TRANSMISSION LOSS IN MODEL PROPELLER WAKES 



511 



age over this entire range of depths, the second effect 

 partially cancels the first one. Moreover, the bubble 

 population found in a destroyer wake may be quite 

 different from that in the wake of the stationary pro- 

 peller (zero slip), and any variation of the relative 

 abundance of bubbles of different sizes is likely to pro- 

 duce frequency-dependent acoustic effects. Hence, 

 the discrepancy noted above is not alarming. 



In the case of the 10-in. propeller (Figure 3), the 

 attenuation falls to almost zero at a depth of 40 ft for 

 all frequencies. This phenomenon is consistent with 

 the results obtained in the model chamber mentioned. 

 In the model experiments it was found that fewer 

 nonpersistent cavities were formed at higher pres- 

 sures. According to the mechanism of bubble forma- 

 tion described in Section 27.1 higher pressure causes 

 a more rapid collapse of the cavities formed, before 

 there is time for a considerable amount of gas to 

 diffuse into them; moreover, cavities containing a 

 given amount of gas are compressed into bubbles of 

 smaller size at higher pressures. 



Observations were also made at two frequencies 

 with transducer and hydrophone in the wake, both 



mounted in the wake axis, but their number is small 

 and no clear-cut conclusions can be drawn from them. 

 There is some indication that for 50-kc sound the out- 

 put of the transducer may be reduced, or "quenched" 

 by the wake, but for 10 kc the "quenching" effect, if 

 it exists at all, is very much smaller than for 50 kc. 

 In summary, the observations of wakes produced 

 by a stationary propeller are in reasonable agreement 

 with those of destroyer wakes, as far as the depend- 

 ence on frequency of the transmission loss is con- 

 cerned. By dividing the attenuations plotted in 

 Figures 3 and 4 by the distance between transducer 

 and hydrophone (6 ft), attenuation coefficients can be 

 computed. For instance, at a depth of 15 ft attenu- 

 ation coefficients of 3.6 and 2.3 db per yd are found 

 for 25-kc sound, which is the same order of magnitude 

 as found for destroyer wakes at speeds of 10 to 15 

 knots. Since the diameter of the wake probably was 

 smaller than the distance from transducer to hydro- 

 phone, the values quoted for the attenuation coef- 

 ficient are actually lower limits; the true attenuation 

 coefficient may have been greater by 50 to 100 per 

 cent. 



