Low-resolution acoustic scattering models: Fluid-filled cylinders and fish with swim bladders

Abstract

Low spatial resolution acoustic scattering models were developed to model the scattering by objects that are less ideal than perfect cylinders, i.e., fish. The models used the first few modes of solutions for finite and bent fluid cylinders [T. K. Stanton, J. Acoust. Soc. Am. 83, 55–63 (1988) and 86, 691–703 (1989)]. Empirical mode attenuation factors and bent cylinders were used to control the spatial resolution. All computations were for backscatter in the far field. First, numerical evaluations were made for frequency domain scattering amplitudes. The scattering amplitudes were inverse Fourier transformed to time domain for identification of the reflection and diffraction components. Gas-filled cylinders represented fish swim bladders. Fish bodies were represented by a liquid-filled cylinder where the liquid had a density and a sound velocity a little greater than water. Gas-filled cylinder: The sum of the zeroth and first mode gave a reflection from the front face and a diffraction from the boundary of the insonified and shadow zone. Summations over 23 modes gave a single reflection and no boundary diffracted wave for ka<20, where k is wave number and a is cylinder radius. Liquid-filled cylinder: The sum of the zeroth and first mode gave reflections from the front and back faces and shadow boundary wave. The shadow boundary wave was larger than the reflected components. The sum of 23 modes gave reflections from the front and back face and no shadow boundary wave for ka<20. Fish model: The theoretical models were compared to measurements of sound scattered by anchovy ([R. H. Love, J. Acoust. Soc. Am. 49, 816–823 (1971) and D. V. Holliday, J. Acoust. Soc. Am. 51, 1322–1332 (1972)]. The matches of data and theory were excellent. Comparison of the bent cylinder theory and measurements to the traditional spherical bubble model showed that the spherical bubble was a poor approximation to the swimbladders of these fish. The research shows that acoustic modeling needs fish length, fish mass, fish and swimbladder morphology, etc.