Echoes from Hollow Aluminum Spheres in Water

Abstract

Recorded farfield echoes from precision hollow aluminum spheres in water, with different shell thicknesses and with either water or air in the hollow interior, are compared with calculated echoes. Steady-state echoes are investigated over a ka range of from 6 to 27, and individual pulsed echoes at ka = 20. Excellent verification is obtained for the features predicted by the theory. It is shown that the echoes are sensitive to changes in the elastic properties of the shell, particularly when the shell is thin. The experiments showed, moreover, that the echoes are dependent on the orientation of the sphere. This is due presumably to small asymmetries in the structure and properties of the shell and emphasizes again the strong interrelation between sonar echoes and the nature of the material in the target.