532 



OBSERVATIONS OF WAKE ECHOES 



6 ft in front of the transducer and in the same orienta- 

 tion as in the actual measurements. Such determina- 

 tions were made for each frequency used in the meas- 

 urements. The results were found to be independent 



Table 10. Direct signal as function of frequency. 



Frequency in kc 

 Direct signal Zo 



30 

 22.5 



40 

 26.2 



50 

 34.0 



60 

 36.8 



of depth, as would be expected, and were reasonably 

 constant from day to day. Relative minor variations 

 are probably attributable to small differences in the 

 spacing of the two instruments. Values of the direct 

 signal, which will be called Za, measured in this way 

 are given in Table 10. On the basis of these results, 

 it is a simple task to calculate the relative intensity 

 of the reflected sound. This, in terras of decibels, is 

 simply Zr — Zo- Table 11 gives the values of Zr — Zo 

 obtained with each of the two propellers at a depth 

 of 10 ft. In arriving at these results values of Zr were 

 read off the smooth curves of Figures 9 and 10; 

 values of Zo were taken from Table 10. 



The intensities of the scattered sound at other 

 depths are, of course, less than these, in accordance 

 with the way in which the curves of Figures 9 and 10 

 drop off. It is evident that there is no considerable 



Table 11. Reflected sound as function of frequency. 



effect of frequency between 30 kc and GO kc. The de- 

 crease of the echo intensity with depth is again a 

 manifestation of the influence of increased pressure 

 on the formation and dissolution of bubbles, as in the 

 decrease of the attenuation with depth described in 

 Section 32.5. 



