506 



OBSERVED TRANSMISSION THROUGH WAKES 



the original report to the numerical coefficients of the 

 first terms of equations (10) and (11), but it is stated 

 that the initial values (i = 0) of the transmission loss 

 for individual runs show a scatter of the order of 3 db. 

 Figure 2 gives an idea of the accuracy with which 

 equations (10) and (11) represent the initial trans- 

 mission loss at different speeds and frequencies. 



A higher transmission loss for case (2) source hi 

 wake, than for case (1) source beyond wake, appears 

 to be well estabUshed observationally, but the theo- 

 retical explanation for this systematic difference is not 

 at all evident. With the source located inside the aer- 

 ated water of the wake, air bubbles are Ukely to be 

 held on the face of the transducer by adsorption. 

 There are theoretical reasons for believing that such 

 a layer of adsorbed gas should reduce, or "quench," 

 the output of the transducer, causing an apparent 

 increase of the transmission loss in case (2) . However, 

 it is somewhat surprising that the quenching effect 

 should show a behavior regular enough to follow 

 equation (10). 



The difference in the decay rate for case (2) the 



source in wake and case (1) the source beyond wake 



is 



dH^ dHw 1 n J, • . 



= 1.8 db per mmute. 



dt 



dt 



This difference may not be significant in view of the 

 standard errors of these quantities. However, if it is 

 accepted at its face value, the relative rates of decay 

 are equal to each other, and given for fresh wakes by 

 the equation 



J_ dHu 

 Hy, dt 



1 dH'^ 

 H' dt 



(vfr 



(12) 



This equaUty between the two rates is evident from 

 equatrons (10) and (11) in which corresponding co- 

 efficients have the same ratio of 5/8. According to 

 equation (9) of Section 32.1, the relative rate of decay 

 is a function solely of the rate of disintegration of the 

 bubble population. The physical significance of the 

 observed decay rate will be discussed in Section 34.4. 

 Some incidental information on the transmission 

 loss across wakes has been obtained during measure- 

 ments of the underwater sound output at 5 kc of a 

 destroyer, cruiser, and aircraft carrier ^ observed at 

 varying speeds. Measurements at higher frequencies 

 were also made, but the results are inconclusive as 

 far as the transmission loss across wakes is concerned. 

 All that can be said about the transmission loss at 25 

 and 60 kc is that it is distinctly higher than at 5 kc; 



residual sound intensities, after passage through the 

 wake, in most cases had dropped to the background 

 noise and thus made impossible an evaluation of the 

 transmission loss. Even the 5-kc data, plotted as a 

 function of age of the wake, are rather widely scat- 

 tered. But for each of these vessels the plot is not in- 

 consistent with tentative predictions made from 

 equation (11) above, a fact that is somewhat sur- 

 prising in view of the dimensions of two of these three 

 ships listed in Table 1. The initial transmission loss 



Table 1. Ship dimensions. 



(t = 0) is about 15 db at 5 kc for each vessel, while 

 formula (11) gives 17 db at 25 knots for 5-kc sound. 

 At higher frequencies, the greater absolute value of 

 the transmission loss might faciUtate the detection 

 of possible differences between the destroyer and the 

 larger ships. 



32.3.2 Two- Way Horizontal Trans- 

 mission Loss 



Another method of measuring the horizontal trans- 

 mission loss has been tried out in experiments aimed 

 at a simultaneous determination of wake echo 

 strength and transmission loss.' The E. W. Scripps, 

 running at 9.5 knots on a straight course, laid a wake 

 between the USS Jasper (PYcl3) and a target sphere 

 3 ft in diameter buoyed at a center depth of 6.5 ft. 

 The drop in the apparent target strength after the 

 wake was introduced thus was taken to represent the 

 two-way transmission loss across the wake. The wake 

 echo intensity could also be measured on each oscillo- 

 gram, giving the effectiveness of the wake as a 

 scatterer of sound. A plot of the sphere and wake 

 echo levels for one of the 24-kc runs is shown in 

 Figure 7 of Chapter 33. The results are summarized 

 in Table 2; further reference to the decay rate of the 

 echo strength will be made in Section 33.4. 



The 45 and 60-kc data on which the echo strength 



