The original stainless steel cavities had the corners 

 spot-welded to angles made of the same material as the 

 sides. Watertightness was then insured by soft soldering 

 the corners (fig. 1). The supports consisted of four SCAB* 

 rubber blocks, placed as close to the corners as the stabil- 

 ity of the cavity would allow. The transducers were im- 

 mersed in the water as close to the corners as the hum 

 and noise level permitted. 



Several attempts were made to improve the construc- 

 tion of the corners of the cavities. It was assumed that 

 the spot-welded and soldered joint would increase the 

 losses of the cavity. Also, the soft-soldered corner had 

 a tendency to leak in time due to electrolytic action, when 

 the cavities were filled with sea water. No really satis- 

 factory solution was found until the NEL machine shop 

 succeeded in developing a technique whereby the corners 

 were inert arc welded. It was not possible to weld sheets 

 thinner than 24 mils, and the final cavities were there- 

 fore made of this thickness (fig. 2). 



The presence of a few bubbles on the wall did not affect 

 the measurements. A bubble with a resonance frequency 

 close to the measuring frequency of the cavity, however, 

 did have a marked effect. As noted in the introduction, a 

 bubble resonating at 5 kc/s is somewhat larger than 1 mm 

 in diameter and can be detected quite readily and removed. 

 The above frequency corresponds roughly to that of the 

 fundamental mode for the cavity dimensions employed in 

 the sea water measurements reported in section V. A 

 large number of evenly distributed small bubbles will be 

 produced on the walls if air-saturated water is allowed to 

 heat up in a cavity without disturbance, and they will have 

 a marked influence on the measurements. This problem 

 is discussed further in section V. 



'Previously made by B. F. Goodrich Rubber Co., Akron, 

 Ohio. 



14 



