Abstract:
The pulsed echoes returned by several submerged cylindrical shells insonified by the peculiar sound pulses (``clicks``) emitted by dolphins have been examined. These clicks and echoes were collected in a large database as is done in standard dolphin experiments [Animal Sonar: Processes and Performance, edited by P. Nachtigall and P. W. B. Moore (Plenum, New York, 1988)]. The emphasis here is on the processing and physical interpretation of the ultrasonic spectroscopic features in these dolphin-generated echoes. These resonance features actually permit the total characterization of the shells. The spectroscopic ``lines'' (i.e., resonances with widths) in the frequency signatures, as well as other features in the associated time-domain signatures, provide all the ingredients required to determine the size, shape, thickness, shell elastic material, and internal filler material, in all cases. The time and frequency processing of the echoes is explained in detail; it follows the pattern briefly outlined elsewhere [G. C. Gaunaurd, J. Opt. Eng. 31, 2253--2261 (1992); G. C. Gaunaurd and H. Huang, J. Inverse Problems Eng. (in press, 1996)], and it will be illustrated with many measured and computationally predicted graphs. These techniques may provide the basic physical explanation of the dolphin's amazing target-ID feats.