510 



OBSERVED TRANSMISSION THROUGH WAKES 



20 25 



DEPTH IN FT 



Figure 3. Ten-inch propeller, 1,600 rpm. 



ation coefficients of 300 to 6,000 db per kyd. This 

 enormous difference is apparently real but has not 

 yet been explained. 



32.5 TRANSMISSION LOSS IN MODEL 

 PROPELLER WAKES 



Attenuation measurements on wakes of ships under 

 way have been supplemented by experiments with 

 wakes of a stationary model propeller. At the Woods 

 Hole Oceanographic Institution,^ an electrically oper- 

 ated device was constructed for driving submarine 

 propellers at speeds ranging from 266 to 1,600 revolu- 

 tions per minute at various depths. This equipment 

 was used in water 70 ft deep. 



First the relation between sound output and speed 

 of the propellers at constant depth was studied, and 

 the critical speed marking the onset of cavitation was 

 determined. Four propellers ranging from 10 to 20 in. 

 in diameter were employed. The noise level increased 

 sharply whenever the tip speed of the propeller 

 blades exceeded 33 ft per sec. In earlier experiments 

 with 2-in. propellers, mounted in an experimental 

 chamber, a critical speed of 35 ft per sec had been 

 found at the same hydrostatic pressure. The agree- 

 ment between these two figures appears quite satis- 

 factory. 



Precise measurements of the attenuation were ob- 

 tained by an arrangement in which the transducer 

 and hydrophone were mounted on opposite sides of 

 the wake on a pipe frame attached to the boom carry- 

 ing the propeller and held rigid by wire stays. The 

 instruments were 9 ft behind the hub of the propeller. 

 In this way the axis of the wake was made to pass 



20 

 DEPTH IN FT 



Figure 4. Fourteen-inch propeller, 1,600 rpm. 



between the transducer and the hydrophone, which 

 were on opposite sides of it at a fixed distance of 6 ft. 

 This arrangement had the advantage that it was 

 easy to handle and could be used in deep water with 

 complete assurance that the position of the instru- 

 ments relative to the propeller would not change. 

 However, it did not allow any_ variation of the dis- 

 tance between the instruments a'nd the propeller. 

 Hence, it was impossible to determine the decay rate 

 along the wake. 



With this arrangement measurements of sound at- 

 tenuation were made systematically at different 

 depths and with different frequencies. Each measure- 

 ment of attenuation involved the observation of the 

 response of the hydrophone under three conditions: 

 (1) with oscillator on and the propeller at rest; (2) with 

 the oscillator on and' the propeller turning; (3) with 

 oscillator off and the propeller turning. By suitable 

 combination of these data it was possible to correct 

 the observations for the noise produced by the pro- 

 peller. Typical results for the different propellers are 

 illustrated in Figures 3 and 4. 



First, it will be noted that the attenuation in- 

 creases with frequency, being almost absent at 10 kc 

 and rising steadily to 60 kc. This increase with fre- 

 quency, at any fixed depth, is so steep that it defi- 

 nitely exceeds the increase with frequency of the 

 transmission loss through destroyer wakes, which is 

 approximately proportional to the square root of the 

 frequency [see Section 32.3.1, equations (10) and 

 (11)]. Second, at each frequency, the attenuation 

 diminishes considerably with depth. This effect is 

 more pronounced at the higher frequencies. Since the 

 destroyer wakes have an average depth of 20 ft, and 

 the transmission loss through them is a sort of aver- 



