Abstract:

Microbubble plumes are produced when waves break and are convected to depth. What role these microbubble plumes have in the production of sound and the scattering of sound near the sea surface from the low- (20-Hz) to mid- (2-kHz) frequency range? Ocean ambient noise shows a dramatic increase in midfrequency levels when wave breaking occurs. Measurements of scattering from the sea surface have a different characteristic than expected by Bragg scattering from gravity waves, i.e., a large zero Doppler component. If microbubble clouds and plumes with void fractions greater than 0.0001 act as collective resonant oscillators, then noise can be produced and scattering can occur with little Doppler shift but ample spread. This hypothesis was based on the theory that the mixture properties determine the radiation and scattering from such a compact region. Experimental results show that the far field radiation of sound from a compact region can be described by monopole volume pulsation beneath a pressure release surface with the natural frequency described by a modified Minneart formula. Scattering measurements from submerged bubble clouds show a significant low frequency scattering. Experimental evidence and theoretical formulations are consistent with collective phenomenon. [Work supported by ONR.]