TRANSMISSION LOSS ACROSS WAKES 



507 



recovery and decay rates quoted are based were 

 taken quasi-simultaneously by tuning the sonar 

 equipment alternately to the two frequencies for two- 

 minute intervals. The wake and sphere distances for 

 this run are those (juoted in the 45-kc row. The 9-db 

 drop in apparent target strength at 60 kc is based on 

 a separate run, with wake and target distances as 

 stated in the third row of the table. 



32.3.3 Two- Way Vertical Trans- 

 mission Loss 



A recording fathometer has been used for the meas- 

 urement of sound transmission loss in the vertical 

 direction through surface ship wakes.* The fathom- 

 eter was secured on the deck of the submarine USS 

 S-18 (SS123) so as to range upward onto the surface 



Table 2. Effect of wake on sphere echoes. 



Earlier measurements of the Scripps wake * gave 

 a depth of the wake bottom of 13 ft. According to the 

 values quoted in Table 1, the sound beam passed 

 definitely above this bottom depth of 13 ft. However, 

 a short time before the data of Table 2 were obtained, 

 the Scripps had been outfitted with a new engme and 

 propeller, so that the wake dimensions may have been 

 altered to some extent. The present propeller is 3.8 ft 

 in diameter and the shaft is 5.5 ft below water line. 

 Therefore, since the Jasper's sound projector is 15 ft 

 deep, maximum acoustic shadowing of the sphere by 

 the wake could not be expected immediately after 

 the Scripps had passed. These wakes widened later- 

 ally, as measured by the wake echo elongation, at 

 about 6 yd per minute. The same rate of spreading 

 may also be applicable in the vertical sense without 

 necessarily implying that a strongly absorbent "core" 

 of the wake ever moves down to an effective screening 

 position in these experiments. This may account for 

 the low magnitude of the observed transmission loss. 



Similarly the decay rate of the transmission loss 

 dHw/dt is one-half the rate of recovery of the sphere 

 echoes; hence dHw/dt is 0.7 and 0.4 db per minute for 

 24- and 60-kc sound, respectively. These decay rates 

 are much smaller than that of destroyer wakes, which 

 were found to be independent of frequency — 3.0 db 

 per nninute. However, the relative rates of decay are 

 in moderate agreement with those computed from 

 equation (10) for a destroyer speed of 10 knots; 

 numerical values are shown in Table 3. 



of the ocean, the echoes being continuously recorded 

 in the control room. With this arrangement, the 

 effect of a surface ship wake is recorded as the sub- 

 marine passes beneath it. The ocean surface is used 

 as a "standard target." Sample records obtained 

 with this method are shown in Figures 4 to 6 of 

 Chapter 30. 



Table 3. Relative rates of decay. 



Quantitative transmission loss results are obtained 

 from the fathometer records by a special procedure of 

 operating the instrument in conjunction with calibra- 

 tion records made in the laboratory. As the submarine 

 passes beneath a surface ship wake, an attenuator in 

 the receiver-amplifier is adjusted so that the effect of 

 the wake plus the effect of the attenuator is such that 

 a light gray "voltage-sensitive" record of the ocean 

 surface echo is produced on the chart paper. Some 

 practice is required, as very little trial-and-error time 

 is available while the submarine is directly below the 

 wake. 



