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Low-Frequency Resonance Scattering from Acoustically Compact Bubble Clouds.

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Final technical rept.,

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Recent developments indicate that microbubble plumes and layers are produced when waves break and are convected to depth. The fundamental question is what role, if any, is played by these microbubble plumes in the production of sound near the sea surface in the low 20 Hz to mid 2 kHz frequency range Measurements of ocean ambient noise levels show a dramatic increase in mid-frequency ambient noise when wave breaking occurs. Furthermore, measurements of sound scattering from the sea surface has a different characteristic than predicted by Bragg scattering from gravity waves, i.e. there exists a large zero-Doppler component. We hypothesized that if microbubble clouds and plumes with void fractions greater than .0001 act as collective resonant oscillators, then radiated noise can be produced and scattering can occur with little Doppler shift but ample Doppler spread. This hypothesis was based on the theory that when a large number of microbubbles with individual resonance frequencies far above the frequency of excitation are present in an acoustically compact region, the mixture properties determine the radiation and scattering. We present scattering measurements from submerged bubble clouds which show that at the lower frequencies the resonance effect is significant, with the lowest order mode being a monopole which can be modeled as the volumetric pulsation of an acoustically compact sphere. Moreover, when such a cloud is located near the sea surface, a dipole radiation pattern results. Consequently, the scattered intensity is a sensitive function of depth, frequency, surface roughness and grazing angle. We show that for certain sea state conditions, observable anomalous effects can result from transient plumes generated close to the surface.

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  • Acoustics
  • Physical and Dynamic Oceanography

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