ASA 129th Meeting - Washington, DC - 1995 May 30 .. Jun 06

2pSA8. Basic model for acoustic reflection and absorption from multilayered underwater, sound-attenuating composites.

Donald Brill

U.S. Naval Acad., Phys. Dept., Annapolis, MD 21402

Armando Santiago

Naval Surface Warfare Ctr., Annapolis, MD 21402

Guillermo Gaunaurd

Naval Surface Warfare Ctr., White Oak, MD 20903-5640

To reduce backscattered acoustic echoes from submerged elastic structures, these are routinely covered with viscoelastic (i.e, sound-absorbing) composite multilayers. This old and well-studied area is revisited and a basic, simple, analytical model to predict how these composites behave when tested is presented. The multilayer can have N layers of homogeneous (or inhomogeneous) materials with different elastic/viscoelastic properties, all bonded to each other, and the whole, to a backing (metal) plate. The structure is fluid-loaded and externally excited with plane, monochromatic sound waves. We have developed a ``propagator matrix'' approach that yields the reflected, transmitted, and absorbed waves from the structure as a product of the N transfer matrices of the layers. The approach is computerized and used to evaluate and plot the reflectivity and/or absorptivity of the structure as a function of frequency for pertinent (often complex) values of the layers' material parameters and thicknesses. The predictions are experimentally verified for selected test cases (i.e., N=2,3), and the enhancement of the echo-reduction plots at the resonance frequencies of the layers is noted. Additional complexities can be handled in a straightforward fashion. [Work partially supported by the IL-IR Program of the CDNSWC.]