III. DESIGN CONSIDERATIONS 



Several materials and cavity shapes were investigated 

 before arriving at the final design. Both cylindrical and 

 spherical chambers were investigated, as well as rectangu- 

 lar prism forms of various relative dimensions. Some of 

 the more important characteristics of the types of chamber 

 which were considered, and reasons for selecting the 

 rectangular or box-shape form in the final design, are 

 discussed in the following sections. 



CYLINDRICAL AND SPHERICAL CAVITIES 



In the cylindrical cavities the walls were too rigid 

 because of the circular cross section. Hence the bubbles 

 which inevitably formed on the inner wall had a very marked 

 influence on the measurements. The wall could have been 

 made more compliant by introducing corrugations, but this 

 would make it difficult to clean. Moreover, it was found 

 that the seam, produced when a cylinder was fashioned out 

 of a sheet, would disturb the symmetry of the container so 

 much that a poor resonant cavity resulted. Values of Q of 

 the order of 2 000 could be obtained from this type of cavity 

 if seamless containers were used, and if proper care were 

 shown. The term Q referred to above represents the qual- 

 ity factor of the cavity response as defined by 



/o 



A/ 



where / is the resonance frequency for a given mode of 

 oscillation and A/ is the bandwidth of the response spec- 

 trum for uniform power input as measured at the 3 db 

 level below the resonant peak level. The Q value is in- 

 versely proportional to the attenuation coefficient for the 

 cavity or in other words, directly proportional to the 



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