ECHOES FROM MODEL PROPELLER WAKES 



531 



20 30 



DEPTH IN FT 



Figure 9. Dependence of sound scattered from 10- 

 inch propeller at 1,600 rpm on depth below surface. 

 Direct signal constant for each frequency. 



the sound which travels around and through the 

 baffle, and Zz representing the cavitation and pro- 

 peller noise. The true value of the scattered sound in 

 decibels, which we call Zr, is in general different from 

 Zi but may be obtained from it by correction for the 

 direct signal (zi) and for cavitation and propeller 

 noise (zs). It is given by the equation 



10^ 



|2r/lO 



= 10^ 



|Z2/lO 



10 



|Zl/lO 



10^ 



iZa/io 



The results presented below were calculated in this 

 way. It should be pointed out that the effect of Zz was 

 in all cases negligible. 



An interval of a minute or a minute and a half was 

 always allowed between successive determinations to 

 make sure that there should be no residual wake from 

 the previous determination to interfere with the fol- 

 lowing one. Only the 10-in. and 14-in. propellers were 

 used, and only at the highest speed, 1,600 rpm. 

 Under other conditions the scattered sound was too 

 small to measure satisfactorily. 



The results of the study are shown in Figures 9 and 

 10. Although the scatter of the observations is large, 

 particularly with the 14-in. propeller, there can be no 

 question of the general effect. It is evident that there 



Figure 10. Dependence of sound scattered from 14- 

 inch propeller at 1,600 rpm on depth below surface. 

 Direct signal constant for each frequency. 



is a marked decrease in sound scattering with depth. 

 At a frequency of 60 kc the scattered sound is less 

 than 3'lo as much at 60 ft as at 5 ft. In this respect 

 the situation is similar to that observed in the case of 

 attenuation (see Section 32.5). 



The data plotted in Figures 9 and 10 give simply 

 the total reflected sound in decibels at the hydro- 

 phone. They take no account of the strength of the 

 direct signal from the transducer. Since the oscillator 

 was always set to give the same output, this signal 

 may be regarded as constant for each frequency. 

 Consequently at each frequency the change in the 

 decibel level of the reflected signal with depth gives 

 the change in the scattering coefficient. Nevertheless, 

 in order to obtain absolute values of the scattering 

 coefficient and to discover its dependence on fre- 

 quency it is necessary to take into account the 

 strength of the direct signal which would be received 

 by the hydrophone in the absence of a wake at the 

 position of what may be called the "virtual image" of 

 the hydrophone with respect to the wake. This is a 

 point at the same distance from the wake as the 

 hydrophone, but on the opposite side of it. It was 

 estimated to be 6 ft away from the transducer. With 

 this in mind, throughout the study, daily determina- 

 tions were made of the response of the hydrophone 



