Volume 104, Issue 5, November 1998
Index of content:
- UNDERWATER SOUND 
104(1998); http://dx.doi.org/10.1121/1.423854View Description Hide Description
A nuclei size measurement technique is developed, based on a dispersion relation for propagation of sound waves through a bubbly liquid. This is used to relate the attenuation and phase velocity of a sound wave to the bubble population, leading to two integral equations. These equations are ill posed, and require special treatment for solution. Algorithms based on a minimization method that imposes a number of physical constraints on the solution, rendering the equation well posed, are developed. The procedure is first tested on analytical data with varying artificial noise added, and found to be successful in recovering the bubble density function, and to perform much better than other published solution techniques. Then, bubbles were generated using electrolysis and air injection through porous tubes, and bubble populations measured. Short monochromatic bursts of sound at different frequencies were emitted and received using hydrophones. The received signals were then processed and analyzed to obtain the attenuation and phase velocity. The void fraction and known experimental errors were also obtained and were fed as constraints to the inverse problem solution procedure. This resulted in bubble populations which compare favorably to those obtained by microphotography.
Thin-sediment shear-induced effects on low-frequency broadband acoustic propagation in a shallow continental sea104(1998); http://dx.doi.org/10.1121/1.423855View Description Hide Description
Low-frequency (6.3–630 Hz) acoustic broadband propagation loss data collected in areas of a shallow continental sea where the seabed is composed of hard substrates covered by thin (less than 40 m) deposits of unconsolidated sediment are presented. Data are shown to exhibit characteristics consistent with loss due to coupling to interface waves at the sediment–substrate interface [K. E. Hawker, J. Acoust. Soc. Am. 65, 682–686 (1979)]. In instances of a physically thin sediment cover, loss is also attributed to coupling to guided elastic waves in the sediment [S. J. Hughes et al., J. Acoust. Soc. Am. 88, 283–397 (1990)]. Interpretations are supported by numerical modeling of propagation loss using a recently developed wave number integration code for horizontally layered fluid–solid media, and geoacoustic models consistent with geophysical data from the sites of acoustic experiments.