SELECTION OF TESTING SITE 



39 



required, in addition to the transducers used in the 

 tests, a considerable amount of auxiliary electrical 

 equipment such as amplifiers, modulators, imped- 

 ance bridges, and recorders. This equipment should 

 all be located in relative proximity to the transducers 

 which are used, so that transmission losses occurring 

 in the propagation of electrical signals from trans- 

 ducers to measuring instruments are as small as pos- 

 sible. If possible, it is most satisfactory to have the 

 electrical measuring equipment directly over the 

 testing location, although signals may be transmitted 

 from the end of a short pier without serious loss. 

 Transmission of signals from a barge to shore, how- 

 ever, may not be as satisfactory as is to be desired. 

 Thus, the location of the test equipment should be 

 taken into account in selecting a test site. 



5.2.3 



Small Lakes as Testing Sites 



In Sections 5.2.3, 5.2.4, and 5.2.5, the relative ad- 

 vantages and disadvantages of the various available 

 testing sites are discussed. USRL has largely con- 

 fined its testing to small lakes and indoor tanks. The 

 former have been found to be a relatively satisfactory 

 type of site for calibration testing. While they have 

 certain disadvantages, the methods developed to 

 compensate for these have proved quite successful. 



The principal disadvantage of a small lake is its 

 relatively shallow depth and. in some cases, its small 

 size. This usually limits the testing distance that can 

 be used, principally because of reflections from the 

 surface and bottom. However, methods described 

 later in this chapter have made it possible to elimi- 

 nate, to a considerable extent, the effect of these re- 

 flections on tests, except at relatively low frequencies 

 (principally below 10 kc). On the other hand, a small 

 lake has the advantages of low ambient noise level 

 and the possibility of working in all but very bad 

 weather. 



If testing is done from a pier, as is usually the case 

 at Mountain Lakes and Orlando, the problem of 

 rigging for tests is fairly easily solved, and equipment 

 may be located on shore without introducing serious 

 transmission line problems. Small lakes are also usu- 

 ally free from rapid currents and tides, thus simplify- 

 ing the problem of rigging even more. While 

 temperature gradients of sizable magnitude occur in 

 small lakes, the small testing distances which are 

 used (rarely over 50 feet) reduce the effect of this 

 factor to relative insignificance. b A general disadvan- 



tage of all outdoor testing sites is the fact that the 

 temperature of the water cannot be controlled, as it is 

 determined by climatic and weather conditions. 



In general, the same considerations apply to ponds 

 as to small lakes, but the relatively smaller size in- 

 troduces the difficulty that reflections from the shore 

 may be quite serious, especially in such tests as the 

 rear response of highly directional projectors. On the 

 whole, however, small lakes probably provide the 

 most generally satisfactory outdoor testing sites for 

 calibration testing. 1 ' 



524 Large Bodies of Water and Rivers 

 as Testing Sites 



The principal advantage of a large body of water, 

 such as the ocean or a large lake, is the great depth 

 which usually can be attained. This reduces interfer- 

 ing reflections and permits the use of greater testing 

 distances, thereby eliminating proximity effects in 

 calibration. 



The number of disadvantages is quite large, how- 

 ever. The ambient noise level is generally high be- 

 cause of large waves and water traffic. Accessibility 

 becomes a serious problem and is usually severely 

 limited by weather conditions. The rigging of test 

 devices at sufficient depths to realize the advantage 

 of deep water is a major problem. Long rigid suspen- 

 sions must be employed and these must be of suffi- 

 cient strength to withstand the bending moments 

 induced by rolling and pitching of the vessel from 

 which the instruments are suspended. Large vessels 

 must be employed if the greater testing distances are 

 to be attained, while if two separate vessels are used 

 to support the receiver and transmitter, the problem 

 of maintaining relative orientations becomes exceed- 

 ingly serious. The effect of thermal gradients also 

 becomes important. On the whole, it may be said that 

 such difficulties make it inadvisable to employ large 

 bodies of water as test sites for calibration work, 

 although they are essential to operational testing. 



Rivers suffer many of the disadvantages of oceans 

 or large lakes, such as a high ambient noise level due 

 largely to water traffic, frequent periods of inacces- 



i' Other laboratories have sometimes indicated considerable 

 trouble caused by temperature gradients, but this has not been 

 the experience at the USRL test stations. 



'■ Experience has indicated that there is no appreciable dif- 

 ference in the behavior of underwater sound equipment in fresh 

 water as compared to sea (salt) water. 



