Chapter 30 

 TECHNIQUE OF WAKE MEASUREMENTS 



MOST OF THE MEASUREMENTS of Submarine and 

 surface vessel wakes discussed in Chapters 26 

 to 35 have been made by University of Cahfornia 

 Division of War Research [UCDWR] or by Navy 

 observers at the U. S. Navy Radio and Sound 

 Laboratory [USNRSL] in San Diego. The instru- 

 ments and physical principles applied to acoustic 

 observations of wakes do not differ essentially from 

 those employed in other underwater sound measure- 

 ments described in Chapter 4, Chapter 13, and 

 Chapter 21. It is unnecessary, therefore, to introduce 

 here detailed descriptions of instruments and their 

 theory. But before discussing the results, some general 

 features of the experimental work at San Diego on the 

 acoustic properties of wakes will be reviewed. 



30.1 LISTENING AND ECHO RANGING 



Listening through a wake to a ship under way, or 

 to a mechanical noisemaker, constitutes the simplest 

 type of acoustic observation of a wake. The presence 

 of a wake manifests itself by a reduced sound level at 

 the receiving hydrophone, compared with the same 

 level with no wake interposed. Such observations of 

 the acoustic screening effect are the incidental result 

 of numerous measurements of the sound output of 

 ships. But, in order to obtain quantitative results, 

 it is desirable to use as sound source a transducer or 

 mechanical noisemaker of constant power output, 

 instead of the noise from the screws of a ship. To- 

 gether with a hydrophone of constant sensitivity, 

 this equipment makes possible determination of the 

 transmission loss which sound undergoes in passing 

 through a wake. 



Echoes returned by wakes can be studied by lis- 

 tening or by using objective records of the current 

 generated in the receiving channel of the transducer. 

 While the second method is indispensable for the 

 determination of sound intensities, it does not tell 

 anything about the small changes in frequency that 

 are caused by the relative motion of target and trans- 



ducer. The acoustic doppler effect is helpful in dis- 

 tinguishing between the echo from a wake, which is 

 nearly stationary, and the echo from the wake-laying 

 vessel. This distinction is occasionally of practical 

 interest, as in the study of the rather weak wakes 

 produced by submarines in submerged level runs. In 

 such cases it may be useful to preserve an audible 

 record, in the form of a phonograph record, of the 

 wake echo. The supersonic echo obtained aboard the 

 experimental vessel is transmitted by short-wave 

 radio to the laboratory ashore, where the phono- 

 graphic recording can be done more conveniently 

 than on a rolling and pitching vessel at sea. 



30.1.1 Sound Range Recorder Traces 



At San Diego it is a standard procedure in all wake 

 work to secure echo records with a sound range 

 recorder of the type in general tactical use. These 

 chemical recorder traces are highly useful for a rapid 

 estimate of the range of the wake and of the decay of 

 its strength. As the chemically treated recording 

 paper is unrolled, with the machine open, the ob- 

 server makes pencil notes on the margin of the record 

 concerning the work in progress, such as the begin- 

 ning and ending of the oscillographic recording, 

 changes of the sound frequency used, and other de 

 tails. Thus the chemical recorder traces also provide 

 a graphical log of the operations. 



The general appearance of wake echoes on the 

 sound range recorder paper is illustrated by the 

 photographic reproductions of original records shown 

 in Figures 1 and 2. They are records of wakes laid 

 by the auxiliary yacht E. W. Scripps between the 

 echo-ranging vessel, the USS Jasper (PYcl3) and a 

 target sphere buoyed at a center depth of 6 ft below 

 the surface, in the course of experiments described 

 in detail in Sections 31.2 and 32.3.2. The lower part of 

 Figure 1 shows the sphere echo alone. Immediately 

 after passage of the Scripps through the sound beam, 

 there appears a strong wake echo and the strength 



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