Excitation of low‐frequency modes of solid cylinders by evanescent and ordinary propagating waves.

Abstract

When using sound to detect objects buried beneath the seafloor, often the acoustic source has a large horizontal stand‐off distance. In such situations there is evidence that the incident acoustic wave in sand can have a significant evanescent component. In prior work, organ‐pipe modes of water‐filled shells were excited in a laboratory simulation, the most significant result being the “double spatial decay rate” effect [Marston et al., J. Acoust. Soc. Am. 122, 3034 (2007)]. To further understand the coupling by evanescent waves into low‐frequency modes of cylinders, backscattering by small solid aluminum cylinders was studied with ordinary‐wave illumination. Free‐field experiments identified resonances worthy of investigation in an evanescent wave experiment. Several of the features identified have been reproduced in FEM‐COMSOL calculations by Williams of APL‐UW. For both ordinary and evanescent waves, strong modes were often excited when the cylinder was highly tilted. A further understanding of the modal features was gained by evaluating the temporal variance of the response. Furthermore, the aluminum cylinders also showed an enhanced spatial decay rate when compared to that of the evanescent soundfield. [Work supported by ONR.]