4§2 



TECHNIQUE OF WAKE MEASUREMENTS 



duced to approximately + 1 degree. The maximum 

 error is, therefore, + 6 degrees and the probable error 

 + 3.5 degrees for data taken prior to April 1944, and 

 + 5 degrees and ±3 degrees, respectively, for data 

 taken subsequently. 



At very short ranges there is another correction 

 which may have to be applied because of parallax 

 resulting from horizontal spacing between pelorus 

 position and projector axis. On the Jasper this cor- 

 rection amounts to 2.5 degrees for the aft projector 

 and 3.5 degrees for the forward projector, when the 

 target is 100 yd away and bears either 90 or 270 

 degrees relative to the sending vessel. 



Finally there are training errors due to rolling and 

 pitching of the sending vessel. This error can be 

 serious at close range since rolls of 45 degrees have 

 been experienced on the Jasper and rolls of over 20 

 degrees are common in moderate weather. For the 

 same vessel the pitching angle is of considerably 

 smaller magnitude than that of the roll, rarely ex- 

 ceeding 7 degrees. Installation of a device to record 

 angle of roll, pitch, projector heading, target bearing 

 and ship's heading for each sound pulse emitted, has 

 been of great help in recognizing and rejecting 

 acoustic observations that have been impaired by 

 serious training errors. 



30.2.2 



Field Calibration 



The transmitting system's absolute output has to 

 be checked at the beginning and end of each day's 

 operation and also during the operation if excessive 

 variations are encountered. For this purpose an aux- 

 iliary transducer, whose performance is known from 

 absolute caUbration in the testing laboratory, is 

 lowered into position by means of a special boom 

 which pivots at the rail and swings down to projector 

 depth. To check the actual output of the transducer 

 in use, it is trained on the auxiliary transducer to 

 give a maximum generated voltage; and from the 

 laboratory calibration of the auxiliary transducer, the 

 sound field pressure is determined. For the inverse 

 calibration process, the known power output of the 

 auxiliary transducer is received by the working trans- 

 ducer and the generated current is recorded as in field 

 work. Any appreciable deviation of any of the read- 

 ings from those normally experienced requires an im- 

 mediate investigation to determine the source of the 

 difficulty. 



According to the experience of the San Diego group 

 with the JK projectors, the standard deviation of the 



output pressure level was only 1.3 db over a period of 

 15 months,and 0.4 to 1.0 db for groups of consecutive 

 cahbrations within that sequence. However, the 

 standard deviation of the sensitivity of the receiving 

 channel was 3.9 db for the same period and varied 

 from 0.4 to 2.0 db for groups of calibrations within 

 that period. The causes of this variation are un- 

 known. In the course of one day, changes in overall 

 sensitivity, which is the sum of the projector output 

 and the sensitivity of the receiving channel, are 

 negligible; changes in output level did not show any 

 correlation with changes in receiver sensitivity. Also, 

 over the whole period under discussion, changes in 

 output level are not correlated — or at most are 

 weakly correlated — with changes in receiver. sensi- 

 tivity. All these observations refer to 24-kc sound. 

 Incomplete evidence suggests that the performance 

 of 60-kc sound gear is even more variable. 



The existence of large caUbration errors is also 

 suggested by certain discrepancies among the San 

 Diego data on the target strength of spheres, which 

 are discussed in Section 21.4.3. 



30.2.3 Oceanographic Factors 



The weather and state of the sea appears to have 

 some influence on the formation and gradual dis- 

 solution of wakes. It is advisable, therefore, to keep 

 a careful record of the circumstances prevailing at the 

 time of the observations. The momentary oceano- 

 graphic conditions have a profound effect also upon 

 the propagation of underwater sound. Hence, it has 

 become a standard practice to secure bathythermo- 

 grams before and after each set of acoustic observa- 

 tions. The transmission loss in the ocean intervening 

 between sound gear and wake, which must be known 

 in order to correct the measured data, is difficult to 

 determine directly. So far, acoustic observations on 

 wakes have not reached such a high degree of pre- 

 cision as to make it imperative, as in the measure- 

 ment of target strengths, to determine the transmis- 

 sion loss in the ocean simultaneously with the wake 

 observations. For details on the technique of meas- 

 uring the transmission loss consult Chapter 4. 



Perhaps the most serious disturbances of under- 

 water sound measurements are the rapid and un- 

 predictable changes of the transmission loss, generally 

 referred to as fluctuations and described in Chapter 7, 

 which may amount to many decibels over intervals of 

 only a few seconds. The only way to minimize their 



