The discharge of the hose was always kept below the water 

 level in the cavity, and no bubbles were ever observed 

 coming out of the hose. This sampling method was used 

 exclusively in all of the subsequent work, but it was felt 

 that a less "violent" sampling method would be preferable. 



Selection of Cavity for Measurements 



Five cavities were available for the data collection, as 

 mentioned earlier. It became apparent however, that only 

 cavity No. 2 was suited for measurements on the tower. 

 Cavity No. 1 was too big for one person to handle, consider- 

 ing the rather limited space available. The difficulties 

 encountered when cleaning the walls and stirring the content 

 to insure homogeneity were also considerable. The cavities 

 smaller than cavity No. 2 were likewise difficult to clean 

 because of their small size and also because the walls were 

 not sufficiently flexible. 



The influence of the transducers was also marked for 

 cavities 3, 4, and 5 probably partly because of the greater 

 transducer dimensions relative to the wavelength for the 

 cavity. This is also partly because the transducers could 

 not be decoupled to as great an extent as with cavity No. 2, 

 since the thickness of the stainless steel sheet corresponds 

 to a greater fraction of a wavelength. 



It was decided to use cavity No. 2 exclusively for the 

 data collection. This implies that it is not possible to 

 deduce very much about the variation of the attenuation 

 with frequency. However, even if the same sample had 

 been used in all five cavities, the frequency range is not 

 great. Moreover, transferring the sample from one cavity 

 to another would undoubtedly have a very marked influence 

 on the properties of the sea water. An attempt was made, 

 however, to get data at different frequencies by measuring 

 the reverberation time of not only the fundamental mode, 

 but also two higher modes. If the coordinate axes 1, 2, 3 

 are identified with length, width, and depth of the cavity, 

 then the modes measured can be labelled: 



63 



