxiliary transducer of known power out- 

 put and of known sensitivity is used. 



30.2 OPERATIONS AND MEASUREMENTS 



Besides the acoustic measurements proper, the 

 study of wakes requires the determination of various 

 auxiliary data. In the first place, the geometric co- 

 ordinates of the part of the wake to which the 

 acoustic data refer have to be known accurately. If 

 the distance from the stern of the ship to the point 

 where the sound beam strikes the wake is known, the 

 age of the wake at the point of measurement may be 

 found by dividing this 'distance astern by the' speed- 

 of the wake-laying vessel and computations will be 

 facilitated by use of Figure 3 in Chapter 35. Since 

 the instrumental characteristics of the sound gear 

 employed may undergo slow changes, it may become 

 necessary to calibrate the gear immediately before or 

 after the observation. Furthermore, there are a 

 number of variable oceanographic factors whose in- 

 stantaneous values have to be taken into account in 

 interpreting the acoustic measurements. 



In addition to the wake-laying vessel, acoustic 

 measurements on wakes require one vessel for echo 

 ranging and an additional vessel when a transmission 

 run is made in order to measure the horizontal trans- 

 mission loss. For measurements of the transmission 



