BUBBLE MEASUREMENT 



MODE I -l-l 

 f=MEASURING FREQUENCY 

 WITHOUT GLASSROD f=5l26C/S 



Figure 15. Excess attenuation coeffi- 

 cient for mode 1-1-1 due to presence 

 of air bubble vs. bubble diameter 

 (using data of fig. 5, 6, 7). 



later on. E the output from the receiving hydrophone is 

 recorded as a function of frequency when a double mode 

 prevails, it is always found that there are two resonance 

 peaks close together in frequency. The peak with the lower 

 level is always narrower in bandwidth than the peak with 

 the higher level. This would imply that the lower peak 

 corresponds to a normal mode more loosely coupled to the 

 transducers, but with a smaller damping than the higher 

 peak. It was usually possible to eliminate the mode with 

 higher damping by altering the effective coupling between 

 the transducers and the cavity. The reverberation time 

 measured on the mode with lower damping does not change 

 by this procedure. The mode with the longest reverbera- 

 tion time was therefore used in the few cases where the 

 above corrective method did not work. However this was 



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