368 



DIRECT MEASUREMENT TECHNIQUES 



21.3.1 



San Diego 



At San Diego, 35-mm film, running at a speed of 

 either 2.5 or 12.5 in. per sec, was exposed to traces on 

 a cathode-ray oscilloscope and then processed and 

 read on an illuminated viewer. Peak echo amplitudes 

 were measured in millimeters, corrected where neces- 

 sary for the width of the spot of light on the oscillo- 

 scope screen, and averaged over a series of echoes. The 

 average was then converted to mean-square pressure 

 level in terms of the calibration constants of the 

 equipments. The transducer and accessory equip- 

 ment were calibrated before and after each run with 

 an auxiliary calibrated transducer, lowered on a boom 

 from the side of the Jasper; Section 21.4 comprises a 

 discussion of calibration errors. 



In computing target strengths at San Diego, cor- 

 rection was also made for the deviation of the target 

 from the axis of the sound beam on the basis of beam 

 patterns measured in the laboratorj% In addition, the 

 range was found by measuring the distance between 

 the midpoints of the echo and the signal on the film, 

 and referring to index marks recorded every 50 msec 

 at the bottom of the film, corresponding to range 

 intervals of about 40 yd. From calibration data, the 

 source level was calculated, which together with the 

 echo level, and the transmission loss as measured dur- 

 ing the opening and closing runs or, less accurately, 

 estimated from prevailing oceanographic conditions 

 but neglecting possible surface reflections, gave the 

 target strength. Simultaneous sound-range recorder 

 records provided a convenient check on the oscillo- 

 grams. 



21.3.2 



Fort Lauderdale 



A similar procedure was followed by the groups at 

 Fort Lauderdale in analyzing the echoes obtained 

 there. Here, however, the film moved more slowly, 

 at a speed of approximately 1 in. per sec ; only 50 ft of 

 film could be accommodated inside the camera. Con- 

 sequently, the echoes were compressed horizontally 

 and were less detailed, but were still readily measur- 

 able. 



The fine detail of the oscillograms made at San 

 Diego enabled close determination of echo length as 

 well as a study of echo structure for short pulses; this 

 information was supplemented by an examination of 

 echoes registered on the sound-range recorder. Target 

 strength determinations from the films recorded at 

 Fort Lauderdale, however, may be more accurate 



than that recorded at San Diego since the motion of 

 the transducer was better controlled, the fluctuations 

 smaller, and the values more consistent. 



21.4 



CALIBRATION ERRORS 



Errors in target strengths measured directly must 

 be due to errors in the echo level, the source level, or 

 the transmission loss, since these target strengths are 

 computed from equation (6) in Chapter 19. Incorrect 

 echo level or source level determinations are usuallj^ 

 attributed either to errors in calibration, or to errors 

 in reading the echo level from the trace of the echo 

 recorded oscillographically which UCDWR observers 

 estimate as 2 or 3 db at the most. This section de- 

 scribes errors attributable to calibration of the equip- 

 ment; uncertainties in the evaluations of the trans- 

 mission loss are discussed in Section 21.5. 



21.4.1 Purpose of Calibration 



In target-strength studies, the principal purpose of 

 calibration is not so much the absolute determination 

 of the source level and the absolute determination of 

 the echo level, but rather the measurement of the 

 difference between the two levels. In other words, it 

 is necessary to know only the sum of the transducer 

 output as a projector and response as a receiver if the 

 echo level is measured in terms of the voltage across 

 the terminals of the transducer. Then the difference 

 between the echo level and the source level is simply 

 the difference between (1) the echo level, in decibels 

 above one volt, and (2) the sum of the projector out- 

 put and receiver response of the transducer. 



The latter sum can be obtained by means of 

 auxiliary transducers, without bothering about actual 

 sound pressures. One scheme may employ an auxiliary 

 hydrophone and an auxiliary projector. As a first 

 step, the hydrophone could be lowered from a boom 

 on the echo-ranging vessel, a few yards away from the 

 transducer to be calibrated, and the transducer out- 

 put measured in terms of the response of the hydro- 

 phone. Then the auxiliary hydrophone and the trans- 

 ducer, close together, are both exposed to sound from 

 the auxiliary projector some distance away. Thus, 

 the response of the transducer could be compared 

 with the response of the auxiliary hydrophone ; com- 

 bining the measurements would give the desired cali- 

 bration of the transducer. 



