. Basic methods for the calibration of sonar equipment. TESTING TECHNIQUE. IG. Klfect of testing distance on diiectivitv pattern. (A) testing distance = 9 ft, (li) testini; distance := ft. For this transducer L-J\ = 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 velocit) and in the same phase. Actual instruments only approximate this behavior and in many cases
. Basic methods for the calibration of sonar equipment. TESTING TECHNIQUE. IG. Klfect of testing distance on diiectivitv pattern. (A) testing distance = 9 ft, (li) testini; distance := ft. For this transducer L-J\ = 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 velocit) 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, ha\e intentionally reduced sensiti\ ity 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 conditic^ns 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. Summary of Testing Geometry The important factors which determine the testing geometry to be used in calibration measurements ha\e now been discussed. It remains to summarize a procedure for selecting the optimum testing distance and testing depth. An outline is gi\eii 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
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