Dynamic Behavior of a Three Dimensional Aluminum Truss

The United States Navy sponsored Truss Damping Program is a research study in the area of submarine noise reduction. The research explores the feasibility of effectively attenuating noise associated with vibration of rotating machinery by means of a truss support structure. Such a structure occupies the entire machinery space by supporting all internal equipment. This thesis explores the dynamics of a three dimensional aluminum truss to determine the inherent type and degree of damping. By comparing experiments to analytic predictions, I learn that attenuation is due to strut radiation for all frequencies above a critical value. Close to the source of excitation the measured attenuation rate is higher because two mechanisms occur simultaneously: struts excited in flexure radiate effectively, and energy in the form of predominantly compressional or torsional waves scatters to energy comprising a more equal balance of compressional, torsional and flexural wave types. Other mechanisms of attenuation, including radiation from joints, losses in the interfaces between components and losses to ground through the supports, are negligible in comparison to the attenuation of scattering and strut radiation. From experimental results and supporting theory, I develop basic dynamic design guidelines. In full scale, the architect must maximize the quantity of unobstructed struts so that efficient radiation occurs. Maximizing the joints (minimizing cell size) near vibration sources and designing strut connections at or close to right angles promotes desirable scattering of wave types. Designs must consider global truss dynamic effects associated with relatively low frequency excitation, especially where the truss is effectively decoupled from the submarine's outer shell. These guidelines, when combined with fundamental requirements of submarine design, are useful in developing a specialized truss which provides an effective means of passively damping machinery-borne noise. Thesis Supervisor: Dr. Ira Dyer Title: Professor