BUBBLE PRODUCTION BY CAPILLARY-GRAVITY WAVES

In the absence of whitecapping, other physical mechanisms may contribu te to the generation of high-frequency ambient noise. It has been sugg ested [Longuet-Higgins, in NATO Advanced Research Workshop on Sound Ge neration Mechanisms at the Open Surface (NATO, Geneva, 1987)] that cap illary waves, with surface profiles that are peaked downward in the tr oughs and are relatively flat at the crests, can inject acoustically a ctive bubbles into the ocean, and thus contribute to the ambient noise background. It has been demonstrated in the laboratory that bubble in jection can be generated at the trough of capillary-gravity, short-fet ched waves by blowing air over water contained in a long, narrow tank. Simultaneous in situ acoustic and high-speed video monitoring of the capillary-gravity waves demonstrate that these waves can produce acous tically active bubbles. The generation of capillary waves depends prin cipally upon the surface tension, which can be changed by adding surfa ce-active agents to the water. The bubble production rate per unit are a of these capillary-gravity waves was measured, as well as the depend ence of this rate on wind speed, laboratory wind fetch, and surface te nsion. It was determined that an increase in water salinity and a redu ction in surface tension increases the bubble Production rate. The spe ctra of radiated frequencies ranges from 1 kHz to over 100 kHz with a broadband peak located around 4 kHz. The measured spectral densities w ere weakly related to wind speed. The wind-speed threshold value for b ubble production was determined to be approximately 8.6 m/s (14.6 m/s at 10-m level) in fresh water and salt water, which decreased to 8.1 m /s (13.8 m/s at 10-m level) with a surface tension of 40.5 dyn/cm.