or near-gaussian according to certain tests which will be 

 discussed later. Ambient noise was judged to be "clean" 

 when it was free from ship noise, biological noises, or any 

 man-made sounds when the sample was monitored. A band- 

 pass filter and oscilloscope were used to determine whether 

 60-c/s hum or any other single frequency components were 

 present in the noise sample. 



2. All noise samples should be stationary for their 

 entire length. When the sample is ambient ocean noise, 

 this condition will not in general be true. For a noise 

 sample to be stationary it is necessary for the sample 

 parameters, the means and the variances, to remain un- 

 changed as measured from samples taken at different times. 

 It is possible that no significant difference in the sample 

 parameters will be found if the time between samples is 

 short enough. In a previous study 3 it was concluded that 

 ocean noise is a slowly varying, not a stationary, process. 

 This conclusion was based on a comparison of samples that 

 were 3 or more minutes apart. However, no significant 

 difference was found among the values of some other samples 

 which were only 3 minutes or less apart. Thus it appears 

 reasonable to assume that ocean noise is stationary during 



a short interval of time (less than 3 minutes). 



3. The PDA requires a noise sample of about 30 

 minutes duration for a complete automatic analysis of the 

 amplitudes from -3. 00 to +3. 00 standard deviations. 



The need for a long noise sample that is stationary 

 can be satisfied by recording a short noise sample on mag- 

 netic tape and then making a loop of the tape. A loop length 

 was selected according to the following requirements. 



a. The loop should be short enough so that the noise 

 could be considered stationary and so that the entire loop 

 could be analyzed for each amplitude interval. The PDA 

 (in the particular position used) requires 30 seconds to 

 sweep a range of amplitudes equal to the window width, 

 which is 0. 1 times the rms value of the input signal. A 

 sample length of 7 seconds met all the above requirements 



