DIFFERENCES BETWEEN SOUND-SCATTERING BY WEAKLY SCATTERING SPHERES ANDFINITE-LENGTH CYLINDERS WITH APPLICATIONS TO SOUND-SCATTERING BY ZOOPLANKTON

A modeling study was conducted to determine the conditions under which fluidlike zooplankton of the same volume but different shapes (spheri cal/cylindrical) have similar or dramatically different scattering pro perties. Models of sound scattering by weakly scattering spheres and c ylinders of finite length used in this analysis were either taken from other papers or derived and herein adapted for direct comparison over a range of conditions. The models were examined in the very low- (ka much less than 1, kL much less than 1), moderately low-(ka much less t han 1, kL greater than or similar to 1), and high-frequency regions (k a much greater than 1, kL much greater than 1), where ii is the acoust ic wave number, a is the radius (spherical or cylindrical) of the body , and L is the length of the cylinders (for an elongated body with L/a = 10, ''moderately low'' corresponds to the range 0.1 less than or si milar to ka less than or similar to 0.5). Straight and bent cylinder m odels were evaluated for broadside incidence, end-on incidence, and av erages over various distributions of angle of orientation, The results show that for very low frequencies and for certain distributions of o rientation angles at high frequencies, the averaged scattering by cyli nders will be similar, if not identical, to the scattering tay spheres of the same volume. Other orientation distributions of the cylinders at high frequencies produce markedly different results. Furthermore, o ver a wide range of orientation distributions the scattering by sphere s is dramatically different from that of the cylinders in the moderate ly low-frequency region and in the Rayleigh/geometric transition regio n: (1) the Rayleigh to geometric scattering turning point occurs at di fferent points for the two cases when the bodies are constrained to ha ve the same volume and (2) the functional dependence of the scattering levels upon the volume of the bodies in the moderately low-frequency region is quite often different between the spheres and cylinders beca use of the fact that the scattering by the cylinders is still directio nal in this region. The study demonstrates that then are indeed condit ions under which different shaped zooplankton of the same volume will yield similar (ensemble average) scattering levels, but generally the shape and orientation distribution of the elongated bodies must be tak en into account for accurate predictions. (C) 1998 Acoustical Society of America.