SOUND ATTENUATION 

 (PLANKTON) 



Attenuation of sound by plankton 

 organisms and other particulate matter 

 is being investigated with resonant cavity 

 equipment. The cavity resonators have 

 acoustically soft side walls. They are 

 capable of measuring excess attenuation 

 in the 10- to 200-dB/kyd range, at dis- 

 crete frequencies of 5 and 8 kc/s. 



Distilled water or sea water with- 

 out particulate matter shows no mea- 

 surable attenuation. However, natural 

 sea water with particulate matter and 

 medium to high oxygen saturation shows 

 excess attenuation throughout the entire 

 range of instrumentation (i. e. , 10 to 200 

 dB/kyd). 



The excess attenuation is defined 

 as: 



= /.! _ J_) dB/kyd 



\T, T„ ' 



where T and T t are the reverberation 

 times of distilled and plankton filled 

 water, respectively. 



One investigation was carried out 

 during the "red tide" bloom, a condition 

 caused by an extremely high concentra- 

 tion of the plankton organism, Gonyaulax 

 polyedra. The tower measurements 

 during this "red tide" were supplemented 

 by observations at sea made with a com- 

 pliant-tube resonant cavity. 



The seagoing cavity was constructed as a cage, with 

 the bars made of flattened and therefore compliant 

 tubes. These structures act as pressure-release 

 surfaces in a limited frequency band, but are neither 

 as efficient nor as sensitive as the laboratory equip- 

 ment used on the tower. The results from both the 

 tower and ships did, however, confirm high acoustic 

 attenuation where the plankton concentration is high. 



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