Vibro-acoustic effects of a soft compliant layer on submerged finite cylindrical shells

Abstract

The structural acoustics associated with a compliant material applied to the outer surface of a framed fluid-loaded cylinder is examined. Spatially dense laser Doppler vibrometer (LDV) and near field acoustic holography (NAH) measurements are employed to capture the detailed response under radial point force excitation. In the mid- and high-frequency regimes, the soft, low impedance layer is associated with significant reduction in the normal velocity at all wavenumbers. Essentially, there is very little motion of the outer compliant layer due to the impedance mismatch with the fluid. Further, these data indicate that Bloch wave components are modified somewhat due to reduced fluid-loading effects. However, at low frequencies the normal velocities at the fluid interface are greatly accentuated due to simple resonances associated with the impedance of the acoustic medium, the soft spring-like compliant layer, and the structure. Although these resonances are typically characterized as subsonic, enhanced levels are observed in the farfield. Also, it is found experimentally that the input drive impedance changes with the addition of the compliant layer, thus, modifying the energy delivered to the system by the shaker. The data will be reported along with simple impedance models that illustrate the physics of the behavior.