and this gives a loop size of 52. 5 inches, which was 

 conveniently handled. 



b. The recorded noise on the loop should be con- 

 tinuous, i.e., there should be no blank intervals on the 

 loop, since a blank interval would change the average rms 

 value of the recorded noise. 



A typical analysis procedure was as follows. A portion 

 of data was selected for analysis from the recorded data 

 available. The noise was re-recorded on a loop. The loop 

 was played back at l\ ips and the analysis proceeded as 

 indicated by the diagram in figure 2. The filter was set to 

 the desired bandwidth, and the noise was amplified to 1 volt 

 rms or greater. The PDA was carefully calibrated and 

 adjusted just before each analysis. Its input level of noise 

 was adjusted to 1 volt rms by its potentiometer, thus 

 normalizing its output. 



Probability density of the amplitudes was recorded on 

 the Y scale of the XY recorder and the amplitude around 

 which the probability density was measured was on the X 

 scale. Scale factors were selected to give a deflection of 

 4 inches on the Y scale for a probability density range of 

 to 0. 4, and a deflection of 1 inch per standard deviation of 

 amplitude on the X scale. The automatic sweep time of the 

 PDA was set at X = -3. 00 standard deviations, and would 

 automatically sweep through to X = +3. 00 standard deviations, 

 based on a 1-volt rms input. Total running time was about 

 30 minutes. This procedure was repeated for each band- 

 width on every loop analyzed. 



Table 1 lists the number of samples analyzed from 

 each location, the total number of probability density 

 curves obtained from the samples, and the filter used to 

 analyze these curves. When the Allison Laboratories filter 

 was used, the system cutoff frequency at the low end was 

 about 20 c/s and the upper cutoff frequency was determined 

 by the filter which was set at 2500, 1500, 1200, 6 00, 300, 

 or 150 c/s. The B & K filter was used in both the octave 



10 



