SELECTION OF TESTING SITE 



37 



nearly is this the case. The testing distance is limited, 

 however, by the interfering relied ions which arise at 

 the hounding surfaces of medium. A compromise be- 

 tween these two factors must be effected, and much of 

 the later discussion centers around this point. 



Ambient Noise 



To insure that only the desired signal is being 

 measured, it is necessary that the ambient noise, 

 which is invariably present in any body of water, be 

 of sufficiently low level compared to the level of the 

 desired signal. In this mailer, the best that may 

 usually be achieved is the selection of a site where 

 ambient noise is naturally low. When calibrating re- 

 ceivers, a further gain may be realized by using trans- 

 mitters which can deliver an acoustic signal oi 

 sufficiently high level to make the effect of ambient 

 noise of negligible importance. 



sis Necessity for Testing Technique 



The preceding discussion indicates that the specific 

 manner in which a test is conducted is usually a com- 

 promise among several competing extraneous effects 

 which tend to prevent a measurement under ideal 

 testing conditions. The purpose of a doctrine of test- 

 ing technique is to provide a body of rules to aid in 

 determining the most advantageous compromise in 

 the choice of such factors as testing site, depth, and 

 distance, and thus to permit the most accurate meas- 

 urement possible of the quantities desired. These 

 rules are based partly on theoretical considerations 

 and partly on general experience in such measure- 

 ments. They may play as important a part in deter- 

 mining the accuracy of the results as the quality of 

 the testing equipment available. In fact, a lack of ap- 

 preciation for these rules often introduces far greater 

 errors in a measurement than any other factor. 



52 SELECTION OF TESTING SITE 



521 Available Sites 



The first question which arises in a program of 

 calibration testing is the choice of a site for the meas- 

 urements. Available sites usually fall into the follow- 

 ing five groups: 



1 . Oceans and large lakes. These may be character- 

 ized as bodies of water many miles in extent and 

 more than 100 feet in depth. They are of sufficient 

 size so that, except in relatively calm weather, surface 



waves of more than I foot in height are usually pres- 

 ent. To reach great depths one must usually go at 

 least several thousand feet out from shore. 



2. Small lakes. Inland bodies of water ranging 

 from 200 yards to several miles across may be classi- 

 fied as small lakes. Their depths may vary greatly but 

 rarely exceed 100 feet. Surface waves are relatively 

 small and, in many cases, considerable depth may be 

 reached within 50 to 100 feet from shore. 



3. Natural or artificial ponds. Bodies of water up 

 to about 200 yards across, with various depths usually 

 less than 50 feet and with relatively small surface 

 waves, are classified as natural or artificial ponds. 



4. Rivers. Although rivers vary greatly in char- 

 acter, they have more or less steady water currents. 

 They have depths rarely exceeding 50 feet and, if 

 navigable, usually carry considerable water traffic. 



5. ranks. Tanks are usually internally housed 

 structures of relatively small dimensions. For pur- 

 poses of further discussion, swimming pools may be 

 considered as tanks. 



5.2.2 F actors Entering Into Choice of Site 



Certain factors entering into a choice of testing site 

 in relation to compromises in actual testing have 

 been previously discussed, and are now considered in 

 detail along with other factors. 



Size 



The factor of size enters into the choice of site in 

 two ways: in its direct effect on measurements caused 

 by reflections from the surface, shore, bottom, etc., 

 and indirectly in its determination of other factors 

 such as homogeneity of the medium, ambient noise 

 level, accessibility, and rigging considerations. If 

 these latter considerations were not important, the 

 largest and deepest body of water available would be 

 the most preferred, for it would then approach most 

 closely to an infinite medium, and the effect of in- 

 terfering reflections could be essentially eliminated. 

 Actually, however, a compromise must be effected 

 among these various factors. 



Type of Bottom 



The surface of a body of water being in almost all 

 cases an air-water interface which reflects sound al- 

 most completely, there is not much choice in this con- 

 sideration. However, the type of bottom which is 

 present may often be of importance. Types of bottom 



