Physical Factors 



SOUND ATTENUATION 



(BUBBLES) 



Sound attenuation by bubbles is 

 studied at the tower with a bottom- 

 mounted bubble screen, multiple hydro- 

 phones, and a sound source. The 

 attenuation is influenced by sound fre- 

 quency, size and number of bubbles, 

 and other factors. Attenuation as great 

 as 90 percent was found when using a 

 bubble screen 10-feet long which emitted 

 air at 200 cubic feet per minute against 

 a head of 32 pounds per square inch. 



In addition to greatly attenuating 

 the sound, the bubble screen also caused 

 the colder water near the bottom to up- 

 well- forcing the thermocline in the 

 immediate area to rise. 



Natural bubbles caused by the 

 breaking white caps (and probably by 

 plankton and fish) are recorded by other 

 acoustic means. 



The bubble screen (left on facing page) is created by 

 connecting a hose from the air compressor on the 

 tower to a long multiple-hole pipe (perforations 

 0.8 mm diameter every 13 mm) mounted on the sea 

 floor. The air released causes a bubble screen which 

 extends from bottom to surface. The bubbles expand 

 and sometime coalesce as they approach the surface. 



Sound intensity is measured before and after the 

 sound waves pass through the bubble screen. In one 

 test series, explosive charges (shown to the right) 

 were used as the sound source. The bubble screen 

 greatly reduced the sound intensity. 



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