An XY recorder by Electronic Associates, Inc., was 

 used to record the analog X and Y outputs of the PDA. 



A cathode ray oscilloscope monitored the signal out- 

 put of the filter. 



The counter used responded to frequencies of at least 

 10 Mc/s for use with the PDA. The counter can be used in 

 place of an XY recorder and, in fact, is essential if 

 measurements are to be made at low probability densities. 



Research Techniques 



Data which had been recorded for previous ambient- 

 noise studies were available for this study. These samples 

 had been recorded on 1 Of— inch reels of ^-inch tape, at 3f 

 inches per second, and were from three locations. Two 

 groups had been made in shallow water -- one, about 2 miles 

 from the western side of an island off the coast of Southern 

 California, and the other in the Bering Straits. These con- 

 sisted of short ambient-noise samples recorded at regular 

 intervals throughout the day, so that one reel covered data 

 for one day. The third location represented was in deep 

 water in the North Pacific between Hawaii and Alaska; most 

 of these samples were of longer duration than the other two 

 groups, but covered only a few days. 



Samples of ship noise were desired, so that their 

 probability density curves might be compared with those of 

 "clean" ambient noise. Recordings were made of ships 

 entering San Diego Harbor, with the sampling made at ap- 

 proximately the closest point of approach. These included 

 Navy surface ships, submarines (surfaced), and commercial 

 ships. 



Several factors were considered in choosing the data 

 samples to be used in this study. 



1. "Clean" ambient noise was used to determine 

 whether the distributions of the amplitudes were gaussian 



