488 



TECHNIQUE OF WAKE MEASUREMENTS 



mm- ■■•.: ■ * 



I 



it 1 • 1 • « i • 1 



• 1 ¥ 



Figure 5. Fathometer record of wake echoes from USS Hopewell (DD681). 



ous markings on this record is uncertain; probably 

 they are of instrumental origin. The wake is seen to 

 be 30 ft thick at the maximum point. 



Figure 6 contains two records of the wake (17 and 

 1 1 ft thick, respectively) of the Swing, proceeding at 

 13 knots; these records were not suitable for trans- 

 mission loss calculations, since the amplification was 

 increased to record the cross-sectional geometry of 

 the wake. Comparison of Figures 4 and 6 gives an 

 idea of the variations of wake structure occurring in 

 practice; the vessel and speed are the same for both 

 figures. For the proper interpretation of these cross 

 sections, it should be remembered that the sound 

 beam of the customary fathometer is rather broad, 

 including an angle of about 30 degrees, thus causing 

 the fine structure of the cross section to be smoothed 

 out. 



30.1.3 Oscillograms 



In order to obtain permanent sound intensity 

 records suitable for quantitative measurements, the 



current generated in the hydrophone is amplified and 

 fed into a cathode-ray oscilloscope, the screen of 

 which is photographed continually by a high-speed 

 camera on standard moving picture film, as described 

 in Section 4.3.3, Section 13.1.1, and Sections 21.2.1 

 and 21.3.1. The developed negative shows a con- 

 tinuous trace, representing the varying displacement 

 of the luminous spot from its normal position on the 

 oscilloscope screen. Time marks are photographed at 

 suitable intervals as the film moves along steadily. 

 By appropriate design of the electric circuits the dis- 

 placement of the oscillographic trace is made propor- 

 tional to the amplitude of the incident sound wave. 

 The square of the amplitude of the oscillographic 

 trace, therefore, is proportional to the intensity of the 

 sound wave, at the face of the' hydrophone, multi- 

 plied by a factor depending upon the directivity of 

 the hydrophone. If the sensitivity and the directivity 

 pattern of the hydrophone are known, the scale of 

 ordinates on the oscillogram can be caHbrated in 

 absolute units to yield the sound pressure in dynes 

 per square centimeter. 



