done only if the reverberation trace displayed a straight 

 part of at least 2 5 db. Otherwise it could not, with cer- 

 tainty, be differentiated from the part where the reverbera- 

 tion curve merges into the noise level. 



Possibility of Cavitation 



The maximum sound pressure level in the cavity is not 

 known, but it will be a function of the cavity Q. The highest 

 sound pressure is found in the center when the 1-1-1 mode 

 is utilized, and in the corresponding points for the higher 

 modes. The possibility therefore exists that the water may 

 be cavitating in the pressure maximum or maxima. The 

 earphones (fig. 4B) were therefore always used when a 

 measurement was taken, and it was never possible to detect 

 any indication that cavitation occurred. Reverberation 

 curves taken in rapid succession always gave essentially 

 the same result, and they were always straight with the 

 above mentioned exception. It was concluded that the 

 results were not explainable as being caused by cavitation. 



Anomalous Observation 



A peculiar phenomenon occurred twice during the data- 

 taking period. The cavity had been filled with plankton- rich 

 water and left standing. No measurements were completed 

 because a brisk wind created fairly large ripples on the free 

 water surface and made the water in the cavity turn over 

 slowly. After 5 to 10 minutes the plankton had disappeared 

 from the water, and a few strings of gelatinous matter, 

 3-5 mm in diameter and 20-50 mm long, were found floating 

 in the surface. The gelatinous matter had a very large 

 number of air bubbles embedded in it, and later micro- 

 scopic investigations showed it to contain large quantities 

 of chlorophyll. This has some bearing on a possible source 

 of bubbles which is explored in the discussion to follow. 



72 



