Title: Granular damping at various levels of relative humidity

Author (Talk): John Valenza, Schlumberger-Doll Research

Abstract:

The objective of our research is to gauge the viability of using a granular medium to attenuate structure borne sound. Accordingly, we measure the frequency dependent complex valued effective mass of a granular medium, and demonstrate the transferability of this measurement for predicting the corresponding effect on the acoustic properties of a resonant structure (resonance frequency and damping rate). When a granular medium is vibrated over a broad range of frequencies (< 5 kHz), the frequency dependent effective mass passes through a maximum that is many times greater than the static mass. Since the amplitude of vibration is small (~ nm), the dynamic mass is almost wholly due to the energy necessary to move the particles relative to one another. The high effective mass is due to dissipative mechanisms such as inter-particle friction, and the force necessary to stretch and compress nanoscopic liquid bridges around the particle contacts. As a result of the latter mechanism, the behavior of the granular medium, and the corresponding effect on the acoustic properties of a resonant structure vary with the relative humidity. Accordingly, we present the results of our study at various levels of relative humidity. A prerequisite to demonstrating transferability is that the particulate medium be prepared in a consistent manner for both the measurement of the effective mass, and the measurement of the acoustic properties of the resonant structure. Therefore, we also investigate the effect of sample handling on the reproducibility of the effective mass. Lastly, we employ toy models to extract information about the elastic and dissipative properties of the granular medium.