386 



INDIRECT MEASUREMENT TECHNIQUES 



22.2 



SUBMARINE REFLECTIVIT\ 



Since indirect target strength measurements are 

 measurements not on actual submarines but on their 

 scale models, certain possible corrections must be 

 considered before the results can legitimately be 

 compared with the results of the direct measure- 

 ments. One possible source of error is in the reflec- 

 tivity of the models used as compared with the re- 

 flectivity of actual submarines. 



Since the experiments at UCDWR were qualitative 

 in nature and designed only to determine the principal 

 reflecting surfaces on a submarine, the question of 

 absolute reflectivity is unimportant for those tests. 

 The optical experiments at MIT, however, reported 

 specific target strengths. These results were com- 

 puted from the expression for the target strength of 

 a perfectly reflecting sphere, in other words, a sphere 

 which reflects all the sound striking it without trans- 

 mission or absorption. Since both the submarine 

 models and the spheres were finished in exactly the 

 same way, these target strength results will apply 

 only to perfectly reflecting submarines. 



At USRL, the reflectivity of the hull itself was 

 found to be perfect, within experimental error, over 

 the range of frequencies used. The hollow model was 

 first tested filled with air, then filled with water. No 

 difference was observed in the intensity of the re- 

 flected sound for all frequencies between 50 and 2,000 

 kc. Since reflection from an air-filled hull would be 

 almost perfect, regardless of the transparency of the 

 hull to sound, and since reflection from a water-filled 

 hull submerged in water would come solely from the 

 hull, with no air-water interface to reflect the sound, 

 the experimental results did not justify assuming any- 

 thing less than perfect reflectivity. Thus both the 

 optical and acoustical indirect measurements are 

 based on perfect reflection of the sound striking the 

 submarine; transmission through the hull and ab- 

 sorption in the steel are neglected. 



The steel hull of an actual submarine is also almost 

 perfectly reflecting. Therefore, it appears that the re- 

 sults of the indirect measurements may be inter- 

 preted in terms of sound reflected from actual sub- 

 marines. However, the presence of barnacles, moss, 

 and other marine growth on the hull may appreciably 

 affect the reflectivity. Such an effect would be im- 

 portant for surface vessels or surfaced submarines, 

 where the fouled hulls are exposed to the direct sound 

 beam, but might not be significant for a submerged 

 submarine, since the sound beam might not often 



strike the lower part of the hull where such growths 

 attach themselves. No measurements have been 

 made to ascertain the effect of barnacles and moss on 

 the reflection of sound, but it is not believed to be 

 significant Therefore it appears that reflectivity 

 considerations should not greatlj"^ affect any compari- 

 son between direct and indirect measurements. 



22.3 



WAVELENGTH EFFECTS 



If the indirect measurements of target strengths 

 with submarine models are to be trusted, the experi- 

 ments must be properly scaled, that is, the dimen- 

 sions of the models, the ranges and depths at which 

 the tests are made and all the wavelengths must be 

 reduced by the same factor. This factor was 60 for 

 the acoustical measurements at USRL, and all the 

 quantities relevant to the measurements were changed 

 by this factor. 



At MIT-USL, however, visible light was used. The 

 models used in the optical experiments were from 60 

 to 120 times smaller than the submarines they repre- 

 sented. Assume an echo-ranging frequency of 24 kc, 

 and the corresponding scaled wavelengths would be 

 reduced to 0. 1 cm for a 1 : 60 scale or 0.05 cm for a 

 1 :120 scale. Since the actual wavelengths employed 

 were much shorter, errors might be expected in the 

 results. 



Two errors in particular might be introduced. At 

 certain aspects where the surface of the submarine 

 subtends only a few Fresnel zones at 24 kc, as de- 

 scribed in Sections 20.3 and 20.5, the model subtends 

 many such zones, since the wavelength is much 

 shorter compared with the dimensions of the sub- 

 marine. As a result, the Fresnel integrals approach 

 their asymptotic values, especially for surfaces of 

 large radius of curvature, such as planes or cylinders, 

 which subtend many Fresnel zones. Since the conning 

 tower on a submarine is relatively flat, the optical 

 measurements with very short wavelengths may 

 overemphasize the effect of the conning tower. 



Secondly, nonspecular reflection is less than if the 

 wavelength was properly scaled, by a factor equal to 

 the square root of the ratio of the properly scaled 

 wavelength to the improperly scaled wavelength 

 actually used. This may account for the extremely 

 low target strengths obtained optically at aspects 

 giving very little specular reflection, such as the bow 

 and stern. 



Diffuse reflection or scattering may be excessively 

 large optically since the wavelength may be con- 



