Identification and alteration of global resonance modes in ship structures to reduce sound radiation.

Abstract

A ship hull and foundation structure will have many vibratory resonance modes. During the shipbuilding process on land or in a drydock, it would be highly advantageous to determine which of these modes have a potential for strong sound radiation when the ship is waterborne. This research employs an experimental model of a submarine hull and foundation. Small masses are placed on the foundation and hull, and the effects on the vibration spectra of the structure are studied. Based on spectra response, modes are classified as hull modes, foundation modes, or "global" modes. Global modes are well coupled hull and foundation mode pairs which are closely spaced in frequency. Waterbome testing reveals that global mode pairs are strong waterbome sound radiators. Additionally, the application of small masses is shown to perturb the coupling between the global mode pairs and yield dramatic changes in the radiated sound. Four foundations of differing size are tested and repeatability of the experimental results is confirmed. As the foundation size increases, global modes are identified on the basis of foundation drive point vibration spectra alone, hull vibration data is not required.