530 



OBSERVATIONS OF WAKE ECHOES 



smaller than the spread of the data that it cannot be 

 said that there is any significant difference between 

 the decay rate at 60 kc and the decay rate at 24 kc. 



Figure 8 shows a typical example of the variation 

 of the echo level, or of the wake strength, with signal 

 length. Numerical values of the variation of the echo 

 level, or of the wake strength, with signal length are 

 summarized in Table 8. 



This dependence of W on the signal length was pre- 

 dicted from general theoretical considerations (see 

 Section 33.1.2), and the observed magnitude of the 

 effect permits an estimate of the average concentra- 

 tion of bubbles in a wake (see Chapter 34). The num- 

 ber of observed data is too small to warrant any con- 

 clusions as to the influence of frequency and size of 

 the wake vessel on the pulse length effect. 



Table 9. Wake strength and decay rate, E. W. Scripps. 



Table 9 contains some additional values of W for 

 several wakes laid by E. W. Scripps on a day when 

 the sea was unusually calm. Experimental details 

 concerning these observations have already been 

 given in Section 32.3.2, and the transducers used are 

 listed in Table 3; the echo level-time curve of Run 1 

 of Table 9 is reproduced in Figure 7. 



The average W at 24 kc (mean of Runs 1 and 2) is 

 — 8 db for 3-msec pulses. In order to make this value 

 comparable with the average value of W at 24 kc for 

 the wakes of the Scripps and Jasper in Table 7, which 

 is — 13.6 db, a correction for the difference in signal 

 lengths used must be made; from Table 8 it may be 

 estimated that IFiomsee— M^imsM is of the order of 

 -1-6 db. The corrected Wn kc of Table 9 is then -2 db, 

 or about 12 db greater than Wu kc in Table 7. After 

 the corresponding correction of the 60-kc data of Run 

 3 has been made, Weo kc in Table 9 is still 17 db 

 smaller than its counterpart in Table 7; the origin of 

 this serious discrepancy remains unexplained. As for 

 the minor discrepancy at 24 kc it seems worth men- 

 tioning that the E. W. Scripps had been outfitted 

 with a new propeller and engine in the fall of 1944, 



so that the data in Tables 7 and 9 are not strictly 

 comparable. The decay rates in Table 9 do not differ 

 significantly from the averages for all surface vessels 

 quoted before. 



33.5 ECHOES FROM MODEL PROPELLER 

 WAKES 



At the Woods Hole Oceanographic Institution,* a 

 number of experiments were made on the scattering 

 of sound by the wakes of stationary model propellers. 

 Although the published data do not yield absolute 

 values of the wake strength, they give some interest- 

 ing information on the relative echo intensity as a 

 function of the frequency of sound and of the depth 

 of the propeller. 



In order to measure the scattering, the hydrophone 

 and transducer were mounted on the same side of the 

 wake in a horizontal plane including the wake axis. 

 The axis of the hydrophone was vertical and the 

 transducer was directed toward the wake. Both 

 instruments were secured to a pipe frame and were 

 separated by a baffle, in order to reduce the passage 

 of the direct signal from the transducer to the hydro- 

 phone. The baffle consisted of a sheet of Celotex 32 in. 

 square and }4 in. thick, sheathed with copper; the 

 plane of this sheet was perpendicular to the axis of 

 the wake. This single baffle was found to be preferable 

 to a wedge-shaped baffle composed of two sheets of 

 Celotex making an angle with one another. In order 

 to reduce the direct signal still further, the hydro- 

 phone was partially enclosed in a box lined with 

 Celotex and open on the side toward the wake. The 

 perpendicular distance from the instruments to the 

 wake axis was 5 ft; the plane of the baffle, midway 

 between the instruments, was 10 ft from the plane 

 parallel to it through the propeller. With this arrange- 

 ment, scattering measurements were made in the 

 deep spot 200 ft off the wharf at depths var3dng from 

 5 to 60 ft and at frequencies from 30 to 60 kc. At 

 lower frequencies the reflection was too small to 

 measure. 



Each determination of the scattering involved the 

 measurement of the signal at the hydrophone under 

 three conditions: (1) with the propeller at rest and 

 the transducer on; (2) with the propeller running and 

 the transducer on; (3) with the propeller running and 

 the transducer off. The results of these three measure- 

 ments, in decibels, will be referred to by Zi, z^, and 23, 

 respectively, \vith Z\ representing the direct signal 

 from the transducer in the absence of scattering — 



