374 



DIRECT MEASUREMENT TECHNIQUES 



Figure 8. Surface-reflected sound. 



of a combination of both constructive and destructive 

 interference throughout the duration of the echo, be- 

 tween direct and surface-reflected sound. 



A different effect produced by surface-reflected 

 sound is also indicated by more recent information 

 from San Diego.'' During echo-ranging tests on a sub- 

 marine from 90 to 200 ft deep, double echoes were 

 observed, under certain conditions, on the chemical 

 recorder and on the oscillograph — a strong primary 



echo followed by a faint secondary echo, illustrated in 

 Figure 9. This appearance of double echoes suggests 

 that some of the sound is reflected directly back to 

 the projector to form a primary echo, while some of it 

 is reflected vertically upward to the ocean surface, 

 reflected by the surface back to the submarine and 

 finally back to the projector to form a secondary 

 echo. Quantitative data show that the lapse of time 

 between the primary and secondary echoes is equal 

 to the time necessary for the sound to travel up to 

 the surface and back again, thus confirming this 

 hypothesis. 



Although almost all the sound striking the surface 

 is unquestionably reflected back into the water at 

 some angle, the perfect specular reflection expected 

 from a flat surface seems unlikely at sea. The nor- 

 mally rough surface of the ocean and the presence of 

 air bubbles tend to scatter the sound rather than 

 allow perfect specular reflection at the surface. 

 Further evidence minimizing the effects of surface 

 reflection on target strength values is seen in the 

 excellent agreement between the results of the direct 

 measurements computed neglecting surface reflec- 

 tions, and both the indirect measurements and 

 theoretical calculations, where surface-reflected sound 

 either does not appear or may be readily eliminated. 

 Partly for this reason, surface-reflected sound is 

 neglected in all target strength computations in 

 Chapters 18 to 25. However, the results shown in 

 Figure 2 of Chapter 9 and described in Section 9.2.1 

 suggest that reflection from the ocean surface is 

 frequently very nearly specular. More data are 

 needed to clarify the exact importance of surface- 

 reflected sound in practical echo ranging. At present, 

 the resulting uncertainty of 6 db is about the same as 

 the other uncertainties of observation in target 

 strength measurements. 



21.6 



VARIABILITY OF ECHOES 



Perhaps the largest source of uncertainty in target 

 strength measurements arises from variability of 

 echo intensity. Observed echoes vary widely in two 

 ways (see Section 21.1). Gradual changes in echo in- 

 tensity over a relatively long period of time from a 

 few minutes to hours are called variations. Superim- 

 posed on these variations are marked changes which 

 occur from echo to echo and are called fluctuations. 

 A large part of the variability of echo intensity is due 

 to variability in the sound-transmitting character- 



