particularly for mode 2-1-1. The data accumulated for 

 these two modes have therefore not been corrected for the 

 loss in the cavity itself. 



The block diagram utilized for the calibrations and the 

 captured bubble measurements were the same as shown in 

 figure 4B, except that a quartz crystal controlled electronic 

 counter was connected in parallel with the output terminals 

 of the generator. The frequency was therefore known with 

 an accuracy of ±1 c/s. The circuit used for the measure- 

 ments of the sea water samples is identical to that shown 

 in figure 4B. 



The glass rod shown in figure 2 is very useful as a 

 probe of cavity modes. Any particular mode can be identi- 

 fied by passing a captured bubble through the cavity. The 

 reading on the VTVM instrument will be a minimum when 

 the position of the bubble coincides with a pressure maxi- 

 mum in the cavity. This method was always used to identify 

 the cavity modes, whenever it appeared necessary. The 

 mode with the lowest resonance frequency is of course 

 always mode 1-1-1. 



A few measurements on a captured air bubble are 

 included as an illustration of the capability of the cavity 

 used for the data collection. The original reverberation 

 curves obtained from the Briiel and Kjaer recorder are 

 shown in figures 5 to 7, and the results (discussed in a 

 later section)are plotted in figure 15. It should be noted 

 in passing that the diameter of the bubble for each test was 

 estimated by comparison with the divisions of a metric 

 scale, the bubble being held by the glass rod. The accu- 

 racy is accordingly rather poor. 



Character of the Reverberation Curves 



The reverberation curves do sometime show evidence 

 of "double" moding. The reverberation level diminishes 

 through the first 10-15 db at a faster rate than encountered 



70 



