VI. SUMMARY AND CONCLUSIONS 



PRIMARY RESULTS 



Sea water without suspended particulate matter showed 

 no excess attenuation when compared with distilled water. 

 This is in agreement with earlier work performed at higher 

 frequencies, within the measuring accuracy of the equip- 

 ment utilized. Near shore water with high oxygen content 

 and with suspended particulate matter showed excess 

 acoustic attenuation in the full range of the measuring 

 equipment, i, e., from 10 to 200 db/kyd. The largest atten- 

 uations were measured for water supersaturated with oxygen, 

 but excess attenuation was experienced even when the oxygen 

 content was only 65 per cent of the saturation value. 



The comparison of excess attenuation with suspended 

 particulate matter (primarily plankton) and oxygen content 

 does not indicate a clear-cut correlation, but does suggest 

 the trend indicated above. Perhaps the most striking 

 feature of the measurements is the large range of attenua- 

 tions which can exist in samples whose gross properties 

 appear to be quite similar. 



On the other hand the time sequence of attenuation 

 followed a definite pattern. Whenever any significant 

 amount of attenuation was measured, it would always 

 increase when the sample was stirred. The highest atten- 

 uations were consequently usually measured immediately 

 after sampling. The greater part of the excess attenuation, 

 however, would disappear when the sample was left undis- 

 turbed, but an excess attenuation of 2 to 30 db/kyd almost 

 always remained even after long periods of time (several 

 hours). 



A bubble resonant at 5.2 kc/s will have a diameter of 

 approximately 1.2 mm and is easily visible. It is con- 

 cluded that the attenuation is not caused by bubbles close 

 to resonance, but may have been caused by much smaller 



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