Observation of the acoustic glory: Scattering from an elastic sphere in near backward directions

Abstract

We have measured the scattering for small angles γ (relative to the backward axis) from a fused silica sphere of radius a≈52 mm. Tone bursts in water corresponding to ka≈450 were incident on the sphere; their short duration permitted glory and axial returns to be separated in time. The γ for the probe hydrophone was scanned to test a model [P. L. Marston and L. Flax, J. Acoust. Soc. Am. Suppl. I 68, S81 (1980)] of diffractive effects on backward axial focusing. Observations tend to support the model as adapted to fused silica: (1) from the arrival time, the strongest echo is evidently due to the 4‐chord shear glory ray; (2) its amplitude is ∝J0(kb sin γ) where b is the calculated glory circle radius; (3) its amplitude at γ = 0, though slightly smaller than predicted, exceeds that of the first axial reflection; and (4) the times, amplitudes, and γ dependences of other echoes are correlated to predictions. The first null of the strongest echo occurs at γ≃1°. Consequently, we demonstrate for the first time the diffraction limited backward focusing of echoes from a sphere. [Work supported by ONR. Marston is an Alfred P. Slosh Research Fellow.]