Robustness of target strength of an immersed, hollow ceramic flotation sphere of non-constant shell thickness

Abstract

Deep-sea flotation spheres consisting of an air-filled ceramic shell are made to a very high tolerance, suggesting their application as sonar targets, possibly for calibration use. A previous study documented preliminary measurements made on hollow alumina spheres of nominal diameter 91.44 mm and shell thickness 1.3 mm over the total frequency band 5-150 kHz. Here, the sensitivity of target strength to geometrical and material- property values is quantified by means of the analytical solution for scattering of plane acoustic waves by an ideal hollow, homogenous, elastic, spherical shell. The parameters of variation include the outer diameter and thickness of the spherical shell; mass density and longitudinal- and transverse-wave sound speeds of the shell material; mass density and sound speed of the immersion medium; and interior air pressure. The frequency range of the computations is 0.1-150 kHz. The target strength is observed to be quite sensitive to variations in most of the parameters. However, the sphere can be used as a standard target for sonars with operating frequencies in the band 8-13 kHz and for narrowband scientific echo sounders at certain ultrasonic frequencies.