in the morning by adding sufficient distilled water ice, and 

 the calibration was performed while the cavity was heating 

 up. The cavity walls were carefully wiped whenever a 

 desired temperature was reached, and a series of rever- 

 beration curves were then taken for different distances, D, 

 between the rim of the cavity and the surface of the water. 

 A series at any one temperature always began with a full 

 cavity, and the walls were wiped only at the beginning of a 

 series. The difference in the water temperature between 

 the beginning and the end of a series was always less than 

 1°C, and the temperatures given are the average of the 

 series. 



The calibration data are given in figures 9 to 14. The 

 attenuation coefficient for the cavity, a Q , is plotted as a 

 function of D and of the resonance frequency/. Figures 9 

 and 10 apply to mode 1-1-1, or the fundamental mode. 

 Figures 11 and 12 correspond to mode 2-1-1 while figures 

 13 and 14 pertain to mode 1-2-1. Figure 9 shows a (D) for 

 different temperatures. There is a discouragingly large 

 spread between the different points, and it appears that the 

 case D equal to \ inch below the rim is particularly poor. 

 Replotting the same points, but this time using the resonance 

 frequency as the independent variable, led to figure 10. The 

 scatter has to a great extent disappeared. An investigation 

 showed the cause of this behavior to rest with the spacing 

 between the cavity and the supporting table. The cavity is 

 approximately 14 mm above the table, and a quarter wave- 

 length in air is about 18 mm at 5000 c/s. The distance 

 between the table and the cavity could be increased by block- 

 ing the cavity supports up, and decreased by inserting a \ 

 inch aluminum plate between the cavity and the table. It 

 was found that the distance actually improved the cavity Q. 

 Moving the supports as far as \ inch away from the corners 

 did not affect the losses of the cavity, however. 



In regard to the calibration data for modes 2-1-1 and 

 1-2-1 (figs, 11-14), the attenuation due to the cavity is 

 generally greater than that associated with the fundamental 

 mode (at least for frequencies greater than 5.2 kc/s). 

 In addition, the attenuation varies greatly with frequency, 



66 



