508 



OBSERVED TRANSMISSION THROUGH WAKES 



The procedure is completed by determining, in 

 effect, the amount of amplifier attenuation required 

 to record the unobscured ocean surface at the same 

 density as that of the record taken below the wake. 

 The difference of attenuator settings in the two cases 

 yields the wake transmission loss directly. The actual 

 procedure was complicated by the lack of a calibrated 

 attenuator; the details of the necessary laboratory 

 calibration of the gain control by matching records 

 for different gain settings and echo levels are de- 

 scribed in reference 5. The fathometer record yields 

 an accurate value of wake thickness in each case so 

 that attenuation coefficients can be computed in 

 decibels per foot of wake thickness. 



The coefficient of reflection at the ocean surface 

 cancels from the measured transmission loss, because 

 M affects the sound levels both in and outside the 

 wake in an identical manner, aside from slow varia- 

 tions of the state of the sea. If the ocean surface were 

 a perfect plane, and if the axis of the sound beam 

 impinged upon it perpendicularly, the entire off-axis 

 output of the fathometer would be reflected so as not 

 to return to the transducer. On account of the waves, 

 swells, and other irregularities of the surface, and be- 

 cause of imperfect leveling of the submarine, actually 

 some off-axis sound is reflected back on to the face 

 of the transducer. Hence, it is necessary to keep the 

 submarine at a depth shallow enough to make the 

 central lobe of the soimd beam fall entirely inside the 

 wake. This condition was well fulfilled during these 

 experiments, the results of which will now be 

 described. ' 



Sound of 21 kc was found to undergo an average 

 attenuation of 18 + 3 db during vertical one-way 

 passage through the wakes about 400 yd behind the 

 USS Rathburne (APD25, ex-DD113), USS Hopewell 

 (DD681) and USCGC Ewing, traveling at speeds of 

 10 to 13 knots. Combining these total attenuations 

 with the wake depths h for the vessels, accurately 

 determined from the same records and already dis- 

 cussed in Section 31.3, average attenuation coef- 

 ficients in the vertical direction in decibels per yard 

 could be computed and were found to be 3.0 + 0.6 db 

 per yd for the Rathburne and Hopewell and 4.8 + 1.5 

 db per yd for the Ewing. These are grand averages, 

 disregarding differences in the distance astern, which 

 are unknown in many cases, and disregarding devia- 

 tions of the point of measurements from the center of 

 the wake; moreover, some "knuckles" are included 

 with the straight runs. If only data referring to 

 know n distances astern and to the center of straight 



wakes are retained, all observations applying to 

 wakes laid by the Hopewell are eliminated. Plotting 

 as a function of the distance astern, the attenuation 

 coefficients for the wake of the Rathburne, running at 

 a speed of 10 knots (corresponding to a screw-tip 

 speed of 52 ft per sec), the following linear interpola- 

 tion formula is found for the range from 100 to 

 800 yd: 



Hu 



= (3.135 + 0.057) -I- (0.093 + 0.018) X 



/distance astern\ „ 



( 100yd J^^^^^y^- (13) 



Since there is apparently no correlation between the 

 total transmission loss in the wake Hx„ and the dis- 

 tance astern, equation (13) implies .that the wake be- 

 comes thinner in the vertical direction as it ages. A 

 similar plot for the Ewing, running at 13 knots, re- 

 veals an enormous variation of the attenuation 

 coefficient ranging from 2.4 to 6.6 db per yd without 

 any clear dependence on the distance astern. The 

 distances astern cannot, however, be very accurately 

 determined in these experiments. There is no obvious 

 explanation why the Ewing data should show a 

 greater scatter, enough to obliterate any dependence on 

 distance astern. The higher value of the attenuation 

 coefficients for the Ewing has been associated tenta- 

 tively with the greater screw- tip speed (112 ft per 

 sec at 13 knots) of this vessel, compared with the 

 two destroyers. No corroboration for this surmise 

 could be found among the observations of the hori- 

 zontal transmission loss through wakes laid by dif- 

 ferent destroyers, already described in Section 32.3.1, 

 although the screw-tip speeds of these vessels ranged 

 from 80 to 137 ft per sec at 15 knots, and from 53 to 

 95 ft per sec at 10 knots. 



It is of interest to compare the attenuation coef- 

 ficient in the vertical direction with that computed 

 from the total transmission loss measured hori- 

 zontally. According to equation (11) in Chapter 32, 

 the one-way horizontal transmission loss through the 

 wake 400 yd behind a destroyer traveling at 10 knots 

 is about 20 db for 21-kc sound. The width of this 

 wake is about 75 ft, according to Figure 22 of refer- 

 ence 1. Hence, the horizontal attenuation coefficient 

 is about 0.9 db per yd, or about one-third of the 

 vertical one, as reported above for the destroyers 

 Rathburne and Hopewell. This discrepancy is probably 

 real, but hardly disturbing. In fact, the average at- 

 tenuation coefficient would be expected to be smaller 



