Acoustic scattering from swim-bladder fish at low frequencies

Models of acoustic resonant scattering from swim-bladder fish are sensitive to the choice of damping coefficient and resonance frequency. Existing theory does not provide a consistent representation of the role played by tissue surrounding the swim bladder in determining this coefficient and empirical fits are therefore often used to adjust model parameters for specific examples. In an attempt to provide a firmer theoretical basis for the interpretation of experimental results, it is shown how the elastic properties of the tissue can be combined with the swim bladder in a consistent way. Although the analysis must eventually be extended to swim bladders of arbitrary shape, for simplicity the essential features are illustrated with a spherical geometry. The results emphasize the need for accurate measurement of the bulk elastic properties of tissue, specifically the complex shear modulus.