38 



TESTING TECHNIQUE 



available may be classified as mud, sand, and rock. 

 The last is the least desirable, since it is usually a good 

 reflector. A soft mud bottom which does not contain 

 gas bubbles is probably the most desirable of the 

 three. However, most soft mud bottoms contain or- 

 ganic material whose decay produces bubbles of gas. 

 Part of this gas remains entrapped in the mud and 

 makes it a relatively good reflector. 



A fine sand or silt may be fairly absorbing. It has 

 been found, for example, that the fine sand bottom 

 at Lake Gem Mary in Orlando is a better absorber 

 for sound than the soft mud bottom at Crystal Lake 

 in Mountain Lakes. The inferiority of the latter has 

 been traced to the presence of gas bubbles in the 

 bottom. No satisfactory method has been found to 

 inhibit permanently the decay which produces the 

 bubbles, but repeated dredgings can keep the bottom 

 in a satisfactory state. 



Transmission and bottom reverberation measure- 

 ments at sea also have indicated that rock, sand, and 

 mud are successively better absorbers, in the order of 

 listing. 



Ambient Noise 



The ambient noise present in the water at a testing 

 site determines the minimum signal pressure which 

 can be measured at the locations. Ambient noise 

 is usually not a significant factor in measurements 

 except when the transmitters used have a sound out- 

 put limited to low values. Ambient noise also deter- 

 mines the lower limit at which inherent self-noise 

 measurements on transducers may be made. 



The ambient noise at a testing site may be due to 

 many factors. It may be produced by waves on the 

 surface or lapping against the shore (particularly in 

 rough weather), underwater life, water traffic, air- 

 borne sound, rain, or various other sources. Hence, 

 to keep the ambient noise as low as possible, one 

 should choose a location where waves are relatively 

 small, no water traffic is present, and noise-producing 

 underwater life is absent. Small lakes, ponds, or tanks 

 meet these conditions best. 



Accessibility and Weather 



An obvious but important consideration in choos- 

 ing a testing site is the accessibility of the site itself. 

 If one goes to a large lake or ocean and testing is done 

 far from shore, the matter of transportation to the 

 testing site becomes important. Furthermore, testing 

 may be made impossible at such a site in any but fair 



weather so that the lime available during the year 

 for testing may be relatively short. In small lakes, 

 ponds, or tanks, this is much less of a problem, and it 

 has been found that on small lakes testing may pro- 

 ceed satisfactorily except during heavy storms. A 

 homely but significant factor, if testing is done from 

 ships or barges in large bodies of water, is the possi- 

 bility of seasickness among the testing crew in any 

 sort of weather where pitching or rolling of the vessel 

 occurs. 



Temperature Gradients 



Temperature gradients in water are usually of the 

 same order of magnitude, regardless of the size of the 

 body of water. The general effect of temperature 

 gradients is the refraction of the sound beam. The 

 most effective remedy is to work -at testing distances 

 as short as possible. Consequently a location where 

 other factors allow a short testing distance is in gen- 

 eral more satisfactory from the point of view of tem- 

 perature gradients. Climatic conditions conducive 

 to minimizing temperature gradients in water are to 

 be preferred. These include cloudy days, rough water, 

 or ice on the surface. The last two interfere, of course, 

 with other aspects of calibration. 



Rigging Considerations 



Bv rigging is meant the general mechanics of hold- 

 ing transducers and auxiliary equipment in a particu- 

 lar location and with particular orientation. The 

 testing site has some influence on the problems of 

 rigging. For many tests, it is necessary to maintain 

 instruments at a constant distance apart and to main- 

 tain constant their relative orientation. This problem 

 is simplified if the rigging is done from a rigid struc- 

 ture. Therefore, piers are more satisfactory testing 

 locations than ships or barges, so far as ease of rigging 

 is concerned. When rigging is done from a ship or 

 barge, the supporting structures require additional 

 rigidity because of pitching and rolling of the vessel. 

 In addition, a pier allows vertical hanging of an in- 

 strument with comparative ease. The greater the 

 testing depth, the more difficult it is to provide a 

 rigid structure to maintain instruments at their loca- 

 tions. Thus, the advantages of working at great depth 

 to avoid surface reflections are partly vitiated by the 

 additional difficulties in rigging. 



Location of Test Equipment 



In the performance of calibration tests, there is 



