DESIGN CONSIDERATIONS 



Early measurements made at the Sweetwater Calibration 

 Station showed that pressure was dependent upon the wall 

 thickness of the rubber. It was also observed that the best 

 waveform was obtained from spherical sources. A local 

 rubber manufacturer was therefore contacted to discuss the 

 problem of fabricating special rubber spheres. The 

 manufacturer agreed to develop special formulas, attempting 

 to find one that would give results equal or superior to those 

 obtained with surgical rubber. NEL designed and built molds 

 for the job (fig. 2 3). In order to obtain a maximum of data 

 in a short time, it was decided to concentrate on a sphere 

 approximately 1 inch in diameter, varying the wall thickness 

 from one model to another to compare results (fig. 24). 

 High-speed photographs (approximately 2500 frames per 

 second) were made of the bubbles produced by these spheres; 

 figure 2 5 is a typical sequence. Later, spheres of other 

 outside diameters and various wall thicknesses were made 

 by the manufacturer for testing (fig. 26). 



Following are the major characteristics of rubber mixtures 

 used in the NEL tests: 





Surgical 

 Rubber 



Reeves 

 Mix 3011. 1 



Reeves 

 Mix 3015. 



i^ 



Reeves 

 Mix 416 



Durometer 

 Hardness 



42 



47 



50 





60 



Specific 

 Gravity 



? 



1.02 



1.02 





1. 11 



Tensile 

 Strength (psi) 



3400 



3310 



4500 





3400 



Elongation (%) 



600 



690 



775 





480 



*The greatest number of tests at NEL have used Reeves Mix 3015. 5 



Directionality at Low Frequency 



The problem of obtaining high directivity at frequencies 

 below 1000 c/s has been a major difficulty for many years 

 because the necessary reflector, to be effective, must be 

 of such large physical dimensions. 



44 



