Chapter 1 

 INTRODUCTION 



By Robert S. Shankland 



Scientific programs generally require accurate 

 standards and techniques of measurement to in- 

 sure quantitative correlation and interpretation ol 

 phenomena under investigation. Only when an un- 

 derstanding of phenomena in quantitative terms has 

 been achieved can accumulated data be effectively 

 and efficiently applied to the design of new equip- 

 ment, to the improvement of present designs, and to 

 the prediction of results obtainable with such gear. 



The program undertaken by Section C-4, later 

 Division 6 of NDRC, included studies and experi- 

 mental investigations on the transmission of sound in 

 ocean waters and the further development of sonar 

 gear. This necessitated the provision of suitable 

 practical standards and a study of measurement 

 techniques. The results accomplished form the prin- 

 cipal subject of this volume. 



In the development of apparatus for service use 

 it is generally true that the final criterion is the effec- 

 tiveness of the equipment under operational condi- 

 tions. In the case of sonar gear, operational tests are 

 not only expensive and time-consuming, but are per- 

 formed at that stage in development when changes 

 in design are most difficult to achieve. Heme, labo- 

 ratory tests under controlled conditions directed 

 toward determining design changes, which will pro- 

 duce maximum effectiveness under operational con- 

 ditions, are a necessary adjunct to a program of 

 research and development. 



DEVELOPMENT OF MEASUREMENT 

 AND CALIBRATION TECHNIQUES 



Establishment of Standard 

 Sound Fields 



1.1. 



To make the results of the several laboratories en- 

 gaged in subsurface warfare research directly com- 

 parable, it was necessary to reduce their test data to 

 common terms. Also, as the program progressed, it 

 was necessary to work continually toward higher 

 standards of accuracy. The Underwater Sound Ref- 

 erence Laboratories [USRL] were assigned the task 



of establishing reference levels, and of making cali- 

 brated standards available to the other laboratories. 



Originally, this standardization was based upon 

 the characteristics of hydrophones (developed in co- 

 operation with the Bell Telephone Laboratories, Inc. 

 [BTL]), whose absolute calibrations could be ob- 

 tained from their design or by comparison with fairly 

 well established standards for air acoustics. Later, it 

 was found that the reciprocity method of calibration 

 provided improved accuracy and a simplified pro- 

 cedure. 



The reciprocity method of calibration had been 

 suggested by Ballantine and MacLean, 71 - 77 but had 

 received little attention until it was applied to the 

 establishment of fields in underwater sound. Its adop- 

 tion by the USRL has facilitated accurate calibration 

 of standards for frequencies ranging from about 10 c 

 to 2.5 mc, with the possibility of attaining much 

 higher frequencies. The lower frequency limit was 

 determined by the difficulty of making low-frequency 

 measurements in a shallow lake and not by failure 

 of the reciprocity method. In the range from 100 c 

 to 100 kc, this method is accurate to within ±1 db. 

 A comparison of standard levels obtained by reciproc- 

 ity calibration at USRL with those independently 

 established at British laboratories showed excellent 

 agreement. Other calibration methods (low fre- 

 quency pressure tank built by BTL and the CMF 

 self-calibrating condenser hydrophone, built by the 

 Massachusetts Institute of Technology [MIT]) suc- 

 cessfully extended the low frequency limit to below 

 1 c. 



1.1.2 Development of Standard 



Instruments 



Although the BTL, operating under an OSRI) 

 contract, was largely responsible for the development 

 of hydrophones and projectors suitable for use as 

 standards, many other laboratories constructed 

 standards suited to the particular applications in 

 which they were interested. Instruments, employing 

 piezoelectric crystal, magnetostrictive, condenser and 

 electrodynamic coupling, cover collectively the 1 c 



