raise its sound output many decibels The findings, therefore, 



pointed to the necessity of maintaining a continuing program 



of measurement coordinated with a program of corrective measures. 



An entirely different set of sound sources, those occur- 

 ring in nature s were also studied by Division 6 For these 

 background noises, which interfere with the detection of wanted 

 sounds, there are many causes, ranging from breaking waves to 

 marine life 9 such as porpoises, snapping shrimp and croakers. 

 The data on biological noise were used by the Bureau of Ordnance 

 in the design of acoustic mines 



Standards and Calibration Studies, 



Standard methods for the measurement of underwater sound 

 are important in two general ways c First, they assist in the 

 development and production of sonar equipment; and second., 

 they are essential for the checking and monitoring of the per- 

 formance of the acoustical gear at sea c 



The devices and gear used by the Navy in combat were the 

 result of a coordinated effort by many groups leading through 

 many stages from research finally to operation at sea. At 

 many points in this sequence, especially during the research 

 stage and the development of pre-production models, experience 

 proved that progress was greatly accelerated when quantitative 

 measuring techniques were available to obtain reliable physi- 

 cal data expressed in the terms and units standard for the 

 art o It was also important that gear installed on shipboard 

 be periodically checked by standard measuring techniques to 

 be sure that it was functioning properly^. The instruments 

 used in these monitoring measurements had to be calibrated 

 in terms of the standards used for the whole field of under- 

 water soundo 



With this basic situation in mind<, NDRC when it undertook 

 in 1941 to co-operate with the Navy in underwater sound re- 

 search and development gave prompt consideration to the further 

 development of instrumentalities and techniques for precision 

 measurements in underwater soundo The program of investiga- 

 tion and development was conducted by the Underwater Sound 

 Reference Laboratories^ established by the Columbia University 

 Division of War Research under a contract with OSRD USRL's 

 director was Dr„ R, F, Shanklando In planning the NDRC pro- 

 gram, full use was made of the practices currently employed 



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