TESTING TECHNIQUE 



Figure 16. F.tfect of testing distance on directivity pattern. (A) testing distance = 9 ft. (B) testing distance = 1.5 ft. 

 For this transducer Z.-/X = 5 ft- 



case but, as in all other cases where corrections are 

 applied, caution is necessary. The calculations are 

 carried out on the basis of the transducer's behaving 

 in a certain well-defined manner. For example, it is 

 assumed that all parts of a piston move with the same 

 velocity and in the same phase. Actual instruments 

 only approximate this behavior and in many cases 

 depart significantly from it. The amplitude of a 

 piston is often smaller at the edge than at the center, 

 or the piston may actually break up into areas which 

 oscillate out of phase. Instead of having uniform 

 sensitivity, lines are often made up of an array of 

 discrete elements and are often shaded or tapered, 

 that is, have intentionally reduced sensitivity at the 

 ends in order to suppress side lobes. The validity of 

 the theoretical formulas is then questionable. 



One must conclude, as a general rule, that a theo- 

 retical correction of more than 5 db is open to con- 

 siderable question, even if all other criteria as to 

 applicability of the theoretical correction are favor- 

 able. If possible, test conditions should be selected so 

 that corrections for spherical wave effects are avoided. 



When corrections must be made, the charts in Fig- 

 ures 13, 14, and 15 giving the corrections for a pres- 

 sure-gradient device, a circular piston, and a line will 

 be found useful. 



5.4.8 



Summary of Testing Geometry 



The important factors which determine the testing 

 geometry to be used in calibration measurements 

 have now been discussed. It remains to summarize a 

 procedure for selecting the optimum testing distance 

 and testing depth. An outline is given in the follow- 

 ing. 



1. Select the testing depth. If the reflection coeffi- 

 cient of the bottom is known, Figure 12 may be used. 

 If the coefficient is not known, a testing depth of from 

 Vi to 3 A tne water depth is usually satisfactory. If the 

 water is very deep, the greatest depth consistent with 

 satisfactory rigging of the instruments is desirable. 



2. If both instruments are nondirectional, as small 

 a testing distance as possible is desirable. It should 



