INTRODUCTION 



to 2.5 mc frequency range. Many of these instru- 

 ments have flat responses over wide frequency ranges. 

 The 3A type crystal hydrophone, for example, has a 

 flat response from about 100 c to 25 kc, and the XMX 

 crystal hydrophone has a Hat response within ±2 db 

 from below 100 c to about 100 kc and hence were 

 useful not only for single frequency measurements, 

 but also for recording transients, and for measuring 

 signals covering a wide frequency band. In the ma- 

 jority of cases, these instruments were also character- 

 ized by mechanical ruggedness and stability of 

 calibration. Considering the fact that acoustic stand- 

 ards for underwater applications were almost non- 

 existent before 1940, the rapidity and comprehensive- 

 ness of subsecpient developments in the art are 

 noteworthy. 



1.1.3 The Objectives of Tests and 



Calibrations 



Although the necessity for development and pre- 

 production tests of underwater sound gear was soon 

 recognized, the specific objectives of such tests and 

 calibrations became apparent with further develop- 

 ment. Although this problem is still not completely 

 solved, much has been learned concerning the signifi- 

 cance of the various factors which characterize effi- 

 cient operation of sonar gear. Because of the large 

 number of tests required and the limited facilities 

 available, it was necessary to concentrate on the 

 determination of these factors which depend upon 

 the application for which the equipment was de- 

 signed. While factors such as frequency response, 

 directivity, impedance, signal-to-noise ratio, and 

 efficiency were of significance in the majority of meas- 

 urement programs, other quantities including rear 

 response, side lobes, tuning, harmonic distortion, 

 and variability with temperature and pressure often 

 were of equal importance. 



Quantitative relationships between the foregoing 

 quantities and the operational efficiency of under- 

 water sound devices were only imperfectly under- 

 stood at the initiation of the program. Theoretical 

 and experimental studies which were undertaken 

 provided a more exact understanding of these rela- 

 tionships and a sounder basis for compromise be- 

 tween competing factors in design, and, in turn, a 

 basis for the selection of important parameters to be 

 measured in development and calibration tests. 

 Thus, the relationship which existed between devel- 



opment, calibration and testing, and operational and 

 tactical application of the equipment, was of ines- 

 timable value in unifying the program as a whole, 

 and in directing it toward its prime objectives. 



111 Testing Technique 



With reference standards available, and with in- 

 creased knowledge of significant parameters, the 

 technique of actually performing a calibration test 

 remained to be considered. Because these parameters 

 were characteristic of the devices under test, and had 

 to be distinguished from extraneous effects produced 

 by the test location and by the equipment involved, 

 a large part of the effort of the USRL, as well as that 

 of other laboratories engaged in equipment develop- 

 ment, was directed toward devising measurement 

 techniques. The principal problems in testing gear 

 in laboratory sites arise from the limited extent of 

 the testing medium. Acoustic reflections from the 

 surface, bottom, and shores of the body of water in 

 which the tests are made and the limited testing dis- 

 tance available required the development of means 

 for eliminating the effect of these factors. To this 

 end baffles and screens, directional sources, proper 

 orientation of instruments, pulses and noise bands 

 have been used for eliminating the effect of reflec- 

 tions, and spherical wave corrections have been 

 applied to compensate for effects caused by short 

 testing distance. In addition, equipment was de- 

 signed to reduce the time and labor necessary for 

 performing tests with maximum use of available 

 facilities. Among devices in this category are auto- 

 matic recorder devices (linear and polar), special 

 amplifiers, arrangements for rapid changes in driv- 

 ing and receiving impedances, adjustable and inter- 

 changeable rigging gear, and computing aids to 

 facilitate the reduction of data. Techniques for test- 

 ing in indoor tanks under varying conditions of tem- 

 perature and pressure, for testing at high power 

 inputs, and for measuring impedance under various 

 conditions were developed as the need arose. With 

 experience, the technique of testing underwater 

 sound devices grew as an art as well as a science. 

 Close liaison with developments in the field of under- 

 water sound was essential to insure the adaptability 

 of testing techniques and facilities. This flexibility 

 made possible the realization of the potentialities of 

 laboratory testing in the development of underwater 

 sound devices. 



