Effect of Stirring 



Some observations can be made on the basis of figure 

 32. Even very gentle stirring always increases the excess 

 attenuation, if any is present at all. The sample was always 

 stirred very thoroughly when collected, due to the method 

 of collection by pumping from the sampling depth. Accord- 

 ingly, the largest attenuation was usually measured immed- 

 iately after sampling. Different degrees of stirring were 

 tried after the samples were collected, ranging from a 

 thorough stirring with the squeegee to a very gentle stirring 

 with a thermometer. Care was taken not to work any air 

 bubbles into the water when stirring with the thermometer, 

 and the organic material that sometimes settled down to the 

 bottom of the cavity was never disturbed by this action. 

 The attenuation would usually decrease with time, if the 

 sample was not disturbed in any way, but there was almost 

 always residual attenuation of 2 to 30 db/kyd, when the 

 sample had possessed appreciable attenuation initially. 



Bubbles did sometimes appear on the sides of the 

 cavity, but wiping them off generally increased rather than 

 decreased the attenuation, presumably due to the unavoid- 

 able stirring of the sample. On a few occasions bubbles 

 were observed floating close to the center of the cavity and 

 very large attenuations were measured under these circum- 

 stances. Only samples for which no air bubbles could be 

 seen (in the presence of a strong light beam) have been 

 included in the data collection. This limits the possible 

 bubbles to 0. 1 mm or less, the largest observable diam- 

 eter being somewhat dependent upon the kind and amount of 

 suspended matter. Reverberation curves taken in rapid 

 succession give the same value for the reverberation time. 

 It may therefore be assumed that a single measurement of 

 attenuation does not change the acoustic properties of the 

 sample noticeably, but it does not assure that the sample 

 is unaffected by a sound field sustained over an appreciable 

 time. 



87 



