TESTS AND MEASUREMENTS 



Shallow Water Tests 



All initial tests of the pneumatic sound source were 

 conducted in shallow water at the Navy Sweetwater Calibra- 

 tion Station. These tests can be summarized by stating 

 that the average peak sound level was around 100 db/dyne/cm^ 

 at 1 meter for the early thin-walled spheres and surgical 

 rubber tubing used. Hundreds of combinations were tried. 

 Since the resulting frequencies were all around 100 c/s or 

 below, it was apparent that, to obtain a true picture, much 

 deeper water would be necessary to study the device. For 

 this reason all the later measurements were made in deep 

 water at the NEL Pend Oreille Calibration Station in Idaho. 



Deep-Water Measurements 



As mentioned earlier, elaborate preparations were made 

 to measure both the underwater spark source and the pneumatic 

 device at various depths and to obtain as much high-speed 

 photographic data as possible. The large rigging to accomplish 

 this was shown in figures 9 and 10. Photographs of the 

 phenomena connected with the oscillating bubble are shown in 

 figure 33 which was taken 200 feet below the surface, and in 

 figure 34 which was taken at a depth of 300 feet. These are 

 thought to be the first pictures of such phenomena taken at 

 such great depth. It will be noted that the sphere reaches 

 the same external dimension regardless of the depth. This 

 is because for a given initial diameter, wall thickness, and 

 type of material, the spheres rupture at a nearly constant 

 differential pressure. To date, the maximum depth at 

 which measurements have been made is 600 feet. 



One important finding at the greater depths was that 

 the air initially entrapped in the sphere and inflating line 

 was compressed allowing the sphere to be forced into the 

 inflating fitting, thus weakening a small area of the rubber 

 wall prior to normal inflation. This problem was eliminated 

 by making the ball on the end of the molding mandrel remov- 

 able and allowing it to remain in the sphere. This provided 

 several advantages since it eliminated the entrapped air 

 and provided support to the inside of the sphere when it 

 was under compression from the outside. The ball has 

 not interfered with the inflation process. 



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