ASA 128th Meeting - Austin, Texas - 1994 Nov 28 .. Dec 02

4aPAa2. Backscattering enhancements and the impulse response of shells: Observations and ray theory.

Philip L. Marston

Greg Kaduchak

Dept. of Phys., Washington State Univ., Pullman, WA 99164-2814

David H. Hughes

Naval Res. Lab., Washington, DC 20375-5350

Observations of backscattering by thin spherical shells in water were carried out using tone bursts and a novel broadband transient source [G. Kaduchak, Ph.D. dissertation, WSU (1994)]. Ray theory pertaining to some of the underlying features observed will be discussed including: (a) a bipolar specular feature of the impulse response sensitive to the shell's mass-per-area; (b) a high-frequency enhancement of the tone burst response due to a backwards wave [G. Kaduchak et al., J. Acoust. Soc. Am. (in press)]; (c) the coincidence frequency enhancement of the tone burst and impulse responses associated with the a[sub 0-] wave; (d) low-frequency features evident in the impulse response associated with the effect of curvature on the a[sub 0-] wave (or what some authors describe as the ``Junger wave''); and (e) periodic s[sub 0] wave packets giving resonance dips up to moderate frequencies. Feature (b) is sometimes called the thickness quasiresonance and for the stainless steel shell studied the ray theory uses the s[sub 2] leaky Lamb wave properties in the negative group-velocity region. The development of ray theory for situations where conventional thin shell theory is not applicable will be reviewed. [Work supported by ONR.]