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.]