78 



EXPERIMENTAL PROCEDURES 



the sound level at 100 yd to the source level as 

 defined m Section 4.1. 



WHOI used a related method of calibration until 

 the summer of 1945. For each individual run, the ob- 

 served sound field levels were each increased by 

 20 log R and plotted against range. The resulting 

 points between a range of 100 yd and the range where 

 the observed intensity was 40 db less than the in- 

 tensity at 100 yd were then fitted by inspection with 

 a straight line. This line was extrapolated to a 

 range of 1 yd to give the presumable sound level (in 

 decibels above 1 nv) at that range. 



More recently, both institutions have put into use 

 new methods, which are designed to obtain a calibra- 

 tion directly in terms of the short-range (1 to 10 yd) 

 sound level. These methods are of two kinds, which 

 may be described as unaided calibrations and as 

 calibrations with the help of standards. As an example 

 of unaided calibration, WHOI floats one of the re- 

 ceiving hydrophones out to a distance where it can be 

 picked up safely by the crew of the sending ship. The 

 hydrophone remains connected by cable with the re- 

 ceiving ship, and its output is recorded by the same 

 equipment used in the transmission runs. The hydro- 

 phone is then secured at a measured distance of a few 

 yards from the face of the projector. The projector is 

 trained on the hydrophone, and a signal is put into 

 the water and received by the hydrophone. At this 

 short range, the effect of the surface is minimized by 

 the directivity of the projector, and tests have shown 

 that the sound field obeys the inverse square law 

 within the limits of observational accuracy. This 

 method of calibration would be expected to yield the 

 most accurate and most trustworthy results. A sub- 

 stantially identical method is occasionally employed 

 by UCDWR for checking the results of other calibra- 

 tion methods. The unaided methods are not always 

 practical in the field since in a heavy sea the transfer 

 of a hydrophone from one ship to the other may not 

 be possible. At best, the hydrophone transfer is a 

 cumbersome and time-consuming maneuver. Never- 

 theless, unaided calibration is standard procedure at 

 WHOI. 



Most of the field calibrations at UCDWR are now 

 made with the help of calibration standards, sound 

 units which are designed primarily for this purpose 

 and which are more stable than other units. Typical is 

 a UCDWR procedure which involves the use of two 

 OAX transducers; these transducers are HUSL de- 

 signs. One of these two transducers is kept aboard 

 the receiving vessel, the other aboard the sending 



ship. Aboard the sending ship, where the projector 

 source strength S is to be determined, the OAX is 

 used as a hydrophone. It is attached to a boom which 

 can be swung over the side and which insures that the 

 OAX is always at the same distance (13 ft) from the 

 face of the keel-mounted JK projector. The projector 

 is trained on the OAX, and the voltage generated 

 across the terminals of the OAX unit by the sound 

 field of the JK projector is measured. Aboard the 

 receiving ship, where the hydrophone response S is 

 sought, the second OAX unit is hung over the side 

 amidships to a depth where it clears the keel. The 

 receiving hj'drophone is also hung over the side, on 

 the other side of the ship, and is lowered to the same 

 depth as the OAX. The distance between the two 

 imits is, therefore, with fair accuracy, the beam width 

 of the receiving ship. The OAX is then energized as 

 a projector with a standard power input, and the gen- 

 erated voltage across the terminals of the receiving 

 hydrophone measured. If these two tests lead to the 

 same results or very nearly the same results day after 

 day, it is assumed that all four units are constant. 

 Large jumps (several decibels) are presumably indi- 

 cations that either one of the four sound heads or the 

 electrical follow-up (amplifiers and associated equip- 

 ment) has changed its characteristics. If the change 

 cannot be assigned to the electrical follow-up, it is 

 assumed that the standard OAX units have remained 

 unchanged and that either the JK power output or 

 the receiving hydrophone response has changed. In a 

 word, the characteristics of the projector and hydro- 

 phone used in the transmission runs are measured in 

 terms of auxiliary standards, which are presumed to 

 be stable. The standards themselves are thoroughly 

 tested every few months at a special calibration 

 station. 



For a time, WHOI also used a similar method of 

 calibration that depended upon the use of HUSL 

 monitor standards. This method of calibration was 

 later abandoned by that group in favor of unaided 

 calibration. 



Clearly, not one of the calibration methods which 

 have been described is both convenient and wholly 

 satisfactory as a method for translating observed 

 hydrophone voltages into accurate estimates of the 

 absolute transmission loss. Yet, until electroacoustical 

 equipment is developed which can be relied on to re- 

 main stable, field calibration remains a necessity. It 

 is to be hoped that rapid and adequate procedures of 

 calibration will be developed in the future. 



Once the equipment is calibrated, the transmission 



