430 BELL SYSTEM TECHNICAL JOURNAL 



become separated. Secondly, the peripheral gap cuts through the surface 

 currents at points of high density for TM modes and minimum density of 

 TE Omn modes and hence is a form of mode suppression. Thirdly, the gap 

 greatly simplifies the mechanical design of a movable end plate by elimi- 

 nating the need of physical contact with the side wall of the cylinder. 



A similar gap may be used at the other end. This facilitates "levelling" 

 of a false bottom in the cavity. 



Fig. 14 — Panel view of a 9 kinc echo box. 



Back Cavity Effects 



The cavity with a peripheral gap may give rise to further spurious reso- 

 nances in the region behind the reflecting surface (known as the back cavity) 

 if these responses are not damped. The addition of a lossy material such 

 as bakelite or carbon loaded neoprene in the back cavity is a successful sup- 

 pression method. 



Cylinder Tolerances 



The geometry of the structure is very important in realizing'the poten- 

 tial Q of the cavity. The theoretical computations are based on a perfect 

 right circular cylinder which in practice is seldom achieved. Distortions 

 occur in various forms: e.g., the cylinder instead of being round may be 

 elliptical; the ends may not be perpendicular to the axis of the cyUnder or 

 not parallel to each other; surface irregularities may be present causing field 

 distortions within the cavity. Many of these effects have been minimized 

 by requiring adherence to close dimensional tolerances. 



For some designs, the requirement on the paralleUsm of the end plates is 

 greater than can be commercially produced. An adjustable mechanism for 



