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Understanding acoustic scattering from objects placed on the interface between two media requires incorporation of scattering off the interface. Here, this class of problems is studied in the particular context of a 61 cm long, 30.5 cm diameter solid aluminum cylinder placed on a flattened sand interface. Experimental results are presented for the monostatic scattering from this cylinder for azimuthal scattering angles from 0 degrees to 90 degrees and frequencies from 1 to 30 kHz. In addition, synthetic aperture sonar (SAS) processing is carried out. Next, details seen within these experimental results are explained using insight derived from physical acoustics. Subsequently, target strength results are compared to finite-element (FE) calculations. The simplest calculation assumes that the source and receiver are at infinity and uses the FE result for the cylinder in free space along with image cylinders for approximating the target/interface interaction. Then the effect of finite geometries and inclusion of a more complete Green's function for the target/interface interaction is examined. These first two calculations use the axial symmetry of the cylinder in carrying out the analysis. Finally, the results from a three dimensional FE analysis are presented and compared to both the experiment and the axially symmetric calculations.

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Linear wave propagation through a bubbly liquid has seen a resurgence of interest because of proposed "corrections" to the lowest-order approximation of an effective wave number obtained from Foldy's exact multiple scattering theory [Foldy, Phys. Rev. 67, 107 (1945)]. An alternative approach to wave propagation through a bubbly liquid reduces the governing equations for a two-phase medium to an effective medium. Based on this approach, Commander and Prosperetti [J. Acoust. Soc. Am. 85, 732 (1989)] derive an expression for the lowest-order approximation to an effective wave number. At this level of approximation the bubbles interact with only the mean acoustic field without higher-order rescattering. That is, the field scattered from a bubble may interact with one or more new bubbles in the distribution, but a portion of that scattered field may not be scattered back to any previous bubble. The current article shows that modifications to the results of Commander and Prosperetti lead to a new expression for the effective wave number, which properly accounts for all higher orders of multiple scattering.